Unbleached paperboard

Abstract

There is provided a method for manufacturing an unbleached paperboard comprising a top layer, a middle layer and a back layer, said method comprising: - forming a top layer web from a first pulp slurry having a headbox consistency of 0.05-0.5 %; - forming a middle layer web from a second pulp slurry; - forming a back layer from a third pulp slurry; - joining the top layer web, the middle layer web and the back layer web such that the middle layer web is positioned between the top layer web and the back layer web to produce a multilayered web; and - drying the multilayered web; wherein 100% of the fibres of the first pulp slurry are unbleached softwood fibres and the top layer has a grammage of 60 g / m2 or less when measured according to ISO 536:2019.

Description

UNBLEACHED PAPERBOARDTECHNICAL FIELD

[0001] The present disclosure relates to the field of paperboards and in particular to liquid packaging boards.BACKGROUND

[0002] The increasing focus on sustainable and environmentally friendly packaging solutions has led to growing interest in paperboard-based liquid packaging as an alternative to plastic. Liquid packaging boards (LPBs) are widely used for beverages, dairy, and other liquid products, traditionally made using bleached white paperboards. However, the bleaching process involves significant chemical use and environmental impact. Thus, there is still a need for further development of the paperboards used as liquid packaging boards.SUMMARY

[0003] Brown paperboards, derived from unbleached or lightly bleached fibers, may provide a more sustainable option by reducing chemical usage and enhancing recyclability. Additionally, their natural appearance may align with consumer preferences for eco-conscious packaging. However, there is still a need for further development to use brown paperboards as liquid packaging boards. By providing an unbleached paperboard having a top layer made out of 100 % unbleached softwood, the present inventors realized that they could further reduce the environmental impact of the brown paperboards while maintaining satisfactory strength and surface properties.

[0004] According to a first aspect, there is a method for manufacturing an unbleached paperboard comprising a top layer, a middle layer and a back layer, said method comprising:- forming a top layer web from a first pulp slurry having a head box consistency of 0.05-0.5 %;- forming a middle layer web from a second pulp slurry;- forming a back layer web form a third pulp slurry;- joining the top layer web, the middle layer web and the back layer web such that the middle layer web is positioned between the top layer web and the back layer web toproduce a multilayered web; and- drying the multilayered web;wherein 100 % of the fibres in the first pulp slurry are unbleached softwood fibres and the top layer has a grammage of 60 g / m2or less when measured according to ISO 536:2019.

[0005] Producing an unbleached paperboard according to the method of the first aspect results in a paperboard using a lower amount of fibres and pulping chemicals during production while improving the bending resistance index and maintaining satisfactory strength and surface properties. The material reduction enabled by the present disclosure results in a reduction in cost and environmental impact. Using a combination of 100 % softwood fibres in the top layer together with a head-box consistency of 0.05-0.5 % enables a good formation of the top layer while having satisfactory surface properties.

[0006] The multilayered web may be dried to a moisture content is 4-10 %, such as 5-8 %.

[0007] The unbleached softwood fibres in the first pulp slurry may have a Schopper-Reigler (SR) value according to ISO 5267-1:1999 value of 18-39, such as 21-34, such as 22-32 after repulping according to ISO 5263-1:2004. This SR value may enable better formation and improved surface properties.

[0008] The second pulp slurry and / or third pulp slurry may comprise unbleached softwood fibres and / or unbleached hardwood fibres. At least 70 %, such as at least 80 %, such as at least 90 %, of the fibres in the second pulp slurry and the third pulp slurry are unbleached fibres. The percentage is based on the total amount by weight of fibres in the respective pulp slurry.

[0009] Preferably, at least 80 % of the fibres in the unbleached paperboard are unbleached fibres, such as at least 85 %, such as at least 90 %. The percentage is based on the total amount by weight of fibres in the unbleached paperboard. This results in a reduction in the amount of chemicals used during production and thus a reduction of the environmental impact.

[0010] The second pulp slurry may comprise CTMP, chemical pulp such as kraft pulp and / or broke. Preferably, the pulp in the second pulp slurry is a mixture ofunbleached chemical pulp and broke. More preferably, the chemical pulp is softwood chemical pulp.[oon] The third pulp slurry may comprise chemical pulp such as kraft pulp and / or broke. Preferably, the pulp in the third pulp slurry is a mixture of unbleached chemical pulp and broke. More preferably, the chemical pulp is softwood chemical pulp.

[0012] The unbleached paperboard may comprise at least 70 % of softwood fibres, preferably 80 % softwood fibres based on the total amount by weight of fibres in the unbleached paperboard.

[0013] The head box consistency of the first pulp slurry may be o.1-0.5such as o.1-0.4 such as 0.1-0.3 %. This consistency enables a particularly good formation of the top layer.

[0014] The first, second and / or third pulp slurry may comprise additives such as alkyl ketene dimer (AKD), alkyl succinic anhydride (ASA), rosin size, starch and / or fillers. More preferably, the first, second and third pulp slurry comprise AKD, ASA and / or rosin size.

[0015] AKD may be present in an amount of o.2-2.5 kg / tonne fibres per pulp slurry, such as 0.2-2. o kg / tonne fibres per pulp slurry, such as 0.2-1.8 kg / tonne fibres per pulp slurry. When using the method according to the first aspect to produce an unbleached paperboard, the amount of sizing agent, i.e., AKD, ASA and / or rosin size can be reduced while still maintaining the hydrophobic effect. Thus, a reduction in cost and environmental impact may be obtained without compromising the properties of the unbleached paperboard.

[0016] According to a second aspect, there is provided an unbleached paperboard comprising a top layer, a back layer and a middle layer positioned in between the top layer and the back layer wherein the top layer:- comprises fibres and optionally additives, and 100 % of the fibres in the top layer are unbleached softwood fibres;- has a grammage of 60 g / m2or less when measured according to ISO 536:2019; and - is formed using a head box consistency of 0.05-0.5 %.

[0017] An unbleached paperboard according to the second aspect uses a lower amount of fibres and pulping chemicals during production while maintainingsatisfactory strength and surface properties. The material reduction enabled by the present disclosure results in a reduction in cost and environmental impact.Furthermore, using a combination of 100 % softwood fibres in the top layer together with a head-box consistency of 0.05-0.5 % enables a good formation of the top layer while maintaining satisfactory surface properties and reducing the environmental impact.

[0018] The top layer may have a grammage of 58 g / m2or less, 55 g / m2or less when measured according to ISO 536:2019.

[0019] The back layer and / or middle layer may comprise unbleached softwood fibres and / or unbleached hardwood fibres. At least 70 %, such as at least 80 %, such as at least 90 %, of the fibres in the back layer and the middle layer are unbleached fibres. The percentage is based on the total amount by weight of the fibres in the respective layer.

[0020] Preferably, at least 80 % of the fibres in the unbleached paperboard are unbleached fibres, such as at least 85 %, such as at least 90 %. This results in a reduction in the amount of chemicals used during production and thus a reduction of the environmental impact. The percentage is based on the total amount by weight of the fibres in the respective layer.

[0021] The middle layer may be formed using chemi-thermomechanical pulp (CTMP), chemical pulp such as kraft pulp and / or broke. Preferably, the middle layer is formed using a mixture of unbleached chemical pulp and broke. More preferably, the chemical pulp is softwood chemical pulp.

[0022] The back layer may be formed using chemical pulp such as kraft pulp and / or broke. Preferably, the back layer is formed using a mixture of unbleached chemical pulp and broke. More preferably, the chemical pulp is softwood chemical pulp.

[0023] The unbleached paperboard may comprise at least 70 % of softwood fibres, preferably 80 % softwood fibres based on the total amount by weight of fibres in the unbleached paperboard.

[0024] The unbleached softwood fibres in the top layer may have a Schopper-Reigler (SR) value according to ISO 5267-1:1999 value of 18-39, such as 21-34, suchas 22-32 after repulping according to ISO 5263-1:2004. This SR value may enable better formation and improved surface properties.

[0025] In a preferred embodiment, the top layer is formed using a head box consistency is o.i-o.5%, such as o.1-0.4such as 0.1-0.3 %. This consistency enables a particularly good formation of the top layer having a grammage of 60 g / m2or less as well as satisfactory surface properties.

[0026] Preferably, the unbleached paperboard comprises additives such as alkyl ketene dimer (AKD), alkyl succinic anhydride (ASA), rosin size, starch and / or fillers. More preferably, the top layer, middle layer and back layer comprise AKD, ASA and / or rosin size.

[0027] AKD may be present in an amount of o.2-2.5 kg / tonne fibres per pulp slurry, such as 0.2-2. o kg / tonne fibres per pulp slurry, such as 0.2-1.8 kg / tonne fibres per pulp slurry. When preparing an unbleached paperboard according to the second aspect, the amount of sizing agent, i.e., AKD, ASA and / or rosin size can be reduced while still maintaining its sizing effect. Thus, a reduction in cost and environmental impact may be obtained without compromising the properties of the unbleached paperboard.

[0028] All embodiments of the first aspect apply mutatis mutandis to the second aspect and vice versa.DETAILED DESCRIPTION

[0029] There is a desire to further reduced the environmental impacts of the brown paperboards. Thus, the present inventors have realized that an unbleached paperboard according to the present disclosure would lead to a lower material consumption such as pulp and chemicals, while maintaining satisfactory strength and surface properties.

[0030] According to the first aspect, there is a method for manufacturing an unbleached paperboard comprising a top layer, a middle layer and a back layer, said method comprising:- forming a top layer web from a first pulp slurry having a head box consistency of 0.05-0.5 %;- forming a middle layer web from a second pulp slurry;- forming a back layer web form a third pulp slurry;- joining the top layer web, the middle layer web and the back layer web such that the middle layer web is positioned between the top layer web and the back layer web to produce a multilayered web; and- drying the multilayered web; andwherein 100 % of the fibres in the first pulp are unbleached softwood fibres and the top layer has a grammage of 60 g / m2or less when measured according to ISO 536:2019.

[0031] Producing an unbleached paperboard according to the method of the first aspect results in a paperboard using a lower amount of fibres and pulping chemicals during production while improving the bending resistance index and maintaining satisfactory strength and surface properties. The material reduction enabled by the present disclosure results in a reduction in cost and environmental impact.

[0032] The following embodiments are meant to exemplify the method according to the first aspect and should not be seen as limiting to the invention.

[0033] The unbleached paperboard of the present disclosure is preferably manufactured in a paperboard machine.

[0034] A first pulp slurry is prepared comprising fibres and optionally additives, wherein 100 % of the fibres in the first pulp slurry are unbleached softwood fibres. The fibres are preferably provided as chemical pulp. The additives maybe AKD, ASA, rosin size, starch and / or fillers. Preferably, the first pulp slurry comprises AKD, ASA and / or rosin size. More preferably, the first pulp slurry comprises AKD in an amount of o.2-2.5 kg / tonne fibres, such as 0.2-2. o kg / tonne fibres, such as o.2-1.8 kg / tonne fibres. .

[0035] A second pulp slurry is prepared, preferably comprising unbleached fibres and optionally additives. Preferably, at least 80 % the fibres in the second pulp are unbleached fibres. The fibres may be provided as chemical pulp and / or broke. The unbleached fibres may comprise unbleached softwood fibres and / or hardwood fibres. Most preferably, the unbleached fibres are provided as chemical pulp and broke. The additives maybe AKD, ASA, rosin size, starch and / or fillers. Preferably, the second pulp slurry comprises AKD, ASA and / or rosin size. More preferably, thesecond pulp slurry comprises AKD in an amount of o.2-2.5 kg / tonne fibres, such as o.2-2.0 kg / tonne fibres, such as o.2-1.8 kg / tonne fibres.

[0036] A third pulp slurry is prepared preferably using unbleached fibres and optionally additives. The unbleached fibres maybe unbleached softwood fibres and / or hardwood fibres. Preferably, the unbleached fibres are provided as chemical pulp and / or broke. Most preferably, the unbleached fibres are provided as chemical pulp and broke. The additives may be AKD, ASA, rosin size, starch and / or fillers. Preferably, the third pulp slurry comprises AKD, ASA and / or rosin size. More preferably, the third pulp slurry comprises AKD in an amount of o.2-2.5 kg / tonne fibres, such as 0.2-2. o kg / tonne fibres, such as 0.2-1.8 kg / tonne fibres.

[0037] After the pulp slurries have been prepared, they may be directed to their respective head boxes. The head box consistency of the first pulp slurry is 0.05-0.5 %, such as o.i-o.5%, such as o.1-0.4such as 0.1-0.3 %. The head box consistency of the second pulp slurry may be 0.15-0.6 %, such as 0.2-0.5 % and / or the head box consistency of the third pulp slurry maybe 0.05-0.5 %, such as 0.1-0.4 %.

[0038] The top, middle and back layer webs are thus formed using the respective head boxes. The dry mass flow through the head boxes may be controlled so that specific grammages of the three layers are obtained. For the top layer the dry mass flow through the top layer head box may be such that a top layer having a grammage of 60 g / m2or less is obtained when measured according to ISO 536:2019.

[0039] Webs may be formed on respective wires for the top layer, the middle layer and the back layer. The top layer web, middle layer web and back layer web may be dewatered.

[0040] The top layer web, middle layer web and back layer web are joined together such that the middle layer web is positioned between the top layer web and the back layer web. The top layer web, middle layer web and back layer web may be joined together via couching to form the multilayer web. Couching maybe performed under reduced pressure.

[0041] The multilayer web may be subjected to drying until a moisture content of 4-10 % such as 5-8 % is obtained. The drying maybe followed by pressing and or calendaring.

[0042] Preferably, the unbleached softwood fibres of the first pulp slurry are not cellulase-treated.

[0043] Further, the unbleached fibres of the second pulp slurry are preferably not cellulase treated.

[0044] Further, the unbleached fibres of the third pulp slurry are preferably not cellulase treated.

[0045] In a particularly preferred embodiment, none of the first, the second and the third pulp slurry comprises cellulase-treated unbleached pulp. As an example, the first, the second and the third pulp slurry may comprise essentially no cellulase-treated pulp.

[0046] A reason for avoiding cellulase-treated pulp is that the cellulase treatment is difficult to implement in full-scale paperboard production.

[0047] According to a second aspect, an unbleached paperboard is provided comprising a top layer, a back layer and a middle layer positioned in between the top layer and the back layer wherein the top layer:- comprises fibres and optionally additives, and 100 % of the fibres in the top layer are unbleached softwood fibres;- has a grammage of 60 g / m2or less when measured according to ISO 536:2019; and - is formed using a head box consistency of 0.05-0.5 %.

[0048] An unbleached paperboard according to the second aspect results in a paperboard using a reduced amount of pulp and pulping chemicals during production while maintaining satisfactory strength and surface properties. The material reduction enabled by the present disclosure may result in a reduction in cost and environmental impact.

[0049] The unbleached paperboard comprises three layers, a top layer, a back layer and a middle layer positioned in between the top layer and the back layer. The top layer comprises fibres and optionally additives. 100 % of the fibres in the top layer are unbleached softwood fibres. The percentage is based on the total amount by weight of fibres in the top layer. That the 100 % of the fibres in the top layer are unbleached softwood fibres is understood to mean that no other type of fibres are present in the top layer. The top layer may, however, comprise additives. Preferably, the unbleached softwood fibres are provided as chemical pulp such as kraft pulp.

[0050] The unbleached fibres used in the present disclosure may be understood to mean pulp that has not been subjected to a bleaching process or has been subjected to a light bleaching process. Preferably, the unbleached softwood fibres in the top layer, middle layer and / or back layer has a Kappa number of at least 40-70, such as at least 45.

[0051] The unbleached softwood fibres in the top layer may have an SR value according to ISO 5267-1:1999 value of 18-39, such as 21-34, such as 22-32 after repulping according to ISO 5263-1:2004. This SR value may enable better formation and improved surface properties.

[0052] The softwood fibres may be selected from spruce, pine or a mixture thereof. Preferably, the softwood is pine.

[0053] The grammage of the top layer of the unbleached paperboard is 60 g / m2or less when measured according to ISO 536:2019, such as 58 g / m2or less, such as 55 g / m2or less. This is typically lower than top layers of bleached paperboards, which typically have a grammage of 70 g / m2or more. A lower limit for the top layer may be 35 g / m2. The low grammage of the top layer may result in a reduction of material use and thus a reduced environmental impact.

[0054] To separate the top layer from the multilayered paperboard and thus facilitate the grammage measurement, 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.

[0055] Alternatively, a surface grinding technique can be used to remove all layers but the layer in question. Such a surface grinding is one of the services that are commercially available at RISE Bioeconomy (formerly Innventia) in Stockholm, Sweden.

[0056] Softwood fibres may provide strength but usually gives rise to poor formation and poor surface properties which is why it is usually mixed with hardwood fibres. By using a low head box consistency when forming the top layer in the present disclosure, 100 % softwood can be used in the top layer while still maintaining good formation. This enables a lowering in the grammage of the top layer while maintaining satisfactory strength and surface roughness of theunbleached paperboard. Thus, the top layer of the unbleached paperboard is formed using a headbox consistency of 0.05-0.5 %, such as o.1-0.5such as o.1-0.4such as o.1-0.3 %• The consistency is given in weight %, i.e., weight of fibers based on the total weight of the pulp slurry.

[0057] The middle layer may comprise unbleached softwood fibres and / or hardwood fibres. Preferably, the fibres in the middle layer are provided as CTMP, chemical pulp such as kraft pulp and / or broke. More preferably, the fibres in the middle layer are provided as chemical pulp and broke. 100 % of the fibres in the middle layer are preferably unbleached fibres. The percentage is based on the total amount by weight of fibres in the middle layer.

[0058] The grammage of the middle layer may be 60-160 g / m2when measured according to ISO 536:2019.

[0059] The back layer may comprise unbleached softwood fibres and / or unbleached hardwood fibres. Preferably, the fibres in the back layer are provided as chemical pulp such as kraft pulp and / or broke. More preferably, the fibres in the back layer are provided as chemical pulp and broke. At least 70 % of the fibres in the back layer are preferably unbleached fibres. The percentage is based on the total amount by weight of fibres in the back layer.

[0060] The grammage of the back layer maybe 40-70 g / m2when measured according to ISO 536:2019.

[0061] At lease 80 % of the fibres in the unbleached paperboard may be unbleached fibres, such as 85 %, such as at least 90 %. The percentage is based on the total amount by weight of fibres in the unbleached paperboard. This results in a reduction in the amount of chemicals used during production and thus a reduction of the environmental impact.

[0062] The unbleached paperboard may comprise at least 70 % of unbleached softwood, preferably 80 % unbleached softwood based on the total amount by weight of fibres in the unbleached paperboard.

[0063] Preferably, the unbleached paperboard comprises additives such as alkyl ketene dimer (AKD), alkyl succinic anhydride (ASA), rosin size, starch and / or fillers. More preferably, the top layer, middle layer and back layer comprise AKD, ASA and / or rosin size.

[0064] AKD may be present in an amount of o.2-2.5 kg / tonne fibres per layer, such as 0.2-2. o kg / tonne fibres per layer, such as o.2-1.8 kg / tonne fibres per layer. When preparing an unbleached paperboard according to the second aspect, the amount of sizing agent, i.e., AKD, ASA and / or rosin size can be reduced while still maintaining its sizing effect. Thus, a reduction in cost and environmental impact may be obtained without compromising the properties of the unbleached paperboard.

[0065] The unbleached paperboard may have a grammage of 150-300 g / m2when measured according to ISO 536:2019

[0066] The unbleached paperboard according to the present disclosure may comprise at least one coating such as a barrier coating on the top layer and / or the back layer.

[0067] The unbleached paperboard may be a liquid packaging board (LPB).

[0068] Preferably, the unbleached softwood fibres of the top layer are not cellulase-treated.

[0069] Further, the unbleached fibres of the middle layer are preferably not cellulase treated.

[0070] Further, the unbleached fibres of the back layer are preferably not cellulase treated.

[0071] In a particularly preferred embodiment, none of the top, the middle and the back layer comprises cellulase-treated unbleached pulp. As an example, the top, the middle and the back layer may comprise essentially no cellulase-treated pulp.

[0072] As stated above, a reason for avoiding cellulase-treated pulp is that the cellulase treatment is difficult to implement in full-scale paperboard production.

[0073] All embodiments of the first aspect apply mutatis mutandis to the second aspect and vice versa.EXAMPLESExample 1

[0074] An unbleached paperboard was produced having three layers and a target grammage of 217 g / m2. The top layer had a target grammage of 50 g / m2, themiddle layer had a target grammage of 122 g / m2and the back layer had a target grammage of 45 g / m2.

[0075] The unbleached paperboard was formed using a paperboard machine using multiple headboxes. Three different pulps slurries were used to form the three different layers. Unbleached pine fibers, provided as a pine kraft pulp, and unbleached broke were used to prepare the unbleached paperboard. The broke came from unbleached board and the unbleached pine kraft pulp was refined to a SR value of 27 when determined according to ISO 5267-1:1999 after repulping according to ISO 5263-1:2004. The compositions of the different pulp slurries were:

[0076] First pulp slurry (top layer): 100 wt.% unbleached pine kraft pulp.

[0077] Second pulp slurry (middle layer): 80 wt. % unbleached pine kraft pulp and 20 wt. % unbleached broke.

[0078] Third pulp slurry (back layer): 70 wt. % unbleached pine kraft pulp and 30 wt. % unbleached broke.

[0079] Papermaking chemicals were added as additives to the pulp slurries. AKD was added in an amount of 1.7 kg / tonne fibre to the first pulp slurry, in an amount of 2.1 kg / tonne fibre to the second pulp slurry and in an amount of 1.4 kg / tonne fibre to the third pulp slurry. Rosin size was added in an amount of ~ 0.5 kg / tonne fibre to each of the three pulp slurries. Starch and alun were further added to all three pulp slurries.

[0080] The pulp slurries were directed to their respective head boxes. The top layer pulp slurry was directed to the top layer head box having a consistency of 0.2 %. The dry mass flow through each of the head boxes was adjusted to reach a target grammage of 50 g / m2for the top layer, 122 g / m2for the middle layer and 45 g / m2for the back layer when dry. The three fibre webs were formed on three different wires from the respective headboxes and dewatered. The formed top layer web, middle layer web and back layer web where then couched together under reduced pressure (15 kPa) such that the middle layer web was positioned in between the top layer web and the back layer web to form a multilayered web. The multilayered web was then directed to a press section and a drying section. The dried unbleached paperboard had a moisture content of 7.5 %. The unbleached paperboard was finally calendared.

[0081] The properties of the unbleached paperboard were characterized and compared to a bleached paperboard (comparative example) having a grammage of 234 g / m2, see Table 1.

[0082] A bleached paperboard is used as a comparative example. The bleached paperboard is a three-layer paperboard. The compositions of the different pulp slurries used for the different layers of the bleached paperboard were:

[0083] Top layer pulp slurry: 65 wt.% bleached pine kraft pulp and 35 wt.% bleached hardwood pulp.

[0084] Middle layer pulp slurry: 66 wt. % unbleached pine kraft pulp and 34 wt. % unbleached broke.

[0085] Back layer pulp slurry: 75 wt. % unbleached pine kraft pulp and 25 wt. % unbleached broke.

[0086] The bleached paperboard was prepared in the same manner as the unbleached paperboard. The top layer of the bleached paperboard comprised only bleached fibres where the fibres were a mixture of hardwood and softwood fibre. The grammage of the top layer was also lower for the bleached paperboard. The target grammage of the top layer of the bleached paperboard was 68 g / m2, 18 g / m2higher than the top layer of the unbleached paperboards. The middle layer and the back layer of the bleached paperboard had the same target grammages as the middle layer and the back layer of the unbleached paperboard, 122 g / m2and 45 g / m2respectively. The comparative bleached paperboard was further coated with a coating having a coat weight of 20 g / m2. The coating does not influence the properties in Table 1. The test points for evaluating the properties were located in the middle of the tested paperboards.

[0087] The grammages of the paperboards and the top layers were measured according to ISO 536:2019.

[0088] The bending resistance and bending resistance index were measured according to ISO 2493-1, tensile strength was measured according to ISO 1924-3 and Bendtsen roughness was measured according to ISO 8791-2.

[0089] Table 1 shows the properties of two unbleached paperboards and one bleached paperboard.* The bleached paperboard was coated.**Lactic acid solution was used as the test solution to be absorbed.***Hydrogen peroxide solution was used as the test solution to be absorbed.****Surface roughness was measured on uncoated paperboards after calendaring.

[0090] Table 1 shows that the tensile strength and bending resistance of the unbleached paperboards are in the same range as the tensile strength and bending resistance of the bleached paperboard even though the grammages of the unbleached paperboards are lower. Furthermore, it is shown that the bending resistance index increased for the unbleached paperboards compared to the bleached paperboard.

[0091] It was further shown that the edge wicking index at 23 °C for the unbleached paperboards was in the same range as for the bleached paperboard while the edge wicking index at 70 °C was improved compared to the bleached paperboard, even though the amount of AKD was lower in the unbleached paperboards compared to the bleached paperboard. Furthermore, the unbleached paperboard also exhibited satisfactory surface roughness.

[0092] This example shows that an unbleached paperboard according to the present disclosure can be obtained having satisfactory strength properties, sizing properties and surface roughness while lowering the material consumption and thus improving the environmental impact.Example 2

[0093] An unbleached paperboard was produced having three layers and a target grammage of 205 g / m2. The top layer had a target grammage of 45 g / m2, the middle layer had a grammage of 115 g / m2and the back layer had a grammage of 45 g / m2.

[0094] The unbleached paperboard was formed using a paperboard machine using multiple headboxes. Three different pulps slurries were used to form the three different layers. Unbleached pine fibers, provided as a pine kraft pulp, and unbleached broke were used to prepare the unbleached paperboard. The broke came from unbleached board and the unbleached pine kraft pulp was refined to a SR value of 27 when determined according to ISO 5267-1:1999 after repulping according to ISO 5263-1:2004. The compositions of the different pulp slurries were:

[0095] First pulp slurry (top layer): 100 wt.% unbleached pine kraft pulp.

[0096] Second pulp slurry (middle layer): 80 wt. % unbleached pine kraft pulp and 20 wt. % unbleached broke.

[0097] Third pulp slurry (back layer): 70 wt. % unbleached pine kraft pulp and 30 wt. % unbleached broke.

[0098] Papermaking chemicals were added as additives to the pulp slurries. The amount of AKD was reduced to 1.3 kg / tonne fibre in the first pulp slurry. AKD was added in an amount of 2.1 kg / tonne fibre to the second pulp slurry and in an amount of 1.4 kg / tonne fibre to the third pulp slurry. Rosin size was added in an amount of~o.5 kg / tonne fibre to each of the three pulp slurries. Starch and alun were further added to all three pulp slurries.

[0099] The pulp slurries were directed to their respective head boxes. The top layer pulp slurry was directed to the top layer head box having a consistency of 0.2 %. The dry mass flow through each of the head boxes was adjusted to reach a target grammage of 50 g / m2for the top layer, 121 g / m2for the middle layer and 45 g / m2for the back layer when dry. The three fibre webs were formed on three different wires from the respective headboxes and dewatered. The formed top layer web, middle layer web and back layer web where then couched together under reduced pressure (15 kPa) such that the middle layer web was positioned in between the top layer web and the back layer web to form a multilayered web. The multilayered web was then directed to a press section and a drying section. The dried unbleached paperboard had a moisture content of 7.5 %. The unbleached paperboard was finally calendared.

[0100] The test points for evaluating the properties were located in the middle of the tested unbleached paperboards. The grammages of the paperboards and the top layers were measured according to ISO 536:2019.

[0101] The bending resistance and the bending resistance index were measured according to ISO 2493-1, tensile strength was measured according to ISO 1924-3 and Bendtsen roughness was measured according to ISO 8791-2.

[0102] The properties of the unbleached paperboards of Example 2 can be seen in Table 2.

[0103] Table 2 shows the properties of two unbleached paperboards.* Lactic acid solution was used as the test solution to be absorbed.**Hydrogen peroxide solution was used as the test solution to be absorbed.***Bendtsen roughness was measured on calendared paperboards.

[0104] Table 2 shows that the unbleached paperboards have satisfactory strength and Bendtsen roughness. Table 2 further shows that the bending resistance index is further increase compared to the unbleached paperboards in Example 1. It is also shown that the edge wicking index is kept low even though the amount of AKD is reduced compared to the unbleached paperboards in Example 1.

[0105] Thus, it is shown that it is possible to further reduced the grammage of the unbleached paperboard and the amount of AKD in the top layer without deteriorating the tensile strength, bending resistance, surface roughness and edge wicking index of the unbleached paperboard.

Claims

CLAIMS1. A method for manufacturing an unbleached paperboard comprising a top layer, a middle layer and a back layer, said method comprising:- forming a top layer web from a first pulp slurry having a headbox consistency of 0.05-0.5 %;- forming a middle layer web from a second pulp slurry;- forming a back layer web from a third pulp slurry;- joining the top layer web, the middle layer web and the back layer web such that the middle layer web is positioned between the top layer web and the back layer web to produce a multilayered web; and- drying the multilayered web;wherein 100 % of the fibres of the first pulp slurry are unbleached softwood fibres, at least 70 % of the fibres in the second pulp slurry are unbleached fibres, at least 70 % of the fibres in the third pulp slurry are unbleached fibres, and the top layer has a grammage of 60 g / m2or less when measured according to ISO 536:2019.

2. The method according to claim 1, wherein the second and / or third pulp slurry comprises unbleached softwood fibres and / or unbleached hardwood fibres.

3. The method according to any one of claims 1-2, wherein at least 80 % of the fibres in the unbleached paperboard are unbleached fibres.

4. The method according to any one of the preceding claims, wherein the first, second and third pulp slurry comprise alkyl ketene dimer, alkyl succinic anhydride and / or rosin size.

5. The method according to claim 4, wherein the alkyl ketene dimer is present in an amount of o.2-2.5 kg / tonne fibres per pulp slurry, such as o.2-2.0 kg / tonne fibres per pulp slurry, such as o.2-1.8 kg / tonne fibres per pulp slurry.

6. The method according to any one of the preceding claims, wherein the head box consistency of the first pulp slurry is o.1-0.5%, such as o.1-0.4 %, such as o.1-0.3 %•7. The method according to any one of the preceding claims, wherein:the unbleached softwood fibres of the first pulp slurry are not cellulase- treated;the unbleached fibres of the second pulp slurry are not cellulase-treated; and / orthe unbleached fibres of the third pulp slurry are not cellulase-treated.

8. An unbleached paperboard comprising a top layer, a back layer and a middle layer positioned in between the top layer and the back layer, wherein the top layer:- comprises fibres and optionally additives and 100 % of the fibres in the top layer are unbleached softwood fibres;- has a grammage of 60 g / m2or less when measured according to ISO 536:2019; and- is formed using a head box consistency of 0.05-0.5 %and wherein at least 70 % of the fibres in the back layer are unbleached fibres and at least 70 % of the fibres in the middle layer are unbleached fibres.

9. The unbleached paperboard according to claim 8, wherein 80 % of the fibres in the unbleached paperboard are unbleached fibres.

10. The unbleached paperboard according to any one of claims 8-9, wherein the back layer and / or middle layer comprises unbleached softwood fibres and / or unbleached hardwood fibres.

11. The unbleached paperboard according to any one of claims 8-10, wherein the top layer has a grammage of 58 g / m2or less, such as 55 g / m2or less when measured according to ISO 536:2019.

12. The unbleached paperboard according to any one of claims 8-11, wherein the unbleached paperboard comprises at least 70 % of softwood fibres, preferably 80 % softwood fibres based on the total amount by weight of fibres in the unbleached paperboard.

13. The unbleached paperboard according to any one of claims 8-12, wherein the unbleached softwood fibres in the top layer has a Schopper-Reigler (SR)value according to ISO 5267-1:1999 of 18-39, such as 21-34, such as 22-32 after repulping according to ISO 5263-1:2004.

14. The unbleached paperboard according to any one of claims 8-13, wherein the head box consistency is o.i-o.5%, such as o.1-0.4such as 0.1-0.3 %.

15. The unbleached paperboard according to any one of the claims 8-14, wherein the top layer, middle layer and back layer comprise alkyl ketene dimer, alkyl succinic anhydride and / or rosin size.

16. The unbleached paperboard according to claim 15, wherein the alkyl ketene dimer is present in an amount of o.2-2.5 kg / tonne fibres per layer, such as o.2-2.0 kg / tonne fibres per layer, such as o.2-1.8 kg / tonne fibres per layer.

17. The unbleached paperboard according to any one of the claims 8-16, wherein:the unbleached softwood fibres of the top layer are not cellulase-treated; the unbleached fibres of the back layer are not cellulase-treated; and / or the unbleached fibres of the middle layer are not cellulase-treated.

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