Multilayer paperboard

Abstract

There is provided a method of forming a multilayer paperboard comprising a top layer, a back layer and a middle layer, wherein the method comprises the following steps: - providing a top layer furnish comprising at least 50% by dry weight kraft pulp; - providing a middle layer furnish comprising 25-75% by dry weight of maple chemithermomechanical pulp (CTMP) and 15-65% by dry weight of broke pulp; - adding a cationic retention aid to the middle layer furnish in an amount of 250-700 g / tonne based on dry weight of the middle layer furnish; - adding an anionic retention aid to the middle layer furnish in an amount of 420-700 g / tonne based on dry weight of the middle layer furnish; - providing a back layer furnish comprising at least 80% by dry weight kraft pulp; - form a top layer from the top layer furnish, a middle layer from the middle layer furnish, and a back layer from the back layer furnish; and - form a multilayer paperboard from the top layer, middle layer and back layer so that the middle layer is arranged between the top layer and back layer.

Description

MULTILAYER PAPERBOARDTECHNICAL FIELD

[0001] The present disclosure relates to the field of paperboard and in particular to multilayer paperboard comprising maple CTMP in the middle layer.BACKGROUND

[0002] Paperboards are commonly made of one layer or out of two or more layers, wherein the latter is referred to as multilayer paperboards. Multilayer paperboards are used in for example packaging due to their mechanical properties.

[0003] For multilayer boards having three layers, mechanical strength is often provided through construction of an I-beam structure, with chemical pulp in the outer layers and bulky mechanical pulp in the middle layer.

[0004] The mechanical properties are related to the density, a higher density typically provides a higher strength. However, a higher density also requires more fibres, which is negative from a cost and environmental perspective.

[0005] Therefore, there is a desire to produce paperboards with improved strength without increasing the fibre consumption at the same time.SUMMARY

[0006] There is an objective of the present disclosure to provide a method for producing a multilayer paperboard comprising three layers having improved strength properties, in particular improved strength properties in the Z direction.

[0007] Accordingly, the present disclosure provides the following listing of itemized embodiments:1. A method of forming a multilayer paperboard comprising a top layer, a back layer and a middle layer, wherein the method comprises the following steps: providing a top layer furnish comprising at least 50% by dry weight kraft pulp; providing a middle layer furnish comprising 25-75% by dry weight of maple chemithermomechanical pulp (CTMP) and 15-65% by dry weight of broke pulp; adding a cationic retention aid to the middle layer furnish in an amount of 250- 700 g / tonne based on dry weight of the middle layer furnish;adding an anionic retention aid to the middle layer furnish in an amount of 420- 700 g / tonne based on dry weight of the middle layer furnish; providing a back layer furnish comprising at least 80% by dry weight kraft pulp; form a top layer from the top layer furnish, a middle layer from the middle layer furnish, and a back layer from the back layer furnish; and form a multilayer paperboard from the top layer, middle layer and back layer so that the middle layer is arranged between the top layer and back layer.2. The method of item 1, wherein the cationic retention aid is cationic polyacrylamide (cPAM), polyethyleneimine (PEI) or cationic starch.3. The method of item 1 or 2, wherein the anionic retention aid is silica microparticles or bentonite clay.4. The method of any one of the preceding items, wherein starch is added to the middle layer furnish.5. The method of item 4, wherein the amount of starch added to the middle layer furnish is of 1.5-7.0 kg / tonne based on dry weight of the middle layer furnish, such as 2.0-6.0 kg / tonne based on dry weight of the middle layer furnish.6. The method of any one of the preceding items, wherein the middle layer is formed from the middle layer furnish by a headbox having a vertical slice opening of 38-26 mm, such as 31-26 mm, such as 30-27 mm.7. The method of any one of the preceding items, wherein the maple CTMP provided in the middle layer furnish has a Canadian standard freeness (CSF) measured according to ISO 5267-2:2001 of 300-600 ml, such as 300-500 ml, such as 310-450 ml.8. The method of any one of the preceding items, wherein the length-weighted average fibre length of said maple CTMP as measured according to ISO 16065-2:2014 is equal to or below 1.0 mm, such as equal to or below 0.9 mm.9. The method of any one of the preceding items, wherein the maple CTMP in the middle layer furnish has been refined to 10-30 kWh / t.10. The method of any one of the preceding items, wherein the broke pulp in the middle layer furnish has been refined to 20-40 kWh / t.11. The method of any one of the preceding items, wherein the maple CTMP content in the middle layer furnish is 40-70 wt% by dry weight of the furnish, such as 50-65 wt% by dry weight of the furnish, such as 52-61 wt% of the furnish.12. The method of any one of the preceding items, wherein the content of broke pulp in the middle layer furnish is 30-60 wt% by dry weight of the furnish, such as 35-50 wt% by dry weight of the furnish, such as 39-48 wt% by dry weight of the furnish.13. The method of any one of the preceding items, wherein the cationic retention aid is added in an amount of 330-650 g / tonne based on dry weight of the middle layer furnish, such as 330-620 g / tonne based on dry weight of the middle layer furnish, such as 350-600 g / tonne based on dry weight of the middle layer furnish.14. The method of any one of the preceding items, wherein the anionic retention aid is added in an amount of 420-650 g / tonne based on dry weight of the middle layer furnish, such as 450-630 g / tonne based on dry weight of the middle layer furnish.15. The method of any one of the preceding items, wherein the multilayered paperboard is calendered.16. The method of item 15, wherein the line load in the calendering is 30-60 kN / m, such as 35-55 kN / m, such as 38-50 kN / m.17. The method of any one of the preceding items, wherein starch is sprayed between the layers of the multilayer paperboard.18. The method of item 17, wherein the amount of spray starch is o.8-2.3 g / m2, such as 1.3-2.3 g / m2, such as 1.5-2. o g / m2.19. The method of any one of the preceding items, wherein the solids concentration in the white water is below 650 mg / L, such as below 600 mg / L.20. The method of any one of the preceding items, wherein the top layer furnish comprises at least 70% by dry weight kraft pulp, such as at least 85% by dry weight kraft pulp.21. The method of any one of the preceding items, wherein the top layer furnish comprises at least 50% by dry weight hardwood kraft pulp.22. The method of any one of the preceding items, wherein the back layer furnish comprises at least 80% by dry weight softwood kraft pulp.23. The method of any one of the preceding items, wherein the back layer furnish comprises at least 85% by dry weight kraft pulp, such as at least 90% by dry weight kraft pulp.24. The method of any one of the preceding items, wherein the kraft pulp in the top layer furnish is bleached.25. The method of any one of the preceding items, wherein the kraft pulp in the back layer furnish is unbleached.26. The method of any one of the preceding items, wherein the Parker Print Surf (PPS) roughness measured according to ISO 8791-4:2007 of the top layer of the paperboard is below 1.5 pm, such as below 1.3 pm.27. The method of any one of the preceding items, wherein the top layer is coated with at least one coating.28. The method of item 27, wherein the back layer is coated with at least one coating, such as a single-layer coating.29. The method of item 27 or 28, wherein the top layer is coated with more than one coating, such as a dual-layer coating or a triple-layer coating.30. The method of any one of the preceding items, wherein the grammage according to ISO 536:2020 of the paperboard is 150-500 g / m2, such as 180-480 g / m2.31. The method of any one of the preceding items, wherein the density according to ISO 534:2011 of the paperboard is 680-820 kg / m3, such as 690-810 kg / m3, such as 710-800 kg / m3.32. The method of any one of the preceding items, wherein the thickness according to ISO 534:2011 of the paperboard is 220-8000 pm, such as 250-750 pm.33. The method of any one of the preceding item, wherein the internal bond strength, Scott Bond, according to Tappi T 569 Scott type of the paperboard is at least 150 J / m2, such as at least 170 J / m2.34. The method of any one of the preceding items, wherein the ZD-tensile strength according to Tappi T 541 z-direction test of the multilayer paperboard is at least 400 kPa, such as at least 430 kPa, such as at least 450 kPa.35- The method of any one of the preceding items, wherein the geometrical shortspan compression test (SCT) index according to ISO 9895:2008 of the multilayer paperboard is at least 22 Nm / g.36. The method of any one of the preceding items, wherein the Bendtsen surface roughness according to ISO 8791-2:2013 of the top layer is below 300 ml / min, such as below 150 ml / min.37. The method of any one of the preceding items, wherein the Bendtsen surface roughness according to ISO 8791-2:2013 of the back layer is below 2000 ml / min, such as below 1500 ml / min.38. The method of any one of the preceding items, wherein the geometric bending resistance index measured according to ISO 2493-2:2020 of the multilayer paperboard is at least 9 mNm6 / g3, such as at least 10 mNm6 / g3.39. The method of any one of the preceding items, wherein the specific formation according to SCAN-P 92:09 for the middle layer is 0.40-0.70 Vg / m, such as 0.45- 0.60 Vg / m.40. The method of any one of the preceding items, wherein the top layer furnish, middle layer furnish and back layer furnish are each provided from separate headboxes on separate forming wires and thereafter combined into the multilayer paperboard.41. A multilayer paperboard comprising a top layer, a back layer and a middle layer, wherein:- the middle layer is arranged between the top layer and the back layer;- the top layer is formed from a top layer furnish comprising at least 50% by dry weight kraft pulp;- the middle layer is formed from a middle layer furnish comprising 25-75% by dry weight of maple chemithermomechanical pulp (CTMP), 15-65% by dry weight of broke pulp, a cationic retention aid in an amount of 250-700 g / tonne based on dry weight of the middle layer furnish and an anionic retention aid in an amount of 420-700 g / tonne based on dry weight of the middle layer furnish;- the back layer is formed from a back layer furnish comprising at least 80% by dry weight kraft pulp.42. Use of a multilayer paperboard produced according to item 41 in packaging.BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Fig 1 shows ZD-tensile strength plotted against density of a multilayer paperboards with varied source of CTMP in the middle layer.

[0009] Fig 2 shows geometric SCT-index for multilayer paperboards of different thicknesses with varied source of CTMP in the middle layer.

[0010] Fig 3 shows Bendtsen surface roughness of the top layer of multilayer paperboards of different thicknesses with varied source of CTMP in the middle layer.

[0011] Fig 4 shows PPS roughness of the top layer of multilayer paperboards of different thicknesses with varied source of CTMP in the middle layer.

[0012] Fig 5 shows specific formation number of the middle layer of multilayer paperboards of different thicknesses with varied source of CTMP in the middle layer.DETAILED DESCRIPTION

[0013] As a first aspect of the present disclosure there is provided a method of forming a multilayer paperboard comprising a top layer, a back layer and a middle layer, wherein the method comprises the following steps: providing a top layer furnish comprising at least 50% by dry weight kraft pulp; providing a middle layer furnish comprising 25-75% by dry weight of maple chemithermomechanical pulp (CTMP) and 15-65% by dry weight of broke pulp; adding a cationic retention aid to the middle layer furnish in an amount of 250- 700 g / tonne based on dry weight of the middle layer furnish; adding an anionic retention aid to the middle layer furnish in an amount of 420- 700 g / tonne based on dry weight of the middle layer furnish; providing a back layer furnish comprising at least 80% by dry weight kraft pulp; form a top layer from the top layer furnish, a middle layer from the middle layer furnish, and a back layer from the back layer furnish; andform a multilayer paperboard from the top layer, middle layer and back layer so that the middle layer is arranged between the top layer and back layer.

[0014] 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). When producing HT-CTMP a higher preheating temperature, typically above 170 °C, is used to produce the pulp. CTMP can be produced from various sources including both softwood, such as spruce, pine, hemlock and balsam fir, and hardwood, such as birch, eucalyptus and maple. The CTMP can be either bleached or unbleached.

[0015] The maple CTMP content in the middle layer furnish is 25-75% by dry weight. Typically, the content is 40-70 wt% by dry weight of the middle layer furnish, such as 50-65 wt% by dry weight of the middle layer furnish, such as 52-61 wt% of the middle layer furnish. Typically, maple CTMP constitutes at least 90 % by dry weight, such as at least 95 % by dry weight such as 100 % by dry weight of the total CTMP content.

[0016] The maple CTMP provided in the middle layer furnish typically has a Canadian standard freeness (CSF) of 300-500 ml measured according to ISO 5267- 2:2001, such as 310-450 ml measured according to ISO 5267-2:2001. Typically, the maple CTMP in the middle layer furnish has been refined to 10-30 kWh / t. Typically, the length-weighted average fibre length of said maple CTMP as measured according to ISO 16065-2:2014 is equal to or below 1.0 mm, such as equal to or below 0.9 mm. Measurements of pulp lengths can be conducted with an instrument MAP-Q from Valmet OY if the measurements are conducted off-line in lab scale. Alternatively, the measurements can be conducted on-line, and the instrument MAP from Valmet OY can in such case be used.

[0017] Retention aid(s) are typically added to furnishes in board manufacturing. It is generally a desire to keep the amount of retention aid(s) as low as possible from a cost perspective. Addition of a cationic retention aid to the middle layer furnish containing maple CTMP in an amount of 250-700 g / tonne based on dry weight of themiddle layer furnish in combination with addition of an anionic retention aid to the middle layer furnish in an amount of 420-700 g / tonne based on dry weight of the middle layer furnish has been identified as key features by the inventors to provide an improvement of ZD-tensile strength without at the same time increase the density of paperboard. There are almost endless combinations one can make when changing different parameters in a paperboard machine, but the inventors have realized that this specific combination of features provides the desired effect. The cationic retention aid is typically cationic polyacrylamide (cPAM), polyethyleneimine (PEI) or cationic starch, and the anionic retention aid is typically silica microparticles or bentonite clay. In one embodiment, the cationic retention aid is cPAM or PEI and the anionic retention aid is silica microparticles. Typically, the cationic retention aid is added in an amount of 330-650 g / tonne based on dry weight of the middle layer furnish, such as 370-620 g / tonne based on dry weight of the middle layer furnish, such as 400-600 g / tonne based on dry weight of the middle layer furnish. Typically, the anionic retention aid is added in an amount of 450-650 g / tonne based on dry weight of the middle layer furnish, such as 500-630 g / tonne based on dry weight of the middle layer furnish.

[0018] The solids concentration in the white water is typically below 650 mg / L, such as below 600 mg / L. The white water is the aqueous solution that drains from the wires that the furnishes has been applied to. The lower the concentration in the white water, the more efficient the retention is working. The middle layer is formed from the middle layer furnish by a headbox typically having a vertical slice opening of 38-26 mm, such as 31-26 mm, such as 30-27 mm. A lower slice opening increases the concentration in the headbox resulting in less water to dewater, which thereby facilitates dewatering.

[0019] Typically, the top layer furnish, middle layer furnish and back 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 three furnishes are provided from a multi-layer headbox directly onto one forming wire.

[0020] The strength of the middle layer may be further improved by addition of starch to the middle layer furnish. Starch is preferably added in an amount of 1.5-7.0 kg / tonne based on dry weight of the middle layer furnish, such as 2.0-6.0 kg / tonnebased on dry weight of the middle layer furnish. To further improve the strength of the paperboard, there is typically a starch addition by spraying between the furnishes prior to combining. Typically, the amount of spray starch applied between the middle layer furnish and top layer furnish is o.8-2.3 g / m2, such as 1.3-2.3 g / m2, such as 1.5-2. o g / m2. In one embodiment, there is more starch added between the middle layer furnish and the top layer furnish than between the middle layer furnish and the back layer furnish. In such case, the addition between middle layer furnish and back layer furnish is typically o.2-2.0 g / m2.

[0021] Broke pulp is formed from trimmings and / or produced, but damaged, board, and, consequently the broke pulp has essentially the same pulp composition as the paperboard. The content of broke pulp in the middle layer furnish is 15-65% by dry weight. Typically, the content of broke pulp in the middle layer furnish is 30- 60 wt% by dry weight of the furnish, such as 35-50 wt% by dry weight of the furnish, such as 39-48 wt% by dry weight of the furnish. Typically, the broke pulp in the middle layer furnish has been refined to 20-40 kWh / t.

[0022] The top layer furnish comprises at least 50% by dry weight of kraft pulp. The proportion of kraft pulp in the top layer furnish may be even higher, such as at least 70% by dry weight kraft pulp, such as at least 85% by dry weight kraft pulp. The proportion of kraft pulp in the back layer furnish may be at least 85% by dry weight kraft pulp, such as at least 90% by dry weight kraft pulp. The top layer furnish typically comprises at least 50% by dry weight hardwood kraft pulp. The top layer furnish may comprise a mix of hardwood and softwood kraft pulp, such as at least 50% by dry weight hardwood kraft pulp and at least 20% by dry weight of softwood kraft pulp, such as at least 65% by dry weight hardwood kraft pulp and at least 20% by dry weight of softwood kraft pulp. The back layer furnish typically comprise a higher content of softwood kraft pulp than the top layer furnish. The back layer furnish may comprise at least 80% by dry weight softwood kraft pulp. The kraft pulp in the top layer furnish as well as the bottom layer furnish may be bleached or unbleached. In one embodiment, the kraft pulp in the top layer furnish is bleached and the kraft pulp in the back layer furnish is unbleached. In another embodiment, the kraft pulp in the top layer furnish and back layer furnish is bleached. In yet another embodiment, the kraft pulp in the top layer furnish and back layer furnish is unbleached.

[0023] The top layer is typically coated with at least one coating. The coating typically comprises pigments and binder. The back layer may also be coated with at least one coating. The top layer is typically 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. For the 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 typically conducted after coating. Drying is typically performed with non-contact drying, such as IR and / or hot air, or contact drying, such as a drying cylinder, 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 includes: 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.

[0024] The CTMP and the kraft pulp are typically virgin pulps. Alternatively, in one embodiment, at least one of the virgin pulps is partly replaced by secondary fibres.

[0025] Typically, the pulps are market pulps. One or more of the pulps may also be never-dried pulps.

[0026] The multilayer paperboard can for example be used as a containerboard, such as linerboard, or a cartonboard, such as a liquid packaging board (LPB).

[0027] Surface properties of the paperboard includes Bendtsen surface roughness. Bendtsen surface roughness is measured according to ISO 8791-2:2013. The Bendtsen surface roughness of the top 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 top layer is lower than the surface roughness of the back layer since the top 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 top layer of the paperboard is typically below 1.5 pm, such as below 1.3 pm. Yet another surface property is specific formation number measuredaccording to SCAN-P 92:09. Formation is a measure of how even the surface is, i.e. that there are as few fibre flocs as possible and that the fibres are evenly distributed in all directions. A low formation is beneficial for printing as the surface is more even. The specific formation for the middle layer is typically 0.40-0.70 Vg / m, such as 0.45- 0.60 Vg / m. The specific formation number of the whole board as measured on the top layer of the paperboard is typically 0.5-0.7 Vg / m. The multilayered paperboard is typically calendered. Calendering is beneficial for surface smoothness. The line load in the calendering is typically 30-60 kN / m, such as 35-55 kN / m, such as 38-50 kN / m.

[0028] The internal bond strength, Scott Bond, according to Tappi T 569 Scott type of the paperboard is at least 150 J / m2, such as at least 170 J / m2. The ZD-tensile strength according to Tappi T 541 z-direction test of the multilayer paperboard is at least 400 kPa, such as at least 430 kPa, such as at least 450 kPa. The geometrical short-span compression test (SCT) index according to ISO 9895:2008 of the multilayer paperboard is 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 paperboard. In particular the strength properties in the z-direction and geometrical SCT-index are mechanical properties that benefit from the usage of maple CTMP as particular source for the CTMP.

[0029] The geometric bending resistance index measured according to ISO 2493- 2:2020 of the multilayer paperboard is typically at least 9 mNm6 / g3, such as at least 10 mNm6 / g3.

[0030] The density according to ISO 534: 2011 of the paperboard is typically 680- 820 kg / m3, such as 690-810 kg / m3, such as 710-800 kg / m3.

[0031] The grammage according to ISO 536:2020 of the paperboard is typically 150-500 g / m2, such as 180-480 g / m2.

[0032] The thickness according to ISO 534:2011 of the paperboard is typically 220-800 pm, such as 250-750 pm.

[0033] As a second aspect of the present disclosure there is provided a multilayer paperboard comprising a top layer, a back layer and a middle layer, wherein:- the middle layer is arranged between the top layer and the back layer;- the top layer is formed from a top layer furnish comprising at least 50% by dry weight kraft pulp;- the middle layer is formed from a middle layer furnish comprising 25-75% by dry weight of maple chemithermomechanical pulp (CTMP), 15-65% by dry weight of broke pulp, a cationic retention aid in an amount of 250-700 g / tonne based on dry weight of the middle layer furnish and an anionic retention aid in an amount of 420-700 g / tonne based on dry weight of the middle layer furnish;- the back layer is formed from a back layer furnish comprising at least 80% by dry weight kraft pulp.

[0034] The examples and embodiments discussed above in connection to the first aspect applies to the second aspect mutatis mutandis.

[0035] As a third aspect of the present disclosure there is provided the use of a paperboard according to the second aspect in packaging.

[0036] The packaging may be a package for food or liquids. Alternative, the packaging is a package for non-food products.

[0037] The examples and embodiments discussed above in connection to the first and second aspect applies to the third aspect mutatis mutandis.EXAMPLESManufacturing of a a-lavered paperboard

[0038] In a full-scale paperboard machine 3-layered boards were produced. The top layer was coated with a dual-layer coating with a grammage of about 25 g / m2and the back layer with a single-layer coating with a grammage of 6 g / m2.

[0039] The uncoated paperboard was calendered using a line load of 40-60 kN / m.

[0040] The pulp composition in the top layer furnish was a mix of 25 wt% bleached softwood kraft pulp (SW kraft) and 75 wt% bleached hardwood kraft pulp (HW kraft). The pulp composition in the back layer furnish was 100 wt% of SW kraft.

[0041] The pulp composition in the middle layer furnish was a mix of 53 wt% CTMP and 47 wt% broke pulp, except in one middle layer furnish that was a mix of 60 wt% CTMP and 40 wt% broke pulp. The wood source of CTMP was a70wt% / 30wt% mixture of birch and spruce, too wt% spruce, a 6swt% / 35wt% mixture of spruce and birch, a 75wt% / 25wt% mixture of maple and spruce, or too wt% maple. The refining energy of the CTMP was about 15 kWh / tonne and the refining energy of the broke pulp was about 30 kWh / tonne. Anionic retention aid (silica microparticles) as well as cationic retention aid (cationic polyacrylamide (cPAM)) were added to the middle layer furnish in varied dosages as shown below.

[0042] The top layer furnish, middle layer furnish and back layer furnish were each provided from separate headboxes on separate forming wires and thereafter combined. The vertical slice opening of the headbox forming the middle layer was varied from 37 to 26 mm. The furnishes were dewatered in a duo former.

[0043] The total thickness of the multilayer paperboard was 405 pm, 355 pm, 305 pm, 290 pm or 255 pm, wherein maple CTMP was used in paperboards of thicknesses of 290 pm and 255 pm.Fibre properties

[0044] The CTMP of different wood sources had the properties presented in Table 1 below. Length-weighted average fibre length was measured according to ISO 16065-2:2014.Table 1. Properties of the different CTMPs used.Runnabilitv evaluation

[0045] The different middle layer furnishes were evaluated with respect to runnability including if it was possible to dewater them. When 350 g / tonne anionic retention aid and 125 g / tonne cationic retention aid was added it was not possible to dewater a mixture having a maple CTMP content of 75 wt%, of the in total 53 wt% CTMP of the furnish when producing a paperboard with a total thickness of 405 pm.

[0046] There are many parameters one can change in a paperboard machine, but the inventors realized that the key parameter was the dosage of retention aids and that when the dosage of cationic and anionic retention aid was increased, it was possible to use 100 wt% maple CTMP with respect to the total CTMP content in the middle layer furnish.

[0047] The complete results are presented in table 2 below. Samples 5 and 6 shows two different addition levels of anionic silica microparticles and cPAM.Table 2. Dewatering test of middle layer furnishes.Strength in z-direction and surface properties

[0048] The produced paperboards were evaluated and results are presented in the figures.

[0049] In the figures, the data points marked accordingly represents:- Filled circles (•): 100 wt% maple CTMP in the middle layer- Filled triangles (A): 70wt% / 30wt% mixture of birch and spruce CTMP in the middle layer- Filled squares (■): 6swt% / 35wt% mixture of spruce and birch CTMP in the middle layer- Tilted squares (♦): 100 wt% spruce CTMP in the middle layer

[0050] In Fig. 1 the ZD-tensile strength is plotted against the density of the paperboards produced. All paperboards containing 100 wt% maple CTMP in the middle layer displayed a higher ZD-tensile strength than the other paperboards independently of the density.

[0051] Hence, the inventors realized that selection of maple CTMP in the middle layer in combination with the presented additions of anionic and cationic retention aids, the strength properties in z-direction of the provided paperboards were improved without increasing the density. Notably, the 70wt% / 30 wt% mixture of birch and spruce, i.e. containing 70wt% of another hardwood CTMP, did not provide the same effect.

[0052] The density was lower in the paperboards produced where the line load in the calender was 40 kN / m in combination with a CTMP content of 60 wt% and a CTMP refining of 10 kWh / t.

[0053] In Fig. 2 the geometrical SCT-index (SCT-index, GM) is plotted for paperboards of varied thickness. The paperboards containing 100 wt% maple CTMP in the middle layer displayed a higher geometrical SCT-index than the other paperboards. Thereby, also regarding geometrical SCT-index the inventors haverealized that usage of maple CTMP provides improved properties and that the particular dosage of retention aids makes it possible to use the necessary high content of maple CTMP in the middle layer.

[0054] Hence, not only the ZD-tensile strength independently of density was improved by using 100 wt% CTMP from maple as CTMP source, but also the geometrical SCT-index, which was made possible by the identification of the retention aids and the contents of those to be added as key parameters for production of middle layers containing 53wt% maple CTMP and 47wt% broke pulp.

[0055] In Figs. 3-5 surface properties of the paperboards are shown. The paperboards having 100 wt% maple CTMP as CTMP source further yielded a low Bendtsen roughness, PPS smoothness as well as formation, which are all properties important for printing.

Claims

CLAIMS1. A method of forming a multilayer paperboard comprising a top layer, a back layer and a middle layer, wherein the method comprises the following steps: providing a top layer furnish comprising at least 50% by dry weight kraft pulp; providing a middle layer furnish comprising 25-75% by dry weight of maple chemithermomechanical pulp (CTMP) and 15-65% by dry weight of broke pulp; adding a cationic retention aid to the middle layer furnish in an amount of 250- 700 g / tonne based on dry weight of the middle layer furnish; adding an anionic retention aid to the middle layer furnish in an amount of 420- 700 g / tonne based on dry weight of the middle layer furnish; providing a back layer furnish comprising at least 80% by dry weight kraft pulp; form a top layer from the top layer furnish, a middle layer from the middle layer furnish, and a back layer from the back layer furnish; and form a multilayer paperboard from the top layer, middle layer and back layer so that the middle layer is arranged between the top layer and back layer.

2. The method of claim 1, wherein the cationic retention aid is cationic polyacrylamide (cPAM), polyethyleneimine (PEI) or cationic starch.

3. The method of claim 1 or 2, wherein the anionic retention aid is silica microparticles or bentonite clay.

4. The method of any one of the preceding claims, wherein the middle layer is formed from the middle layer furnish by a headbox having a vertical slice opening of 38-26 mm, such as 31-26 mm, such as 30-27 mm.

5. The method of any one of the preceding claims, wherein the maple CTMP has a Canadian standard freeness (CSF) measured according to ISO 5267-2:2001 of 300- 600 ml, such as 300-500 ml, such as 310-450 ml..

6. The method of any one of the preceding claims, wherein the maple CTMP in the middle layer furnish has been refined to 10-30 kWh / t.

7. The method of any one of the preceding claims, wherein the broke pulp in the middle layer furnish has been refined to 20-40 kWh / t.

8. The method of any one of the preceding claims, wherein the CTMP content in the middle layer furnish is 40-70 wt% by dry weight of the furnish, such as 50-65 wt% by dry weight of the furnish, such as 52-61 wt% of the furnish.

9. The method of any one of the preceding claims, wherein the cationic retention aid is added in an amount of 330-650 g / tonne based on dry weight of the middle layer furnish, such as 330-620 g / tonne based on dry weight of the middle layer furnish, such as 350-600 g / tonne based on dry weight of the middle layer furnish.

10. The method of any one of the preceding claims, wherein the anionic retention aid is added in an amount of 420-650 g / tonne based on dry weight of the middle layer furnish, such as 450-630 g / tonne based on dry weight of the middle layer furnish.

11. The method of any one of the preceding claims, wherein the solids concentration in the white water is below 95% (w / w), such as below 90 % (w / w), such as below 87% (w / w), such as 70-86 % (w / w).

12. The method of any one of the preceding claims, wherein the density according to ISO 534:2011 of the paperboard is 680-820 kg / m3, such as 690-810 kg / m3, such as 710-800 kg / m3.

13. The method of any one of the preceding claims, wherein the ZD-tensile strength according to Tappi T 541 z-direction test of the multilayer paperboard is at least 400 kPa, such as at least 430 kPa, such as at least 450 kPa.

14. A multilayer paperboard comprising a top layer, a back layer and a middle layer, wherein:- the middle layer is arranged between the top layer and the back layer;- the top layer is formed from a top layer furnish comprising at least 50% by dry weight kraft pulp;- the middle layer is formed from a middle layer furnish comprising 25-75% by dry weight of maple chemithermomechanical pulp (CTMP), 15-65% by dry weight of broke pulp, a cationic retention aid in an amount of 250-700 g / tonne based on dry weight of the middle layer furnish and an anionic retention aid in an amount of 420-700 g / tonne based on dry weight of the middle layer furnish;- the back layer is formed from a back layer furnish comprising at least 80% by dry weight kraft pulp.

15. Use of a multilayer paperboard according to claim 14 in packaging.

Images