Foam-formed paperboard product

Abstract

The present document is directed to a method for producing the outer ply of a paperboard product comprising at least two plies by foam-forming the outer ply. The present document also discloses a paperboard product wherein the outer ply has been foam-formed and has a density of at least 700 kg / m3, and a furnish for foam-forming an outer ply of such a paperboard product.

Description

[0001] FOAM-FORMED PAPERBOARD PRODUCT

[0002] TECHNICAL FIELD

[0003] The present document is within the field of paperboard production. More specifically, the present document is directed to a method for producing an outer ply of a paperboard product comprising at least two plies. The present document also discloses paperboard products produced by such a method and a furnish for foam-forming an outer ply of a paperboard product.

[0004] BACKGROUND

[0005] Paperboard to be converted usually comprises several plies (layers), top and back plies as well as one or several middle plies (midplies). Chemical pulp is oftentimes used in the top and back plies of the paperboard to provide the board with strength and good printing properties, while the midply may contain both mechanical pulp and / or chemical pulp. Bulky fibers such as CTMP pulp (chemi-thermomechanical pulp) is typically used in the midply to provide bulk, rigidity and bending stiffness to the board. It is also normal that the midply or bulk layer has a higher grammage than the outer ply.

[0006] Formation, i.e. local grammage variations, and surface unevenness of the outer plies, and especially the top ply, are crucial properties that have to be controlled to ensure good coating quality. It is usually desirable to avoid any grammage variations and unevenness on the outer plies as these may contribute to optical unevenness of the paperboard product, as will be described in more detail later on. The problem is more obvious if also the midply is rough, as the outer plies might locally penetrate into each other.

[0007] Bulky fibers used for the midply (midplies), which typically are so called high yield pulps with high content of lignin and cellulose also in general have lower brightness. When applying the top fiber layer, which normally is bleached and has a high brightness, on a midply with such bulky fibers, there will be an optical unevenness called white top mottling, which can be defined as optical defects or variations in light absorbance over the unprinted surface when subjected to diffuse or directed light, or simply variations in perceived whiteness.

[0008] Different solutions have been proposed in the prior art to solve the problem with white top mottling. Increasing the grammage of the top ply will solve the problem, whereas this adds costs and slows down production. Alternatively, high reflective pigments in small amounts, e.g. calcined clay or PCC, can be used to decrease white top mottling. However, these pigments are typically expensive and might be difficult to retain to the surface. Pigments are typically very fine and hence might penetrate into the bulk ply. A higher filler content (>10 or >15 wt%) could significantly improve the brightness and reduce white top mottling, but will add other challenges like cost and strength properties or even dusting. Also, the use of a high content and thicker layer of bleached hardwood kraft pulp could solve the problem with white top mottling. Fine fibers like birch and eucalyptus are beneficial to use for the top ply as these give better formation, but their use adds costs and limits also the drainage of the fiber web during manufacturing of the paperboard. The use of a high content of short fibers will also reduce e.g. tear strength and e.g. compression strength.

[0009] White top mottling is not only relevant for uncoated paperboard or containerboard (2 or 3 plies) but also for single or double coated grades.

[0010] In the production of multi-ply paperboard, foam forming has been proposed for the manufacturing of the middle ply to further increase the bulk. Foam forming includes foaming of the furnish and applying the foamed furnish on a wire, where water is drained, followed by pressing and drying. The foaming of the furnish is typically performed using foaming agents.

[0011] Foam forming of the midply has been discussed in earlier publications including US20160186383A1 , WO2015173474A1 , WO2017089507 and EP2949597.

[0012] An object of the present invention is thus to overcome or at least mitigate one or more of the problems described herein.

[0013] SUMMARY

[0014] There is thus a need to find a solution to the problem with white top mottling without decreasing the mechanical properties of the paperboard such as SCT (short-span compression test) or tensile strength, or tear strength.

[0015] The method and products of the present document solves or at least mitigates these problems.

[0016] The present document is directed to a method for producing an outer ply of a paperboard product, said paperboard product comprising at least two plies, characterised in that said outer ply has a density of at least 700 kg / m3, and in that said method comprises foamforming a furnish for said outer ply to provide a foamed furnish comprising fibers and one or more foam-forming agents.

[0017] The present document is also directed to a paperboard comprising a midply and / or a bulk layer and at least one outer ply, the at least one outer ply being a top ply and / or a back ply, wherein said at least one outer ply is foamed formed and has a density of at least 700 kg / m3, preferably wherein the midply and / or bulk layer is / are not foam-formed.

[0018] The present document also discloses a paperboard product obtained or obtainable by a method for producing an outer ply of a paperboard product as defined herein.

[0019] The present document is also directed to a foamed furnish for an outer ply of a paperboard product comprising at least two plies, said foamed furnish comprising: a) a furnish comprising at least 80 wt% of kraft pulp, based on the total dry weigh of said furnish, said kraft pulp having a mean fibre length Lc(l) of at least about 0.9 mm in accordance with ISO 16065-2:2014, preferably wherein at least 10 wt% of the fibers are fibers having a mean fiber length Lc(l) of at least 1 .8 mm, such as from about 1 .8 mm to about 2.5 mm, such as from about 1 .8 mm to about 2.2 mm. b) one or more foaming agent(s); wherein said furnish has a solids content of at least about 1 wt%, said furnish optionally further comprising one or more filler(s) and / or dispersing agent(s).

[0020] Other features and advantages of the invention will be apparent from the following detailed description, drawings, examples, and from the claims.

[0021] DEFINITIONS

[0022] The singular forms “a”, “an” and “the” include plural references unless the context clearly dictates otherwise.

[0023] BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 shows an example of a cross-section structure of three ply paperboard with large grammage variations in top ply (outer ply) that causes white top mottling.

[0025] Figure 2 shows an overview of the protocol used in the experimental section for foamforming of an outer ply.

[0026] DETAILED DESCRIPTION

[0027] The present document is concerned with outer plies of paperboard products comprising at least two plies and how to reduce top white mottling in such outer plies when they are assembled on a midply or bulk layer. In particular, the present document is concerned with how to reduce white top mottling. The present document is thus directed to a method for producing an outer ply of a paperboard product wherein the paperboard product comprises at least two plies (i.e. the outer ply and at least one more ply). The method comprises foam-forming the furnish for the outer ply to provide a foamed furnish comprising fibers and one or more foam-forming agents and forming the outer ply. Thereby the outer ply of paperboard product can be produced to have a density of at least 700 kg / m3, preferably from about 710 to about 1200 kg / m3, even more preferably from about to about 750 to about 1100 kg / m3, such as from about 760 kg / m3to about 1100 kg / m3, or from about 800 kg / m3to about 1100 kg / m3. The basis weight of the outer ply may be 55 gsm (g / m2) or less, such as from about 15 gsm to about 55 gsm, such as from about 20 to about 55 gsm, according to ISO 536:2019. Further properties of the outer ply are given in the below.

[0028] It has been found that the foam-forming of an outer ply for a paperboard product, especially the top ply, effectively addresses issues related to uneven surface and white top mottling. The problem with white top mottling is illustrated in Fig. 1 . Without wishing to be bound by theory, this is because foam-forming enables higher solid content (i.e. consistency) of the furnish which hence allows less infiltration into the bulk layer. In particular, foam-forming of the outer ply reduces problems with uneven surface and white top mottling when the basis weight of the outer ply is decreased, such as to be in the range of < 55 gsm, such as from about 15 gsm to about 55 gsm, such as from about 20 to about 55 gsm. Foam-forming of the outer ply thus reduces local grammage variations on the surface of the outer ply and / or improve the formation (i.e. allows for a lower specific formation value), hence decreasing optical unevenness. In contrast, with conventional water forming, it is more difficult to get good formation when the top ply gets thinner or when formation of the midply or bulk ply is poor. The latter may e.g. be caused by high grammage, or by high content of bulky fibers such as CTMP (chemithermo mechanical pulp) or HT CTMP (high temperature chemithermo mechanical pulp), in the midply or bulk layer. Foam-forming of the outer layer reduce these problems.

[0029] White top mottling is a problem especially when bleached kraft pulp is used in outer plies and / or when unbleached mechanical or kraft pulp is used in the midply or bulk layer. Bleached kraft pulp has typically an ISO brightness of more than 80% according to ISO 2470-1 :2016, whereas the midply or bulk ply might have an ISO brightness below 70% or such as below 65% or 60%. Foam-forming the outer ply of a paperboard product may reduce top mottling when such kraft pulps are used. The brown or brownish midply or bulk layer will thus impact the quality of the outer ply, especially if formation of the top ply is uneven or if the top ply is thin. The use of foamforming the furnish for the outer ply mitigates this problem.

[0030] Mottling can be defined e.g. according to ISO / IEC 24790:2017(en) standard as a measure of the appearance of unintended, aperiodic macroscopic fluctuations of lightness. White top mottling is a type of paper mottling, i.e. variations in light absorbance over the unprinted surface that are seen in diffuse or directed light but typically not in specular angle. Hence it is variations in perceived whiteness. Depending on the scale of variation, it can be called e.g. graininess (small) or cloudiness (large) of the paper. One way to determine the white top mottling is to use the STFI mottling expert, which can be used for greyscale mottling measurements (STFI Mottling Expert v 1.34).

[0031] The paperboard product comprising at least two plies according to the present document comprises at least one outer ply and at least one other ply. In the context of the present document, such a paperboard product comprising at least two plies may also simply be denoted a paperboard product or a multiply paperboard product. An outer ply may be arranged on one or both outer sides of the paperboard product (i.e. as the top ply and the back ply). The outer ply is typically arranged on a midply or bulk layer, but it can also be arranged directly onto a second outer ply which may or may not be foam-formed. The paperboard product may comprise two or more midplies and / or bulk layers. Typically, the outer ply is the top ply of the paperboard product. When the outer ply is the top ply, it also forms the print side of the paperboard product. The outer ply may thus be the print side of the paperboard product. The outer ply may alternatively be a back ply of a paperboard product. The properties of the other ply / plies of the paperboard product but the outer ply are not critical but the foam-formed outer ply according to the present document could be applied to any other ply, such a midply or bulk layer. The skilled person knows how to prepare such midplies and bulk layers. Typically, any midply and / or bulk layer is not foam- formed. Paperboard products according to the present document typically have midplies or bulk layers with a density of less than 750 kg / m3such as less than 650 kg / m3or less than 600 kg / m3or in the range of from about 350 kg / m3to about 550 kg / m3. Typically, a bulk layer comprises high yield pulp and broke, but it can also be made with or without e.g. brown kraft pulp (i.e. unbleached kraft pulp), bleached kraft pulp, and / or CTMP (chemi-thermomechanical pulp, from hardwood and / or softwood). The kraft pulp may thus contain one of these or mixtures thereof. Typically, the ply bond or internal Z-strength of a midply is above 150 kPa, such as above 175 kPa, above 200 kPa, or above 250 kPa according to the SCAN-P 80:98 standard. Typically, when applying a foam-formed outer ply according to the present document to a midply or bulk layer, the strength properties of the resulting paperboard product comprising the outer ply and at least one more ply are maintained.

[0032] The foam-forming of a furnish for an outer ply to provide a foamed furnish comprising fibers and one or more foaming agents may be achieved by adding one or more foamforming agents to a furnish comprising fibers, and then foaming the furnish. Alternatively, the foam-forming may be achieved by first forming a foam by mixing water and one or more foaming agents and then adding fibers to this foam to provide the foamed furnish comprising fibers and one or more foaming agents. Foaming may be achieved by mixing and / or injecting air into the furnish with foaming agents or water with foaming agents. For example, air may be mixed into the furnish or water containing foaming agents by using a moving element or without the use of a moving element. Additionally or alternatively, a high mechanical shear foam generator may be used for generating foaming. Shearing may be e.g. performed at from about 1000 rpm to about 4000 rpm, such as from about 1500 rpm to about 3500 rpm such as about 2500 rpm.

[0033] For example, the foamed furnish comprising fibers and one or more foaming agents may be manufactured by: a) providing a furnish comprising fibers, such as kraft pulp, as defined elsewhere herein; b) mixing and optionally refining the furnish comprising fibers step a); c) adding a foaming agent as defined elsewhere herein and optionally a dispersing agent as defined elsewhere herein to the furnish comprising fibers of step b); d) mixing the resulting mixture of step c) so to provide a first foam, said mixing of step d) preferably being performed by injecting air into said mixture; e) treating the first foam of step d) with a high mechanical shear foam generator, thereby forming a foamed furnish, said high mechanical shear foam generator preferably performing shearing at from about 1000 rpm to about 4000 rpm, such as from about 1500 rpm to about 3500 rpm such as about 2500 rpm. Another example of how the foamed furnish comprising fibers and one or more foaming agents may be manufactured is by: a) preparing a mixture comprising water and one or more foaming agents as defined elsewhere herein and mixing and / or injecting air in to said mixture to prepare a foamed mixture; and b) adding fibers as defined elsewhere herein to said foamed mixture to obtain a foamed furnish; c) optionally adding one or more further ingredients as defined elsewhere herein to said foamed furnish of step b). Optionally step c) may be performed before step b).

[0034] The foamed furnish comprises fibers and one or more foaming agents. Further, the foamed furnish comprises water. Typically, the amount of water in the foamed furnish is from about 94 wt% to about 99.5 wt% based on the total weight of the foamed furnish (wet weight). The foamed furnish may also comprise one or more fillers and / or one or more additives as further explained elsewhere herein. In accordance with the present document, weight percentages may be given based on the total dry weight of the foamed furnish. This means that such percentages are based on the weight of the foamed furnish excluding the water content.

[0035] The foamed furnish for the outer ply typically has a solids content (i.e. consistency) of at least about 0.5 wt%, preferably at least 0.6 wt%, such as 1 wt%, preferably at least about 1 .2 wt%, even more preferably from about 1 .3 wt% to about 6 wt%, based on the total weight of the foamed furnish (wet weight). Such a solid content allows for improved evenness of the outer ply and less infiltration into the midply and / or bulk layer. The use of foam-forming for the outer ply enables the use of a higher solids content than with conventional water forming without losing formation. The improved formation and higher solids content also improves the drainage and formation of the multiply structure of the paperboard product. The solids content comprises fibers and fillers. Up to about 30 % of the solids can be fillers. The use of foam-forming of the furnish for the outer ply allows a higher content of solids to be used in the furnish.

[0036] The foamed furnish typically comprises from about 70 wt% to about 99.5 wt% fibers based on dry weight of the furnish. Typically, the fibers are cellulose fibers. Typically, the foamed furnish comprises at least about 70 wt% kraft pulp, such as at least 80 wt%, based on the total dry weight of the foamed furnish. The kraft pulp typically has a SR value of 25-55, such as from about 25 to about 40. The kraft pulp is preferably bleached kraft pulp. The kraft pulp is preferably hardwood pulp, such as birch or eucalyptus. The use of foam-forming also allows the use of softwood kraft fibers in the furnish for the outer ply, such from about 0.1 wt% to about 100 wt% of softwood fibers, based on the total weight of fibers in the foamed furnish. The use of softwood fibers (instead of hardwood fibers) increases the strength properties, especially the tear strength of the paperboard.

[0037] The fibers of the foamed furnish may have a mean fibre length Lc(l) of at least about 0.8 mm, such as at least about 0.9, at least about 1 .0 mm, at least about 1 .1 mm, at least about 1 .2 mm, or at least about 1 .3 mm in accordance with ISO 16065-2:2014 and as determined e.g. with Valmet FS5 for samples taken in the headbox. The maximum length of the fibers is typically about 6 mm. At least 5 wt% (dry weight), such as at least 10 wt%, of the total content of the fibers may be fibers having a mean fiber length Lc(l) of at least 1 .8 mm, such as from about 1 .8 mm to about 2.5 mm, such as from about 1 .8 mm to about 2.2 mm, according to ISO 16065-2:2014 and as determined e.g. with Valmet FS5 for samples taken in the headbox. Foam forming allows the use of a high content of such longer fibers, such as from about 5 wt% to about 80 wt% (dry weight) of the fibers, such as from about 10 wt% to about 70 wt%, such as from softwood, without the problem of uneven surface and white top mottling. Normally, long fibers would give increased formation and higher risk of white top mottling, especially at lower grammages. With the foam-forming method for producing an outer ply according to the present document, an outer ply with a lower grammage, such as in the range of < 55 gsm, such as from about 15 gsm to about 55 gsm, such as from about 20 to about 55 gsm, may be produced. However, the present method for producing a foam-formed outer ply also improves the formation (i.e. lower specific formation value) and / or improves the tensile strength also for outer plies having shorter mean fiber length. Typically, at least 60 wt% (dry weight based on the total content of fibers in the foamed furnish or outer ply), such as at least 70 wt%, at least 80 wt%, at least 90 wt% or at least 95 wt% of the fibers in the foamed furnish have a mean diameter of at least 10 |j.m as determined by ISO 16065-2:2014 and as determined e.g. with Valmet FS5.

[0038] A strength agent may also be added to the furnish. Such strength agents are well-known to the skilled person. The strength agent may e.g. be, but is not limited to, refined or microfibrillated cellulose, a polysaccharide, such as (cationic) starch, sodium carboxymethyl cellulose, derivatives of polysaccharides, such as enzyme treated starch, and / or mixture of starches. The refined cellulose may have a Schopper Riegler (SR) value of above 60 as measured according to ISO 5267-1 :1999. All SR values referred to herein are as measured according to ISO 5267-1 :1999. Preferably, such refined cellulose is a microfibrillated cellulose (MFC), which may have a SR value of 85-100. The foamed furnish may alternatively or in addition comprise highly refined cellulose (HRC), which may have a SR value of 60-85. Without being bound to any theories, it is believed that the addition of from about 0.1 wt% to about 10 wt% of cellulosic strength agent, based on the total dry weight of the foamed furnish, can adjust foam and / or bubble size and / or also provide strength as compensation when using the foaming agent, as foaming agents may act as debonders decreasing the strength of the outer ply. Even though the use of MFC in paperboard products may contribute to the strength properties of the paperboard it may also reduce the porosity of the paperboard. Foam-forming of the outer ply allows for the use of MFC in the outer ply without losing too much porosity. Typically, the surface roughness (Bendtsen roughness) of the outer ply is from about 100 ml / min to about 500 ml / min, such as from about 200 ml / min to about 400 ml / min, such as from about 150 ml / min to about 300 ml / min, as measured by the Bendtsen method according to ISO 8791-4:2021.

[0039] The foamed furnish typically comprises about 10 wt% or less of refined cellulose such as MFC and / or HRC (highly refined cellulose), such as from about 0.1 wt% to about 10 wt% refined cellulose, such as MFC and / or HRC, or about 7 wt% or less refined cellulose, such as MFC and / or HRC, such as from about 0.1 wt% to about 7 wt% refined cellulose, such as MFC and / or HRC, or 0 wt% refined cellulose, such as MCF and / or HRC, based on the total dry weight of the foamed furnish. When different types of refined cellulose, such as MFC and HRC, are present in the foamed furnish, these values refer to the amount of the different types of refined cellulose, such as MFC and HRC, taken together. HRC is typically made up of cellulose having a reduced fiber width and MFC is typically made up of cellulose particle fibers or fibrils with at least one dimension in the range of 20-1000 nm. MFC typically has a SR value of 85-100. HRC typically has a SR of about 60-85. Thus, refined cellulose as referred to in the present document typically has a SR value of 60-100 and the foamed furnish (or the outer ply) may thus comprise the above referred to amounts of refined cellulose having a SR value of 60-100.

[0040] As mentioned above, the foamed furnish may comprise from about 1 wt% to about 30 wt% of fillers, preferably from about 1 wt% to about 20 wt%, such as from about 1 wt% to about 15 wt%, and even more preferably from about 1 wt% to about 10 wt%, based on the total dry weight of the foamed furnish. By using foam-forming of the outer ply in accordance with the present document, the retention of fillers is improved, whereby less amount of fillers can be added to the furnish for the outer ply. Also, foam-forming of the outer ply allows for more cost-efficient use of high scattering pigments or fillers such as calcined clay, clay, PCC, TiO2or combinations thereof or hybrid particles.

[0041] The foamed furnish may also comprise a total amount of from about 0.01 wt% to about 3 wt% of one or more dispersing agents, such as an anionic, amphoteric, or non-ionic polysaccharide, such as carboxymethyl cellulose (CMC), based on the total dry weight of the fibers of the foamed furnish. The use of dispersing agents may reduce fiber-friction and thus less flocculation, and increase foam quality, including re-pulpability.

[0042] The total amount of foaming agents in the foamed furnish may be from about 0.01 wt% to about 10 wt%, such as from about 0.05 wt% to about 5 wt%, based on the total dry weight of the foamed furnish. One or more different types of foaming agents may be used. The foaming agent may be a surface-active chemical selected from one or more of anionic surfactants, non-ionic surfactants, zwitterionic surfactants, and polymeric surfactants.

[0043] More specific examples of surfactants that may be used as foaming agents in accordance with the present document are e.g. sodium alkyl sulphates such as sodium dodecyl sulphate (SDS), Sodium Lauryl Sulphate or e.g. polymeric surface-active chemicals such as hydrophobically modified cellulose derivatives or polysaccharides such as n-OSA starch, hydrophobic modified ethylhydroxyethyl cellulose or synthetic polymers like PVOH having degree of hydrolysis less than 92%.

[0044] Due to the good retention achieved by foam-forming, the amount of retention chemicals may be reduced. The foamed furnish may comprise 0.1 wt% or less, such as 0.05 wt% or less, 0.01 wt% or less or about 0 wt% of retention chemicals, such as nanoparticles, based on the dry weight of the foamed furnish. Also, retention chemicals can disrupt the effectiveness of foaming agents, making it especially advantageous to avoid their use.

[0045] The method for producing an outer ply of a paperboard product may further comprise adding the outer ply produced to another ply, such as a midply or bulk layer to produce a paperboard product comprising at least two plies. Further, surface sizing and / or pigment coating may be applied on the outer ply.

[0046] In the method producing an outer ply of a paperboard product of the present document, a foamed furnish for the outer ply of the paperboard product may be applied on a fiberbased web or directly on the wire. The skilled person knows how to apply a furnish for an outer ply on a fiber-based web or directly on a wire.

[0047] When the foamed furnish is applied to a fiber-based web, this can be a wet or dry fiberbased web. The solid content of a wet fiber-based web is typically 15 wt% or less based on the total weight of the wet fiber-based web. Such a fiber-based web will form the midply or bulk layer of the paperboard product. The furnish for the fiber-based web typically comprises 5 vol% or less air, typically 1 vol% or less, such as about 0.35 vol% (as calculated for the furnish in the headbox).

[0048] The foamed furnish may alternatively be applied directly onto a wire. In this case, preferably the foamed furnish is applied to a first wire, whereas a second furnish, which will form the midply or bulk layer, is formed with a separate headbox and on a separate wire. The foamed furnish and the second furnish may then be drained or dewatered partly on their respective wires to a consistency of 15-25 wt% before being laminated together or couched together in the forming section.

[0049] For example, when the paperboard product has three plies (including the foam-formed outer ply), the three plies may be formed on separate wires and forming sections. Thereafter the plies may be applied to each other, such as by wet lamination where wet webs are applied to each other and then pressed together before subjecting them to a wet press dewatering step. Alternatively, when the paperboard product has two plies, these plies may be produced with one or two headboxes on one or two wires. In this case, the print side is the foam-formed outer ply, whereas the other ply is a bulk layer.

[0050] Another example is to use two headboxes of which the first makes foam-formed outer ply and a midply, whereas the other makes the back ply (which also may be a foam-formed outer ply according to the present document).

[0051] After the foamed furnish has been applied to a fiber-based web or a wire, the fiber-based web with the outer ply or outer ply (if the foamed furnish was applied directly to the wire) are dewatered. Such dewatering may further comprise steps of pressing and / or drying. Foam-forming of the outer ply according to the present document allows a more even porous structure to be obtained than water-formed outer plies, which makes the foam- formed outer plies easier to dewater in the press section. Also, the higher solids content of the foamed furnish enables a higher solids content after the press section. Further, as the foam-formed outer ply is more porous than water-formed outer plies, allowing a furnish comprising a higher amount of refined pulp to be used without the problem of steam pressure building up in the middle of the outer ply or paperboard causing problems in the drying section. This more porous structure enables higher press solids after press section. Pressing may involve a total press impulse of less than 150 kPa*s, preferably less than 120 kPa*s, preferably less than 110 kPa*s, more preferably less than 100 kPa*s, but preferably higher than 30 kPa*s or 40 kPa*s. The press impulse is calculated by dividing the linear load with the speed. The press section may comprise a shoe press nip or a shoe press nip and one or more further press nips, whereof at least one and preferably all of the nips are double felted. The invention allows for the use of a lower press impulse and still achieve a high solid content before the drying section.

[0052] The method for producing an outer ply of a paperboard product comprising at least two plies, may thus comprise the steps of: i) providing a fiber-based web or a wire; ii) providing a foamed furnish for an outer ply of said paperboard product, said foamed furnish comprising fibers and one or more foam-forming agents; iii) applying said foamed furnish to said fiber-based web or wire of step i), thereby forming said outer ply on said fiber-based web or said wire; and iv) dewatering the fiber-based web and / or outer ply obtained in step iii).

[0053] In such a method, the provided fiber-based web of step i) may be a midply and / or bulk layer. Such a midply and / or bulk layer is preferably not foamed formed. The dewatering step iv) may comprise pressing the fiber-based web with the outer ply obtained in step iii) such as with a total press impulse of less than 150 kPa*s, preferably less than 120 kPa*s, preferably less than 110 kPa*s, more preferably less than 100 kPa*s, but preferably higher than 30 kPa*s or 40 kPa*s.

[0054] In the method for producing an outer ply of a paperboard product according to the present document, the temperature of the headbox for the foamed furnish for the outer ply is typically 50 °C or less, such as about 40 °C or less, such as from about 30 °C to about 50 °C.

[0055] The present document is also directed to a paperboard product comprising at least two plies. Exemplary such paperboard products and some exemplary properties of them have been described above. Typically, the paperboard product comprises a midply and / or a bulk layer and at least one outer ply. As mentioned above, the outer ply of such a paperboard product is foamed-formed and has a density of at least 700 kg / m3, preferably from about 710 to about 1200 kg / m3, even more preferably from about to about 750 to about 1100 kg / m3, such as from about 760 kg / m3to about 1100 kg / m3, or from about 800 kg / m3to about 1100 kg / m3. In such a paperboard product, the midply and / or bulk layer preferably is / are not foam-formed. A paperboard product of the present document may be obtained or is obtainable by the method for producing an outer ply of a paperboard product comprising at least two plies as disclosed herein. The outer ply of a paperboard product of the present document may have a basis weight of 55 g / m2or less, such as from about 15 g / m2to about 55 g / m2, such as from about 20 g / m2to about 55 g / m2according to ISO 536:2019. The outer ply may comprise at least 70 wt% of kraft pulp, said kraft pulp preferably being a bleached kraft pulp. The kraft pulp of the outer ply may comprise at least 5 wt%, such as at least 10 wt%, of fibers (based on the total content of fibers of the kraft pulp) having a mean fiber length Lc(l) of at least 1.8 mm, such as from about 1 .8 mm to about 2.5 mm, such as from about 1 .8 mm to about 2.2 mm, in accordance with ISO 16065-2:2014. The kraft pulp of the outer ply typically has a Schopper Riegler (SR) value of at least 18, such as from about 25 to about 55, such as from about 25 to about 40, as measured according to ISO 5267-1 :1999. The outer ply may comprise from about 1 wt% to about 30 wt% of fillers, preferably from about 1 wt% to about 20 wt%, such as from about 1 wt% to about 15 wt%, such as from about 1 wt% to about 10 wt%. The kraft pulp of the outer ply of a paperboard product of the present document typically comprises about 10 wt% or less of refined cellulose, such as MFC and / or HRC, such as from about 0.1 wt% to about 10 wt% refined cellulose, such as MFC and / or HRC, or about 7 wt% or less refined cellulose, such as MFC and / or HRC, such as from about 0.1 wt% to about 7 wt% refined cellulose, such as MFC and / or HRC, or 0 wt% refined cellulose, such as MFC and / or HRC, based on the total dry weight of the outer ply. When more than one type of refined cellulose, such as MFC and HRC, are present in the outer ply, these values refer to the amount the different types of refined cellulose, such as MFC and HRC, taken together. Properties of refined cellulose, such as MFC and HRC, are described elsewhere herein.

[0056] The outer ply of the paperboard product may have a brightness of at least about 70 according to ISO 2470-1 :2016, such at least 74 or at least 80. The formation value of the outer ply of the paperboard product may be less than 0.8 gA0.5 / m, preferably less than 0.7 gA0.5 / m, more preferably less than 0.6 gA0.5 / m, more preferably less than 0.5 gA0.5 / m as measured according to SCAN-P 92:09. As mentioned above, the Bendtsen roughness of the outer ply of the paperboard product may be from about 100 ml / min to about 500 ml / min, such as from about 200 ml / min to about 400 ml / min, such as from about 150 ml / min to about 300 ml / min, according to ISO 8791-4:2021 . The tensile index for the outer ply is typically 20 Nm / g or more, such as 22 Nm / g or more, such as 24 Nm / g or more in accordance with ISO 1924-3:2011. The bursting strength of the paperboard product comprising the foam-formed outer ply may be at least about 70 kPa according to ISO 2758:2014. The Scott bond may be 250 J / m2or more, such as from about 300 J / m2to about 650 J / m2according to ISO 16260-2016. Further, foam-forming of the outer ply in accordance with the present document allows for less variations in COBB Unger values and / or a better size retention of the outer ply.

[0057] The outer ply may comprise refined cellulose having a SR value of above 60 as measured according to ISO 5267-1 :1999. Preferably, such refined cellulose is a microfibrillated cellulose (MFC), which may have a SR value of 85-100. The outer ply may alternatively or in addition comprise highly refined cellulose (HRC), which may have a SR value of 60-85. Such refined cellulose may preferably be a microfibrillated cellulose. The outer ply may comprise 0.1 wt% or less, such as 0.05 wt% or less, 0.01 wt% or less or about 0 wt% of retention chemicals.

[0058] Other properties of such a paperboard product comprising a foam-formed outer ply are evident from the above disclosed method for producing an outer ply of a paperboard product due to the properties disclosed for the (foamed) furnish used for producing such an outer ply.

[0059] The present document is also directed to a foamed furnish for an outer ply of a paperboard product comprising at least two plies, said foamed furnish comprising: a) a furnish comprising at least 80 wt% of kraft pulp, based on the total dry weigh of said furnish, said kraft pulp having a mean fibre length Lc(l) of at least about 0.9 mm, such at least about 1 .0 mm, at least about 1.1 mm, at least about 1 .2 mm, or at least about 1.3, in accordance with ISO 16065-2:2014, preferably wherein at least 10 wt% of the fibers are fibers having a mean fiber length Lc(l) of at least 1 .8 mm, such as from about 1 .8 mm to about 2.5 mm, such as from about 1 .8 mm to about 2.2 mm, according to ISO 16065-2:2014; b) one or more foaming agent(s); wherein the furnish having a solids content of at least about 1 wt% based on the total weight of the foamed furnish, said furnish optionally further comprising one or more filler(s) and / or dispersing agent(s) as disclosed elsewhere herein. Further details, content and properties of such a foamed furnish are disclosed in connection with the method for producing an outer ply of a paperboard product according to the present document. Such a foamed furnish may be used in the method for producing an outer ply of a paperboard product disclosed herein. As mentioned above, the foamed furnish typically comprises about 10 wt% or less of refined cellulose, such MFC and / or HRC (highly refined cellulose), such as from about 0.1 wt% to about 10 wt% refined cellulose, such as MFC and / or HRC, or about 7 wt% or less refined cellulose, such as MFC and / or HRC, such as from about 0.1 wt% to about 7 wt% refined cellulose, such as MFC and / or HRC, or 0 wt% refined cellulose, such as MFC and / or HRC, based on the total dry weight of the foamed furnish. When different types of refined cellulose, such as MFC and HRC, are present in the foamed furnish, these values refer to the amount of the different types of refined cellulose, such as MFC and HRC, taken together,

[0060] The invention will be further described in the following examples, which do not limit the scope of the invention described in the claims.

[0061] EXPERIMENTAL SECTION

[0062] 100 % bleached hardwood (birch) will be disintegrated to a consistency (solids content) of 2.8 wt% (wet weight of the furnish before addition of the other components of the furnish) and refined to a SR of 35 (ISO 5267-1 :1999).

[0063] After refining, cationic starch (6kg / tn) will be added to the furnish and then the stock will be fed to a first foam preparation step in which 1% anionic surfactant (Sodium dodecyl sulfate, SDS) will be added to the stock (1% based on fiber amount (w / w)), whereafter air will be injected at high speed and pressure to the stock enabling efficient mixing of fiber- chemicals-surface active agent-air to a first suspension. The high-speed injection will be effected with a device using non-moving elements. After the first foam preparation step, the suspension will be fed into a second foam preparation step, which will be a high shear mechanical foam generator using a rotating element enabling formation of homogenous foam by shearing at 2 500 rpm. After the foam generation, the foam will have a density of about 250 g / L and a temperature of 35 degree C and will then be fed into the pilot paper machine headbox and run on a Fourdrinier type of machine for preparing a 50 gsm web or top ply (outer ply). The moisture content of the wet pressed and dried web will be about 8%.

[0064] Foam forming using the above protocol of order of mixing and foam generation will provide a consistent and good foam, which will result in good specific formation and higher press solids after press section. Also, white water consistency will be low confirming that the above conditions enables significant reduction in retention chemicals or even possibility to run without retention chemicals. The above exemplary process for how a foam-formed outer ply according to the present document can be performed is illustrated in Fig. 2.

[0065] Without being bound to any theories, it is believed that the mixing-foaming performed according to this protocol enables better control of bubble size and foam quality. Typically, a high SR would lead to higher drainage resistance in the press section and hence lower solids before drying section.

[0066] It is to be understood that while the invention has been described in conjunction with the detailed description thereof, the foregoing description is intended to illustrate and not limit the scope of the invention, which is defined by the scope of the appended claims. Other aspects, advantages, and modifications are within the scope of the following claims.

[0067] Unless expressly described to the contrary, each of the preferred features described herein can be used in combination with any and all of the other herein described preferred features.

Claims

CLAIMS1 . A method for producing an outer ply of a paperboard product, said paperboard product comprising at least two plies, characterised in that said outer ply has a density of at least 700 kg / m3, and that said method comprises foam-forming a furnish for said outer ply to provide a foamed furnish comprising fibers and one or more foam-forming agents.

2. The method of claim 1 , wherein said foam-forming to provide a foamed furnish comprising fibers and one or more foaming agents is achieved by adding one or more foam-forming agents to a furnish comprising fibers and then foaming the furnish, or by first forming a foam by mixing water and one or more foaming agents and then adding fibers to this foam.

3. The method according to any one of the preceding claims, wherein the foamed furnish has a solids content of at least about 0.5 wt%, preferably at least 0.6 wt%, such as 1 wt%, preferably at least about 1 .2 wt%, even more preferably from about 1 .3 wt% to about 6 wt%, based on the total weight of the foamed furnish.

4. The method according to any one of the preceding claims, wherein at least 10 wt% of the total content of fibers are fibers having a mean fiber length Lc(l) of at least1 .8 mm in accordance with ISO 16065-2:2014, such as from about 1 .8 mm to about 2.5 mm, such as from about 1 .8 mm to about 2.2 mm.

5. The method according to any one of the preceding claims, where the total amount of foaming agents in said foamed furnish is from about 0.01 wt% to about 10 wt%, such as from about 0.05 wt% to about 5 wt%, based on the total dry weight of the foamed furnish.

6. The method according to any one of the preceding claims, wherein said foaming agent is a surface-active chemical selected from one or more of anionic surfactants, non-ionic surfactants, zwitterionic surfactants, and polymeric surfactants.

7. The method according to any one of the previous claims, wherein the foamed furnish further comprises refined cellulose having a Schopper Riegler value of above 60 as measured according to ISO 5267-1 :1999, the refined cellulose preferably being microfibrillated cellulose.

8. The method according to any one of the preceding claims, wherein said foamed furnish comprise 0.1 wt% or less, such as 0.05 wt% or less, 0.01 wt% or less orabout 0 wt% of retention chemicals, based on the dry weight of said foamed furnish.

9. The method according to any one of the preceding claims, wherein said foamed furnish comprises about 10 wt% or less of refined cellulose, such as microfibrillated cellulose and / or highly refined cellulose, such as from about 0.1 wt% to about 10 wt%, or 0 wt%, refined cellulose, based on the total dry weight of said foamed furnish.

10. The method according to any one of the preceding claims, wherein said method further comprises adding said outer ply to another ply, such as a bulk layer, of said paperboard product.1 1 . The method of any one of the preceding claims, said method comprising the steps of: i) providing a fiber-based web or a wire; ii) providing a foamed furnish for an outer ply of said paperboard product, said foamed furnish comprising fibers and one or more foam-forming agents; iii) applying said foamed furnish of step ii) to said fiber-based web or wire of step i), thereby forming said outer ply on said fiber-based web or said wire; and iv) dewatering the fiber-based web with the outer ply or outer ply obtained in step iii).

12. A paperboard comprising a midply and / or a bulk layer and at least one outer ply, the at least one outer ply being a top ply and / or a back ply, wherein said at least one outer ply is foamed formed and has a density of at least 700 kg / m3, preferably wherein the midply and / or bulk layer is / are not foam-formed.

13. The paperboard product of claim 12, wherein the outer ply of said paperboard product has a basis weight of 55 g / m2or less, such as from about 15 g / m2to about 55 g / m2, such as from about 20 to about 55 g / m2, according to ISO 536:2019.

14. The paperboard product according to claim 12 or 13, wherein at least 10 wt% of the fibers of the paperboard product have a mean fiber length Lc(l) of at least 1.8 mm in accordance with ISO 16065-2:2014, such as from about 1.8 mm to about 2.5 mm, such as from about 1 .8 mm to about 2.2 mm.

15. The paperboard product according to any one of claims 12-14 wherein the outer ply has a brightness of at least about 70 according to ISO 2470-1 :2016, a specific formation value of less than 0.8 gA0.5 / m as measured according to SCAN-P 92:09, a Bendtsen roughness of from about 100 ml / min to about 500 ml / min according to ISO 8791-4:2021 , and / or a bursting strength of at least about 70kPa according to ISO 2758:2014, wherein the outer ply comprises 0.1 wt% or less, of retention chemicals, based on the dry weight of said foamed furnish, a tensile index of 20 Nm / g or more in accordance with ISO 1924-3:2011 , and / or wherein the outer ply comprises at least 10 wt% of fibers having a mean fiber length Lc(l) of at least 1 .8 mm, such as from about 1 .8 mm to about 2.5 mm in accordance with ISO 16065- 2:2014, such as from about 1 .8 mm to about 2.2 mm.

16. The paperboard product according to claim 15, wherein said outer ply of said paperboard product comprises about 10 wt% or less of refined cellulose, such as microfibrillated cellulose and / or highly refined cellulose, such as from about 0.1 wt% to about 10 wt%, or 0 wt%, refined cellulose.

17. A paperboard obtained or obtainable by a method according to any one of claims 1-11.

18. A foamed furnish for an outer ply of a paperboard product comprising at least two plies, said foamed furnish comprising: a) a furnish comprising at least 80 wt% of kraft pulp, based on the total dry weigh of said furnish, preferably wherein at least 10 wt% of the fibers of the kraft pulp are fibers having a mean fiber length Lc(l) of at least 1 .8 mm, such as from about 1.8 mm to about 2.5 mm, in accordance with ISO 16065-2:2014, such as from about 1 .8 mm to about 2.2 mm. b) one or more foaming agent(s); wherein said furnish has a solids content of at least about 1 wt% based on the total weight of the foamed furnish, said furnish optionally further comprising one or more filler(s) and / or dispersing agent(s).

19. The foamed furnish according to claim 18, wherein said foamed furnish comprises about 10 wt% or less refined cellulose, such as microfibrillated cellulose and / or highly refined cellulose, such as from about 0.1 wt% to about 10 wt%, or 0 wt% refined cellulose, based on the total dry weight of said foamed furnish.