Patent Description:
Traditionally, only virgin pulps have been used to form high-quality paperboard. In case of a multi-layered paperboard of at least three layers, which is a common type of paperboard in Europe, chemical pulps, predominantly kraft pulps, are typically used for the outer layers, while a mechanical pulp in combination with a chemical pulp is often used for the middle layer(s). Further, broke pulp, which is inevitably generated in paperboard production, is typically included in the formation of paperboard.

The objective of the present disclosure is to include recycled fibres in paperboard while maintaining or even improving properties characteristic of high-quality paperboard.

Accordingly, the present disclosure provides a method of producing a paperboard comprising a top layer, a back layer and a middle layer, said method comprising the steps of:.

wherein the first fraction constitutes at least <NUM>% by dry weight of the middle layer furnish and the CTMP constitutes at least <NUM>% by dry weight of the middle layer furnish.

The present disclosure further provides a paperboard comprising a top layer, a back layer and a middle layer, wherein the middle layer comprises:.

As shown in the Examples section below, the present disclosure enables the formation of a paperboard of equal bending resistance as that of a prior art paperboard (formed from virgin pulp only) at a lower total fibre consumption. In other words, the present disclosure not only enables the introduction of a significant portion of recycled fibres in high-quality paperboard without sacrificing the bending resistance of the board, but it also enables a reduction of the amount of fibres needed to reach the desired bending resistance.

As first aspect of the present disclosure, there is provided a method of producing a paperboard comprising a top layer, a back layer and a middle layer. As understood by the skilled person, the middle layer is arranged between the top layer and the back layer. The top layer is preferably a white top layer, i.e. a layer formed from bleached fibres. The top layer is typically intended for printing. Hence, the top layer may be provided with a pigment-based coating, which may consist of several sublayers applied in consecutive coating steps.

The first fraction constitutes at least <NUM>% by dry weight of the middle layer furnish, such as at least <NUM>% by dry weight of the middle layer furnish. An upper limit may be <NUM>% by dry weight of the middle layer furnish.

CTMP constitutes at least <NUM>% by dry weight of the middle layer furnish, such as at least <NUM>% by dry weight of the middle layer furnish. An upper limit may be <NUM>% by dry weight of the middle layer furnish.

In one embodiment, said middle layer furnish further comprises broke pulp. The broke pulp may constitute at least <NUM>% by dry weight of the middle layer furnish, such as at least <NUM>% by dry weight of the middle layer furnish. An upper limit may be <NUM>% by dry weight of the middle layer furnish.

In an embodiment, the density according to ISO <NUM>:<NUM> of first fraction is below <NUM>/m<NUM> after sheet forming according to ISO <NUM>-<NUM>:<NUM>. As an example, this density may be <NUM>-<NUM>/m<NUM>.

In one embodiment, the tensile stiffness index according to ISO <NUM>-<NUM>:<NUM> of the first fraction is at least <NUM> MNm/kg after sheet forming according to ISO <NUM>-<NUM>:<NUM>. As an example, this tensile stiffness index may be <NUM>-<NUM> MNm/kg.

The middle layer furnish may further comprise one or more strength agents, such as starch (preferably cationic starch), carboxymethyl cellulose (CMC) and/or microfibrillated cellulose (MFC).

In one embodiment, the paperboard is a liquid packaging board (LPB). In such an embodiment, each of the (fibre-based) layers of the paperboard comprise hydrophobic size. The hydrophobic size is preferably selected from the group consisting of ASA, AKD, rosin size and combinations thereof.

The amount of hydrophobic size in the middle layer furnish may for example be at least <NUM>/tonne dry fibre, such as at least <NUM>/tonne dry fibre. The amount of hydrophobic size in the outer layers is preferably lower than the amount of hydrophobic size in the middle layer.

In a preferred embodiment, the hydrophobic size in each of the furnishes of the method of the first aspect is a combination of AKD and rosin size. When such a combination is used, the headbox pH of the furnishes is preferably in the range of <NUM>-<NUM>, such as <NUM>-<NUM>.

In one embodiment, the first fraction is obtained by hydrocyclone fractionation of the pulp of recycled bleached fibers (see e.g. Example <NUM> below). In such an embodiment, the Schopper-Riegler value measured according to ISO <NUM>-<NUM>:<NUM> is preferably below <NUM>, more preferably below <NUM>.

In another embodiment, the first fraction is obtained by screening of the pulp of recycled bleached fibers (see e.g. Example <NUM> below).

In an embodiment of the method in which the top layer is a white top layer, said method further comprises:.

In this embodiment, second fraction has a Schopper-Riegler value (measured according to ISO <NUM>-<NUM>:<NUM>) above that of the first fraction, e.g. above <NUM>, such as above <NUM>. It follows from the above that the second fraction is preferably obtained by hydrocyclone fractionation of the pulp of recycled bleached fibers. However, it may also be obtained by screening of the pulp of recycled bleached fibers.

In an embodiment, the density according to ISO <NUM>:<NUM> of second fraction is above <NUM>/m<NUM> after sheet forming according to ISO <NUM>-<NUM>:<NUM>. As an example, this density may be at least <NUM>/m<NUM>, such as <NUM>-<NUM>/m<NUM>.

In one embodiment, the tensile stiffness index according to ISO <NUM>-<NUM>:<NUM> of the second fraction is at least <NUM> MNm/kg after sheet forming according to ISO <NUM>-<NUM>:<NUM>. As an example, this tensile stiffness index may be <NUM>-<NUM> MNm/kg.

The first fraction to second fraction mass flow ratio may for example be in the range of <NUM>:<NUM> to <NUM>:<NUM>, such as in the range of <NUM>:<NUM> to <NUM>:<NUM>, such as in the range of <NUM>:<NUM> to <NUM>:<NUM>.

In one embodiment, the second fraction constitutes at least <NUM>% by dry weight of the top layer furnish, such as at least <NUM>% by dry weight of the top layer furnish, such as at least <NUM>% by dry weight of the top layer furnish.

In addition to the second fraction, the white top layer furnish preferably comprises a bleached chemical pulp, such as a bleached kraft pulp. As understood by the skilled person, this chemical pulp is a virgin pulp.

Also, the method of the first aspect may further comprise the steps of:.

In one embodiment, the back layer furnish further comprises broke pulp.

The grammage, excluding any coating layers, of the paperboard formed by the method of the first aspect may for example be <NUM>-<NUM>/m<NUM> when measured according to ISO <NUM>:<NUM>. If pigment-coating layers are included, the grammage may be <NUM>-<NUM>/m<NUM>, such as <NUM>-<NUM>/m<NUM>.

The grammage of the middle layer is preferably higher than the grammage of the top layer. In one embodiment, the grammage of the middle layer is higher than the grammage of each of the top layer and the back layer. As an example, each of the top layer and the back layer may have a grammage below <NUM>/m<NUM>, while the grammage of the middle layer is above <NUM>/m<NUM>, such as at least <NUM>/m<NUM>, such as <NUM>-<NUM>/m<NUM>.

In a particular embodiment, the grammage of the top layer is below <NUM>/m<NUM>, the grammage of the back layer is below <NUM>/m<NUM> and the grammage of the middle layer is higher than the grammage of each of the top layer and the back layer, e.g. above <NUM>/m<NUM>.

In one embodiment, recycled fibres constitute <NUM>%-<NUM>% by dry weight of the total amount of fibres used to form the paperboard according to the first aspect.

In one embodiment, the top layer furnish and the back layer furnish comprise no recycled pulp or broke pulp.

In one embodiment, CTMP constitutes <NUM>%-<NUM>% by dry weight of the total amount of fibres used to form the paperboard according to the first aspect.

In one embodiment, broke pulp constitutes <NUM>%-<NUM>%, such as <NUM>%-<NUM>%, by dry weight of the total amount of fibres used to form the paperboard according to the first aspect.

As understood by the skilled person, the method of the first aspect typically comprises a step of couching to join the paperboard layers to each other.

As a second aspect of the present disclosure paperboard, such as a paperboard obtained according to the first aspect. The paperboard of the first aspect comprises a top layer (preferably a white top layer), a back layer and a middle layer, wherein the middle layer is formed from a furnish comprising a first fraction of a pulp of recycled bleached fibers and chemithermomechanical pulp (CTMP). The first fraction has a Schopper-Riegler value measured according to ISO <NUM>-<NUM>:<NUM> of below <NUM>. The first fraction constitutes at least <NUM>% by dry weight of the middle layer and the CTMP constitutes at least <NUM>% by dry weight of the middle layer.

The embodiments of the first aspect apply to the second aspect mutatis mutandis.

A white recycled paper "White High Grade" was purchased from the company Prezero. The recycled paper was slushed in a pulper for <NUM> minutes to produce a crude recycled pulp that was stored in a tank. The crude recycled pulp was subjected to coarse screening using a screen having <NUM> diameter holes to obtain a recycled pulp and a coarse reject. The main reason for the coarse screening was to remove trash and impurities.

The recycled pulp was fractionated by screening to obtain a long fibre fraction and a short fibre fraction (see Example <NUM> below) or by hydrocyclones to obtain a coarse fibre fraction and a fine fibre fraction (see Example <NUM> below). Unfractionated recycled pulp was however used in Example <NUM>.

In many board applications, the bending resistance may be considered the most important parameter. Hence, the effect on bending resistance of including the recycled pulp in a board structure was calculated using the layer grammage and the density and tensile stiffness index (TSI) values in accordance with the method devised by Carlsson and Fellers in a journal article titled <NPL>). This required the conversion of bending stiffness (Sb) values to bending resistance values (having the unit mN), which was done according to the following formula: Bending resistance (mN) = Sb/<NUM> (see Pappersteknik page <NUM>, Gavelin, FoU medd. <NUM>/<NUM>).

In more detail, it was calculated which board grammage that was needed after replacing a portion of the pulp in a reference board with recycled fibres to obtain the same bending resistance as that of the reference board. The pulp replacements that were made are shown below.

In these calculations, a <NUM>/m<NUM> prior art liquid packaging board structure having the composition of table <NUM> was used as a reference. The pulps of this prior art board structure were the following:.

In the reference board structure and in the modified board structures presented belove, the proportion of broke pulp is about <NUM>% to reflect a full-scale process in which a broke pulp stream corresponding to <NUM>% of the total amount of fibres is generated and subsequently accommodated by the full-scale process.

The recycled pulp was fractionated by screening using a screen having <NUM> diameter holes to obtain a long fibre fraction and a short fibre fraction in a <NUM>:<NUM> mass flow ratio. Properties of the fractions are presented in table <NUM> below.

Notably, the long fraction has higher TSI and lower density than the less refined bleached softwood kraft pulp (BSKP low). Further, the TSI of the short fraction is almost as high as that of the more refined bleached softwood kraft pulp (BSKP high), which is striking given that the short fraction has much lower density than the BSKP high.

To obtain a recycled fibre content of about <NUM>%, the kraft pulp and some of the broke pulp of the middle layer of the reference board was replaced with the long fraction. To still accommodate all the broke pulp, some of it was added to the back layer. The resulting board composition is shown in table <NUM> below.

The grammage needed for board structure <NUM> to reach the same bending resistance as the reference board is presented under Results below.

To obtain a recycled fibre content of about <NUM>%, about half of the kraft pulp of the middle layer and most of the softwood kraft pulp of the top layer of the reference board were replaced with the long fraction and the short fraction, respectively. The resulting board composition is shown in table <NUM> below.

The mass ratio between the long fraction in the middle layer and the short fraction in the back layer is the same as the above-mentioned mass flow ratio (i.e..

<NUM>:<NUM>). Accordingly, the composition of board structure <NUM> is such that the board accommodates all fibres of both fractions. The grammage needed for board structure <NUM> to reach the same bending resistance as the reference board is presented under Results below.

The recycled pulp was fractionated by hydrocyclones to obtain a coarse fibre fraction and a fine fibre fraction in a <NUM>:<NUM> mass flow ratio. Properties of the fractions are presented in table <NUM> below.

Notably, the coarse fraction has much higher TSI and much lower density than the less refined bleached softwood kraft pulp (BSKP low). Further, the TSI of the fine fraction is slightly higher than that of the more refined bleached softwood kraft pulp (BSKP high), which is striking given that the fine fraction has much lower density than the BSKP high.

To obtain a recycled fibre content of about <NUM>%, the kraft pulp and some of the broke pulp of the middle layer of the reference board was replaced with the coarse fraction. To still accommodate all the broke pulp, some of it was added to the back layer. The resulting board composition is shown in table <NUM> below.

To obtain a recycled fibre content of <NUM>%, about half of the kraft pulp of the middle layer and most of the softwood kraft pulp of the top layer of the reference board were replaced with the coarse fraction and the fine fraction, respectively. The resulting board composition is shown in table <NUM> below.

The mass ratio between the coarse fraction in the middle layer and the fine fraction in the back layer is the same as the above-mentioned mass flow ratio (i.e. <NUM>:<NUM>). Accordingly, the composition of board structure <NUM> is such that the board accommodates all fibres of both fractions. The grammage needed for board structure <NUM> to reach the same bending resistance as the reference board is presented under Results below.

To obtain a recycled fibre content of <NUM>%, all of the softwood kraft pulp and most of the hardwood kraft pulp of the top layer were replaced with the (unfractionated) recycled pulp. The resulting board composition is shown in table <NUM> below.

The grammage needed for the second reference board structure to reach the same bending resistance as the reference board is presented under Results below.

Claim 1:
A method of producing a paperboard comprising a top layer, a back layer and a middle layer, said method comprising the steps of:
- providing a first fraction of a pulp of recycled bleached fibers, which first fraction has a Schopper-Riegler value measured according to ISO <NUM>-<NUM>:<NUM> of below <NUM>;
- forming a middle layer furnish comprising a chemithermomechanical pulp (CTMP) and the first fraction; and
- forming said middle layer from said middle layer furnish
wherein the first fraction constitutes at least <NUM>% by dry weight of the middle layer furnish and the CTMP constitutes at least <NUM>% by dry weight of the middle layer furnish.