EP4684058A1

EP4684058A1 - Method for making a multi-layered board

Info

Publication number: EP4684058A1
Application number: EP24714977.6A
Authority: EP
Inventors: Matti Hietaniemi; Asko Karppi; Markus Korhonen
Original assignee: Kemira Oyj
Current assignee: Kemira Oyj
Priority date: 2023-03-21
Filing date: 2024-03-20
Publication date: 2026-01-28
Also published as: WO2024194530A1; KR20250161017A; CN120958197A

Abstract

The invention relates to a method for making a multi-layered board in a board machine. The multi-layered board comprises at least two layers. A first fibre suspension comprising cellulosic fibres is obtained for a first layer and a second fibre suspension comprising cellulosic fibres is obtained for a second layer, wherein the first and second fibre suspensions are different from each other and at least one of them comprises recycled cellulosic fibres. A multi-layered board web is formed by combining the first layer and the second layer with each other before wet- pressing of the web in a press section. Glyoxylated polyacrylamide having a number average molecular weight of at least 15 000 g/mol is added into the first fibre suspension and/or the second fibre suspension before forming the first layer and the second layer.

Description

METHOD FOR MAKING A MULTI-LAYERED BOARD

The present invention relates to a method for making a multi-layered board according to the preambles of the enclosed independent claim.

Board, especially corrugated board, is one of the most used packaging materials in the world due to its low price, light-weight structure and recyclability. Corrugated board typically comprises at least one corrugated sheet of fluting board connected to one or two flat linerboards. In order to obtain optimal strength, stiffness and bulk, required for diverse end use applications, the fluting board and linerboards are often manufactured as multi-layered structures. The layers can contain different chemical additives and/or different fibres, i.e. fibres with different fibre length and/or originating from different sources. For example, linerboard is commonly manufactured by using a long fibre fraction for the top layer and a short fibre fraction for the back layer. In this manner it is possible to optimise the properties of each layer as well as the properties of the multi-layered board itself.

Recycled cellulosic fibres of different origin are commonly used in manufacture of multi-layered boards. During the last decades the recycling degree of the cellulosic fibres has continuously increased. Every time a cellulosic fibre is subjected to a repulping process, its fibre length is generally shortened and its anionic surface charge is reduced. This means that the average fibre length of both the long fibre fraction fibres and the short fibre fraction fibres has been getting shorter when the recycling degree has been increasing. Simultaneously the surface charge of the recycled cellulosic fibres has become less anionic. Shorter fibre length has negative effect on the strength of the multi-layered board while the less anionic fibre surfaces reduce the interaction between the cellulosic fibres and the chemical additives used in the manufacture of board.

During the last decades also the process water cycles in the board manufacture have become more and more closed. This means that minimal amounts of fresh water are introduced into the manufacturing process and the used water is recirculated within the process as much as possible. Most often, closing of the process water cycles leads to the presence of increased amounts of dissolved, anionically charged substances in the process water.

As a result of above developments, the fibre suspensions for board manufacture often comprise recycled cellulosic fibres with short fibre length and low surface charge as well as water phase which is rich in anionically charged disturbing substances. This may lead decrease efficiency of common papermaking chemical additives, such as glyoxylated polyacrylamide, which is obtained by polymerising acrylamide and cationic monomers and then crosslinking the polymer with glyoxal. The function of papermaking chemicals is usually based on electrostatic interactions between the cationically charged groups of the chemical and the anionic groups on the surface of the cellulosic fibre. When using recycled fibres, it has been observed that the expected effects of the chemical additive dosage are not always obtained. It is assumed that the absence of surface charges on cellulosic fibres, combined with the potentially high charge load in the water phase, negatively affect the expected interaction between the chemical additives and the recycled fibres. Reduced interaction may lead, among others, to reduced strength of the multi-layered board. The multi-layered boards can show reduced z-directional internal strength, which is seen as delamination or splitting of the multi-layered board, either at the boundary between the layers or within the individual layer itself. Consequently, there is a need for a method which would enable the production of multi-layered boards, such as fluting or linerboard, having an improved internal strength, even from recycled fibres.

An object of this invention is to minimise or possibly even eliminate the disadvantages existing in the prior art.

Another object of the present invention is to provide a method for improving the internal strength properties of multi-layered board, especially the z-directional internal strength within each layer as well as the strength between the layers, such as interlayer bonding. Yet another object of the present invention is to provide a method, with which the delamination resistance of the layers of the multi-layered fibrous web or board is improved.

These objects are attained with the invention having the characteristics presented below in the characterising parts of the independent claims. Some preferred embodiments of the invention are presented in the dependent claims.

The embodiments mentioned in this text relate, where applicable, to all aspects of the invention, even if this is not always separately mentioned.

In a typical method according to the present invention for making a multi-layered board in a board machine, which multi-layered board comprises at least two layers, the method comprises

- obtaining a first fibre suspension comprising cellulosic fibres for a first layer and a second fibre suspension comprising cellulosic fibres for a second layer, wherein the first and second fibre suspensions are different from each other and at least one of them comprises recycled cellulosic fibres;

- forming the first layer from the first fibre suspension and the second layer from the second fibre suspension,

- forming a multi-layered board web by combining the first layer and the second layer with each other before wet-pressing of the multi-layered board web in a press section of the board machine,

- drying the multi-layered board web, wherein a glyoxylated polyacrylamide having a number average molecular weight of at least 15 000 g/mol is added into the first fibre suspension and/or the second fibre suspension before forming of the first layer and the second layer.

Now it has been surprisingly found that an addition of glyoxylated polyacrylamide having a number average molecular weight of at least 15 000 g/mol significantly improves the z-directional internal strength of the formed layer(s) and improves the delamination resistance of the multi-layered board. It is assumed that when the number average molecular weight is as defined in the present invention, the glyoxylated polyacrylamide is not consumed by the detrimental interactions with anionic disturbing substances in the water phase of the suspension and is able to effectively interact with the scarce anionic charges on the fibre surfaces. The glyoxylated polyacrylamide having the number average molecular weight of at least 15 000 g/mol is able to increase the internal strength of the formed layers, even the strength between two layers, i.e. the interlayer bonding and strength between the first and second layer. Conventionally the z-directional strength is weakest at the boundary between the layers, but the addition of the specific glyoxylated polyacrylamide according to the present invention significantly reduces the risk of splitting or delamination of the multi-layered board. It is assumed that when the number average molecular weight of the glyoxylated polyacrylamide is high enough, there is a sufficient number of polymers capable of reaching over the boundary of the adjacent layers. In general, according to the present invention, the z-directional internal strength can be improved between the layers, i.e. at the boundary of two layers, as well as inside of a certain layer. The obtained improvement can be demonstrated, for example by Scott Bond, z- directional strength and/or peeling force measurements.

In the present context, the number average molecular weight denotes the statistical average molecular weight of the polymer chains, and is defined by: where Mi is the molecular weight of a chain and Ni is the number of chains of that molecular weight.

The z-directional internal strength improvement within the layers of the multilayered board can be described by measuring Scott Bond and/or z-directional tensile strength of the multi-layered board. Scott Bond and/or z-directional tensile strength of the multi-layered board are improved by the method according to the invention. An improved z-directional internal strength may typically lead improved SCT strength results, when buckling of the multi-layered board by weaker internal bonding in a SCT test is reduced or even absent.

The multi-layered board comprises at least two fibre layers, namely a first and a second layer. It is possible that the multi-layered board may comprise one, two or more additional middle layers, located between a first outer layer and a second outer layer. For example, multilayered board sheet may be at testliner or a whitetop liner, comprising two fibre layers. Testliner may comprise long fibre recycled pulp as a top ply (first layer) and short fibre recycled pulp as a back ply (second layer). Whitetop liner may comprise bleached kraft pulp, e.g. short fibre or mixed office waste recycled pulp as a top ply (first layer) and unbleached kraft pulp and/or recycled pulp as a back ply (second layer). According to one embodiment, the multi-layered board may comprise 2 - 5, preferably 2 - 4 layers in total. The number of the layers in the multi-layered board is not limited, and the method according to the present invention is applicable to all kind of multi-layered boards irrespective of the number of the layers. For example, the multi-layered board may comprise three layers: a first and a second layer forming the outer layers, and an additional middle layer located between the first and the second (outer) layer. The first outer layer and the second outer layer form the large surfaces of the multi-layered board.

According to one preferable embodiment of the present invention the glyoxylated polyacrylamide may have the number average molecular weight of at least 20 000 g/mol, preferably at least 25 000 g/mol, more preferably at least 30 000 g/mol. The number average molecular weight may be, for example, in a range of 15 000 - 250 000 g/mol, preferably 20 000 - 200 000 g/mol, more preferably 25 000 - 150 000 g/mol, even more preferably 30 000 - 100 000 g/mol. When the glyoxylated polyacrylamide has the number average molecular weight at least 15 000 g/mol or more, it means that the glyoxylated polyacrylamide comprises less oligomers or polymer material having low molecular weight. Absence of low molecular weight or oligomer size polyacrylamide increases the binding efficiency between the cellulosic fibres, as they do not consume the scarce surface charges on the fibre surfaces. The number average molecular weight of at least 15 000 g/mol may also provide effective binding between the adjacent layers in the multilayered board, i.e. between the first and second layers or between the first layer, additional middle layer(s) and the second layer.

The glyoxylated polyacrylamide used in the present invention may have a weight average molecular weight in a range of 200 000 - 950 000 g/mol, preferably 250 000 - 900 000 g/mol.

In the present application the number average molecular weight Mn and the weight average molecular weight Mw is determined by size-exclusion chromatography (SEC) using Agilent 1100 SE chromatography equipment with integrated pump, autosampler and degasser. Eluent is a buffer solution (0.3125 M CH3COOH + 0.3125 M CH3COONa) with a flow rate of 0.5 ml/min at 35 °C. Typical sample concentration is 2 - 4 mg/ml, with an injection volume of 50 pl. Ethylene glycol (1 mg/ml) is used as a flow marker. Column set consists of three columns (a TSKgel PWXL guard column and two TSKgel GMPWXL columns). Refractive index detector by Agilent is used for detection (T = 35 °C). Molecular weight is determined using conventional (column) calibration with poly(ethylene oxide)/poly(ethylene glycol) narrow molecular weight distribution standards (Polymer Standards Service).

The glyoxylated polyacrylamide used in the present invention may be formed by crosslinking a base polymer of cationic polyacrylamide with glyoxal. The base polymer may be obtained by polymerisation of (meth)acrylamide and 7 - 50 mol- %, preferably 7 - 30 mol-%, more preferably 11 - 25 mol-%, of cationic monomers. The cationic monomers may be selected from diallyldimethylammonium chloride (DADMAC), 3-(acrylamidopropyl)trimethyl- ammonium chloride (APTAC), 3-(methacrylamidopropyl)trimethyl-ammonium chloride (MAPTAC), or any combination thereof. Preferably the cationic monomer is diallyldimethylammonium chloride (DADMAC). The weight average molecular weight of the base polymer of cationic polyacrylamide may be in a range of 30 000 - 250000 g/mol, preferably 50 000 - 200 000, for example 70 000 - 180 000 g/mol. The weight average molecular weight of the base polymer describes the molecular weight of the main chain of the (meth)acrylamide polymer which is a result of the copolymerisation of (meth)acrylamide and cationic monomers. The main chain is substantially linear. Methods for measuring the weight average molecular weight are well known by a skilled person, for example size-exclusion chromatography (SEC), as described above may be used.

The base polymer may be glyoxylated either in a chemical plant and transported to a board mill, or the glyoxylation may be performed on-site in the board mill. Glyoxylation of cationic polyacrylamide base polymer is known as such for a person skilled in the art. The glyoxylated polyacrylamide used in the present invention may have a total glyoxal content of free and reacted glyoxal in a range of 5 - 25 weight-%, preferably 7 - 21 weight-%, more preferably 9 - 18 weight- %, even more preferably 10 - 17 weight-%, calculated from total amount of glyoxal and polyacrylamide. This total glyoxal content provides a reduced amount of free glyoxal in the glyoxylated polyacrylamide used.

The glyoxylated polyacrylamide used in the present invention is cationic. The glyoxylated polyacrylamide may have a charge density in a range of 0.4 - 2.5 meq/g, preferably 0.7 - 2.2 meq/g, more preferably 0.9 - 2.0 meq/g, even more preferably 1.0 - 1.7 meq/g, measured at pH 7. Charge density measurements are made by Mutek PCD 03. When the glyoxylated polyacrylamide has a charge density within the defined ranges, significant improvements in z-directional internal strength, such as SCT and burst strength, have been observed, especially when the fibre suspension(s) comprise recycled fibres, e.g. OCC pulp. Furthermore, the defined charge density ranges provide proper retention of the glyoxylated polyacrylamide without overconsumption of available anionically charged groups or risk for overcation ization of the fibre suspension.

The layers of the multi-layered board are formed from fibre suspensions comprising cellulosic fibres. The first layer is formed from the first fibre suspension, the second layer is formed from the second fibre suspension, and any additional middle layer is formed from an additional fibre suspension. At least the first and second fibre suspensions are different from each other and at least one, preferable both, of them comprises recycled cellulosic fibres. If any additional fibre suspension(s) is/are used for additional middle layer(s), the additional fibre suspension(s) is/are preferably different from the first and the second fibre suspensions and may contain recycled cellulosic fibres.

According to one embodiment, the multi-layered board may comprise at least three layers, wherein the first layer forms a first outer layer, the second layer forms one of the additional middle layer(s), and a third layer forms a second outer layer, which can be same or different from the first outer layer. At least the first and second fibre suspensions are different from each other and at least one, preferable both, of them comprises recycled cellulosic fibres.

In the present context, the term “fibre suspension” is generally understood as an aqueous suspension, which comprises cellulosic fibres and optionally fillers. At least one of the fibre suspensions, preferably two or all of them, comprise or consists of recycled fibres. The recycled fibres may originate from old corrugated container (OCC) pulp, mixed waste (MXW) pulp, old newsprints (ONP) pulp, pulp from household collected waste, pulp from paper containing newsprint and magazines (ONP/OMG), mixed office waste (MOW) pulp, deinked pulp (DIP) or any of their mixtures. According to an embodiment of the invention, the recycled fibres may comprise undeinked recycled fibres. In the present context, the term recycled fibres is understood to encompass also broke, which is recirculated within the board mill back to the pulping stage from the following process stages, e.g. web formation. The fibre suspensions may also comprise cellulosic fibres obtained by mechanical pulping, kraft pulping or chemi-thermomechanical pulping.

According to one embodiment of the invention the cellulosic fibres in the first fibre suspension may have a first length weighted average length and the cellulosic fibres in the second fibre suspension have a second length weighted average length, which are different from each other. This means that the fibre suspension have been prepared by fractioning the cellulosic fibres to at least two different fibre fractions with different length weighted average lengths, and the different fractions have been used to prepare the first fibres suspension and the second fibre suspension. The first length weighted average length and the second length weighted average length may have a difference in a range of 0.1 - 1.5 mm, measured by using Kajaani FSA fibre length analyzer. This means that the first fibre suspension, and thus the first fibre layer, may comprise short cellulosic fibres, whereas the second fibre suspension, and correspondingly the second fibre layer, may comprise longer cellulosic fibres. For example, the first length weighted average length may be in a range of 0.8 - 1 .2 mm and the second length weighted average length may be from >1.2 mm to 2.0 mm, measured by using Kajaani FSA fibre length analyzer. According to another embodiment, for example when the fibre suspensions comprise American Old Corrugated Containerboard (AOCC) pulp, the first length weighted average length may be in a range of 1 .0 - 1 .5 mm and the second length weighted average length may be from >1 .5 mm to 2.5 mm, measured by using Kajaani FSA fibre length analyzer.

According to one embodiment, the first fibre suspension may have a Canadian Standard Freeness CSF <120 ml and the second fibre suspension may have Canadian Standard Freeness CSF >120 ml, measured according to standard ISO 5267-2:2001. Alternatively, the first fibre suspension may have a Canadian Standard Freeness CSF <250 ml and the second fibre suspension may have Canadian Standard Freeness CSF >250 ml, measured according to standard ISO 5267-2:2001.

The glyoxylated polyacrylamide having the number average molecular weight of at least 15 000 g/mol may be added to the first, second and/or any additional fibre suspension. According to one embodiment of the invention the glyoxylated polyacrylamide may be added in the first fibre suspension, in the second fibre suspension or in any additional fibre suspension(s) for any of the additional fibre layer(s). Alternatively, the glyoxylated polyacrylamide may be added in the first fibre suspension, in the second fibre suspension and in any additional fibre suspension(s) for any of the additional fibre layer(s). The freedom to select the fibre suspension to which the glyoxylated polyacrylamide is added, enables increased flexibility for adjusting the addition to suit each individual fibre suspension used. For example, it is possible to add the glyoxylated polyacrylamide only to the fibre suspension comprising short cellulosic fibres, having a length weighted average length <1.5 mm, preferably <1.2 mm, measured by using Kajaani FS fibre length analyzer. When glyoxylated polyacrylamide is added to the fibre suspension comprising short fibres it is possible to maximize the z-directional strength increase obtainable with each polyacrylamide dosage. Alternatively, if the glyoxylated polyacrylamide is added to the fibre suspension comprising long fibres, having a length weighted average length >1 .5 mm, preferably >1 .7 mm, measured by using Kajaani FS fibre length analyzer, it is possible to maximize the absolute z-directional strength effect obtainable with each polyacrylamide dosage.

The first and/or the second fibre suspension, as well as any additional fibre suspension for possible additional layers, such second outer layer and/or an additional middle layer(s), may have an ash content in a range of 10 - 25 weight- % or 15 - 25 weight-%, preferably 17 - 23 weight-%, measured by standard ISO 1762, at 525 °C. The ash content may be, for example, 10 - 20 weight-% or 12 - 18 weight-%. The ash content indicates high amount of inorganic material in the fibre suspension, originating from fillers and/or remains of inorganic coating pigments associated with recycled fibres. Especially fibre suspensions comprising or consisting of short cellulosic fibres having a length weighted average length <1.5 mm, preferably <1.2 mm, measured by using Kajaani FS fibre length analyzer, may have the ash content of 15 - 25 weight-%, preferably 17 - 23 weight-%. The ash content for the fibre suspension is usually measured from the thick stock, before the short circulation of the board machine. The addition of the glyoxylated polyacrylamide having the number average molecular weight of at least 15 000 g/mol is especially advantageous to the fibre suspension having a high ash content, as it is able to provide desired z-directional internal strength.

According to one embodiment of the invention, the first fibre suspension and the second fibre suspension have different ash content. The difference between the ash content of the first fibre suspension and the second fibre suspension may be at least 2 percentage units, preferably at least 4 percentage units. According to one embodiment, the ash content of the first fibre suspension may be 15 - 24 % and the ash content of the second fibre suspension may be 8 - 14 %, measured from the thick stock after machine chest. Alternatively, for example when American OCC is used, the ash content of the first fibre suspension may be 7 - 15 % and the ash content of the second fibre suspension may be 2 - 6 %. Especially when the first fibre suspension and the second fibre suspension have different ash content and comprise cellulosic fibres with different length weighted average lengths, the bonding between the formed layers is conventionally quite low. However, the addition of the specific glyoxylated polyacrylamide according to the present invention significantly improves the bonding between the layers even in this case and reduces the risk of delamination.

The first and/or the second fibre suspension, as well as any additional fibre suspension, for example for a second outer layer or for possible additional middle layer(s), may have a conductivity in a range of 2.5 - 10 mS/cm, preferably 3 - 8 mS/cm, even more preferably 4 - 8 mS/cm.

According to one embodiment the first fibre suspension may comprise or consist of virgin Kraft pulp, virgin chemi-thermomechanical pulp or their mixture, whereas the second fibre suspension comprises or consists of recycled fibres, preferably broke fibres.

According to another embodiment, at least the first fibre suspension comprises or consist of mixed office waste pulp, wherein the formed first layer is a white-top liner layer. According to one embodiment of the invention, the glyoxylated polyacrylamide is preferably added to the first, second, and/or any additional fibre suspension having a consistency >1.6 weight-%, preferably >2 weight-%, more preferably >2.5 weight-% or >3 weight-%. The consistency of the fibre suspension may be in a range of 1.6 - 10 weight-%, more preferably 2 - 4.5 weight-% or 2.5 -4.0 weight-%. This means that the glyoxylated polyacrylamide is added to the so- called thick stock, especially when the fibre stock comprises short cellulosic fibres, which enables effective interaction between the polymer and the short cellulosic fibres of the fibre suspension . Short cellulosic fibres comprise cellulosic fibres have a length weighted average length <1.5 mm, preferably <1.2 mm, measured by using Kajaani FSA fibre length analyzer.

According to one embodiment of the present invention, the glyoxylated polyacrylamide may be added to a fibre suspension having a consistency <1.6 weight-%, preferably <1 weight-%, when the fibre suspension comprises long fibres having a length weighted average length >1.5 mm, preferably >1.7 mm, measured by using Kajaani FS fibre length analyzer. For example, the glyoxylated polyacrylamide may be added to a fibre suspension having the consistency in a arrange from 0.1 - 1 .5 weight-%, preferably 0.2 - 0.9 weight-%.

The glyoxylated polyacrylamide may be added in amount of 0.5 - 5 kg/ton dry fibre suspension, preferably 0.8 - 4 kg/ton dry fibre suspension, more preferably 1 - 3 kg/ton dry fibre suspension. It is possible to add different amounts of glyoxylated polyacrylamide to each fibre suspension.

After the addition of the glyoxylated polyacrylamide to the fibre suspension(s), the fibre suspensions are used to form first, second and any additional layers, which are combined with each other to form the multi-layer board web. Any suitable method and/or unit for forming the fibrous layers and combining them together prior the wet-pressing of the formed multi-layered board web may be used. The layers may preferably be combined when the dryness of the layers is <15 weight- %. For example, the multi-layered board web may be formed from separate first, second and additional layers, formed by multiple separate forming units. Each of the layers is formed from the associated fibre suspension by its own forming unit, whereafter the layers are combined together. According to this embodiment, the individual layers are first formed separately on a wire or the like and combined in a later stage after at least partial draining of the layers. The forming units may comprise head boxes or cylinder formers. The formed multi-layered board web is then subjected to further draining, wet-pressing and drying.

Alternatively, the multi-layered board web may be formed by using multilayer headbox.

According to yet another embodiment the first fibre layer may be formed from the first fibre suspension, at least a part of water is drained from the first layer on a wire section, whereafter the second layer is applied on the surface of the first web and the combined multi-layered board web is subjected to further draining, wetpressing and drying. The second fibre layer applied on the surface of the first layer is not necessarily subjected to the draining before the combining of the layers.

After the first layer, the second layer and any additional layer(s), such as additional middle layer(s), are combined together, the obtained multi-layered board web is subjected to wet-pressing in a press section of the board machine. After the wet-pressing, the multi-layered board web is dried, preferably at least to dryness of at least 75 weight-%, typically to a dryness of 80 - 85 weight-%.

The obtained multi-layered board may have a grammage of at least 70 g/m², preferably at least 85 g/m², more preferably at least 100 g/m² or at least 120 g/m². The grammage may be, for example, in a range of 70 - 800 g/m², preferably 85 - 700 g/m², more preferably 100 - 300 g/m², even more preferably 120 - 150 g/m². The present invention provides multi-layered board with good resistance against delamination or splitting of the layers, even at high grammage. The multi- layered board may be selected from testliner board, fluting board, white top liner board, gypsum board liner or core board liner. According to one preferable example, the basis weight of one of the layers, e.g. top ply or outer ply of corrugated board, may be 35 - 80 g/m².

According to one preferable embodiment a surface size solution may be applied on one or both surfaces of the multi-layered board web after the drying of the web. The surface size solution is impregnated into the first and/or second layer of the multi-layered board and enhances the z-directional strength of the multilayered board. The surface size solution may comprise starch or polyvinyl alcohol, preferably starch. Especially, when the surface size solution comprises starch, the starch may react within the layer with the glyoxylated polyacrylamide, and thus provide increased strength effect for the final multi-layered board, including both z-directional internal strength as well as surface strength. The invention thus provides an advantageous effect since otherwise the surface size starch conventionally penetrates only about 30 g/m², as average, from the surface when applied with a metering size press, increasing the risk of delamination between the layers. According to one preferable embodiment, the surface size solution comprises degraded starch. When the surface size solution comprises starch or degraded starch, it may be applied in an amount of 1 - 12 weight-%, preferably 1 - 10 weight-%, more preferably 2 - 8 weight-%, even more preferably 3 - 7 weight-%, calculated from weight of the multi-layered board.

The making of multi-layered board is free of any creping steps employing a Yankee cylinder, either before or after the drying of the multi-layered board. After the drying, the formed multilayer board may be subjected to corrugation in a conventional corrugator machine, used for making corrugated board.

According to one embodiment, the formed multi-layered board may be used as a gypsum board liner. The multi-layered board may be attached on a large surface of a gypsum board. The improved resistance for delamination makes the board especially suited for this end-use. EXPERIMENTAL

Some embodiments of the invention are described in the following non-limiting example.

Example 1

Example 1 demonstrates the effect of high number average molecular weight of glyoxylated polyacrylamide to the properties of multilayered board.

Chemicals

The tested glyoxylated polyacrylamides are identified in Table 1. The peak molecular weight in Table 1 indicates the molecular weight at the peak of the distribution curve, i.e. the molecular weight of the biggest population of the polymer. GPAM1 and GPAM2 are commercially available glyoxylated polyacrylamides and they are used as reference.

Table 1 . Parameters of tested polymers.

It can be seen from Table 1 that even if the weight average molecular weight of reference GPAM2 is similar to the iGPAMI and iGPAM2, GPAM2 has much lower number average molecular weight than iGPAMI and iGPAM2. The same applies for G PAM 1.

Fibre Suspensions, Formation of Multi-layered Board

Multi-layered board sheets with two layers were prepared by using two different fibre suspensions. First fibre suspension for the top layer was prepared from long fiber Old Corrugated Container board (OCC), and had a length weighted fibre length of 1.4 mm (measured with Kajaani FSA fibre length analyzer) and an ash content of 16 %, measured at 525 °C. The second fibre suspension for the back layer was prepared from short fibre OCC, and had a length weighted fibre length of 1 .2 mm and ash content of 16%, both measured as above.

Long fibre OCC and short fibre OCC were separately wet disintegrated in 3 weigh-% consistency at 70 °C with Noviprofibre-pulper (volume 20 I) for 30 s at speed of 500 rpm and 25 min at speed of 1000 rpm, without soaking. Obtained fibre suspensions were further diluted to 0.6 weight-% consistency with tap water. pH and conductivity of the fibre suspensions were adjusted to 6.8 and 3.0 mS/cm. For conductivity adjustment, addition of a salt mixture containing calcium acetate 70 weight-%, sodium sulphate 20 weight-% and sodium bicarbonate 10 weight- % was made, until target conductivity was reached.

Multi-layered board sheets were formed with a dynamic sheet former (DSF). Chemical additions were made to mixing tank of DSF and the same addition was made to the first and second fibre suspension. Glyoxylated polyacrylamides of Table 1 were used in the experiments, at addition level of 2.5 kg/ton. The addition levels are given as kg of dry chemical per ton dry OCC fibre suspension. Each experiment included addition of retention aids system of cationic polyacrylamide (100 g/t) and silica (400 g/t).

The board sheets were formed as follows:

Second fibre suspension for the back layer was added first to DSF and sprayed to form the back layer. Before drainage, top layer was formed on the back layer by using the first fibre suspension. Water was drained out after spraying of both fibre suspensions was completed. Drum was operated with 1250 rpm, mixer for pulp with 450 rpm, pulp pump with 950 rpm/min, number of sweeps was 100 and scoop time was 60 s. The multi-layered sheet was removed from drum between a wire and 1 blotting paper on the other side of the sheet. Wetted blotting paper and the wire were removed, and the sheet was wet pressed at Techpap nip press with 5 bar pressure with 2 passes, having new blotting paper each side of the sheet before each pass. Sheets were dried in restrained condition in drum dryer. Drum temperature was adjusted to 92 °C and passing time to 1 min. Four passes were made: first two passes with the sheet between blotting papers and 2 passes without.

Grammage of each layer was 70 g/m², thus total grammage of the sheet was 140 g/m².

Sheets were pre-conditioned for 24 h at 23 °C in 50 % relative humidity, according to the standard ISO 187.

After pre-conditioning the sheets were cut. The first parts of the sheets were tested for strength properties without application of surface sizing and the second parts of the sheets were surface sized before testing the strength properties.

Surface sizing was done by using degraded starch solution. Starch solution was a mixture of dextrin starches of C*Film 07311 and C*Film 07312 (50 weight- %+50weight-%). Concentration of the sizing solution was 10 weight-% in all the test points. Solution was kept at 70 °C before the use and approximately 170 g of solution was used for surface sizing in each test point. The application of starch solution was done with Mathis laboratory size-press and the sized sheets were drum-dried. Size-press was washed between the test points. 5 board sheets of A5 size were surface sized in each test point. Size-press and drying parameters are given in Table 2.

After surface sizing and drying, board sheets were taken into climate-controlled laboratory (50 % relative humidity, 23 °C) for conditioning. Table 2 Laboratory size-press and drying parameters used in preparation of multi-layered board sheets.

After conditioning strength properties of the multi-layered sheets were measured for SCT (short span compression strength), burst strength, tensile strength and internal bond strength. The used devices and standards are given in Table 3. The results for measurements are shown in Table 4 for multi-layered board sheets without surface sizing and in Table 5 for surface size board sheets. Table 3 Sheet testing devices and standard methods used.

Table 4 Results for multi-layered board sheets without surface sizing. Table 5 Results for multi-layered board sheets with surface sizing.

It can be seen from Table 4 that iGPAMI and iGPAM2 with high number average molecular weight in general provided higher strength values than conventional GPAM1 and GPAM2. Especially the improvement in tensile index in machine direction may reduce web breaks before surface sizing, which improves overall machine efficiency.

It can be seen from Table 5 that use of iGPAMI and iGPAM2 with high number average molecular weight provided unexpected increase in strength properties when combined with surface sizing in comparison to conventional GPAM1 and GPAM2. iGPAMI with lower charge density seems to be advantageous especially for improving internal bond, which is critical property for process steps in converting, such as corrugation and printing. For testliner, which uses lower strength recycled fiber furnish, it is difficult to achieve similar internal bond strength than with kraftliner made from higher strength virgin fibre, even when size press treatment is used only for strengthening of testliner, not for kraftliner. Tensile strength improvement obtained with iGPAMI may also reduce folding cracking of the corrugated container edges.

Even if the invention was described with reference to what at present seems to be the most practical and preferred embodiments, it is appreciated that the invention shall not be limited to the embodiments described above, but the invention is intended to cover also different modifications and equivalent technical solutions within the scope of the enclosed claims.

Claims

1. Method for making a multi-layered board in a board machine, preferably for improving delamination resistance of the multi-layered board, which multi-layered board comprises at least two layers, the method comprising

- drying the multi-layered board web, characterised in

- adding into the first fibre suspension and/or the second fibre suspension glyoxylated polyacrylamide having a number average molecular weight of at least 15 000 g/mol before forming the first layer and the second layer.

2. Method according to claim 1 , characterised in that a surface size solution is applied on one or both surfaces of the multi-layered board web after drying.

3. Method according to claim 1 or 2, characterised in that the glyoxylated polyacrylamide has the number average molecular weight in a range of 15 000 - 250 000 g/mol, preferably 20 000 - 200 000 g/mol, more preferably 25 000 - 150 000 g/mol, even more preferably 30 000 - 100 000 g/mol.

4. Method according to claim 1 , 2 or 3, characterised in that the glyoxylated polyacrylamide has a total glyoxal content of free and reacted glyoxal in a range of 5 - 25 weight-%, preferably 7 - 21 weight-%, more preferably 9 - 18 weight- %, even more preferably 10 - 17 weight-%, calculated from total amount of glyoxal and polyacrylamide.

5. Method according to any of preceding claims 1 - 4, characterised in that the glyoxylated polyacrylamide has a charge density in a range of 0.4 - 2.5 meq/g, preferably 0.7 - 2.2 meq/g, more preferably 0.9 - 2.0 meq/g, even more preferably 1 .0 - 1 .7 meq/g, measured at pH 7.

6. Method according to any of preceding claims 1 - 5, characterised in that the first and/or the second fibre suspension has an ash content in a range of 15 - 25 weight-%, preferably 17 - 23 weight-%.

7. Method according to any of preceding claims 1 - 6, characterised in that the first and/or the second fibre suspension has a conductivity in a range of 2.5 - 10 mS/cm, preferably 3 - 8 mS/cm, even more preferably 4 - 8 mS/cm.

8. Method according to any of preceding claims 1 - 7, characterised in that the cellulosic fibres in the first fibre suspension have a first length weighted average length and the cellulosic fibres in the second fibre suspension have a second length weighted average length, which are different from each other.

9. Method according to claim 8, characterised in that the first length weighted average length and the second length weighted average length have a difference in a range of 0.1 - 1.5 mm.

10. Method according to claim 8 or 9, characterised in that the first length weighted average length is in a range of 0.8 - 1 .2 mm and the second length weighted average length is from >1 .2 mm to 2.0 mm.

11. Method according to claim 8 or 9, characterised in that the first length weighted average length is in a range of 1 .0 - 1 .5 mm and the second length weighted average length is from >1 .5 mm to 2.5 mm.

12. Method according to any of preceding claims 1 - 11 , characterised in that the glyoxylated polyacrylamide is added in the first fibre suspension or in the second fibre suspension.

13. Method according to any of preceding claims 1 - 11 , characterised in that the glyoxylated polyacrylamide is added in the first fibre suspension and in the second fibre suspension.

14. Method according to any of preceding claims 1 - 13, characterised in that the glyoxylated polyacrylamide is added in amount of 0.5 - 5 kg/ton dry fibre suspension, preferably 0.8 - 4 kg/ton dry fibre suspension, more preferably 1 - 3 kg/ton dry fibre suspension.

15. Method according to any of preceding claims 1 - 14, characterised in that the multi-layered board has a grammage of at least 70 g/m², preferably at least 85 g/m², more preferably in a range of 85 - 700 g/m², even more preferably 100 - 300 g/m², or 120 - 150 g/m².

16. Method according to any of preceding claims 1 - 15, characterised in that the multi-layered board is testliner board, fluting board, white top liner board, gypsum board liner or core board liner.

17. Method according to any of preceding claims 1 - 16, characterised in that the surface size solution comprises degraded starch and is applied in an amount of 1 - 12 weight-%, preferably 1 - 10 weight-%, more preferably 2 - 8 weight-%, even more preferably 3 - 7 weight-%, calculated from weight of the multi-layered board.

18. Method according to any of preceding claims 1 - 17, characterised in that the glyoxylated polyacrylamide is added to the first and/or second fibre suspension having a consistency >1.6 weight-%, preferably >2 weight-%, more preferably >3 weight-%.