Patent Description:
Conventional paperboard for use in liquid food packaging laminates has two outer layers of cellulose fibres (a print layer and a reverse layer) and at least one middle layer of cellulose fibres. The print layer may be provided with a pigment coating for improved printability and/or a more appealing appearance.

<CIT> discloses an oxygen barrier based on modified cellulose fibres. Various applications of the fibre-based barrier are suggested. One of them is as an oxygen-barrier layer in a multi-layered paperboard product. In this application, the paperboard and the oxygen-barrier layer may be produced independently of each other and then laminated together. No characteristics of the paperboard other than ranges for density and thickness are disclosed.

<CIT> discloses a laminate for packaging of liquid food. The laminate comprises e.g. paperboard and an oxygen barrier. In the laminate, the aluminium foil that is traditionally used as the oxygen barrier in liquid food packaging is replaced with a paper-based oxygen barrier, which preferably has a grammage of <NUM>-<NUM>/m<NUM>. It is further stated that the paperboard of the laminate may be a conventional paperboard of suitable packaging quality that has a grammage of <NUM>-<NUM>/m<NUM> (preferably <NUM>-<NUM>/m<NUM>, more preferably <NUM>-<NUM>/m<NUM>).

Further details about a paper on which a barrier for a liquid food packaging laminate can be based are disclosed in <CIT>, which also suggest to downgauge the cellulose-based bulk or core layer of the laminate in comparison to conventional liquid packaging paperboard. In more detail, it is suggested to replace the traditional paperboard with a spacer layer that has lower or no inherent bending stiffness and that is sandwiched between stiffness-enhancing layers. So-called foamed cellulose (a fibrous material) is given as an example of a suitable spacer layer.

The present inventors have realized that in a packaging laminate including a paper-based barrier that makes a significant stiffness contribution, the paperboard should not be stripped down to merely a bulky core. Rather, one of the traditional outer layers shall remain in the paperboard structure, whereas the other traditional outer layer is omitted or replaced with a layer having a traditional middle layer composition. However, the present inventors have also realized that the removal or replacement of the traditional outer layer may cause problems in the paperboard production (particularly in the drying section) since it exposes a layer having a traditional middle layer composition. An example of such a problem is dusting. Starch, fines, MFC, CMC or PVOH is therefore added to the exposed "middle layer" surface in order to reduce the occurrence of such problems.

The omission of one of the traditional outer layers may allow for an increase of the grammage of a bulkier middle layer or the addition of another bulky layer. Hence, the paperboard of the present disclosure is not necessarily thinner than the conventional paperboard. Instead, it may actually be thicker since high-density pulp in the omitted outer layer has been replaced with low-density pulp in the middle layer(s).

In its naked state, the design of the paperboard of the present disclosure makes no sense, but in a laminate also including a paper-based barrier, it is optimized and provides for increased bending stiffness and/or reduced fibre content/consumption.

Accordingly, the following itemized listing of embodiments of the present disclosure is provided:.

As a first aspect of the present disclosure, there is provided a multi-layered paperboard comprising a first cellulose fibre-based layer ("first layer") and a second cellulose fibre-based layer ("second layer"). The second layer has an inner surface facing in the direction of the first layer and an outer surface facing in the opposite direction. The density of the second layer is lower than the density of the first layer. The second layer is formed from a furnish comprising mechanical pulp.

In the context of the present disclosure, density is preferably measured according to ISO <NUM>:<NUM>.

To separate an individual layer from the multi-layered paperboard and thereby facilitate measurements thereon, a FORTUNA Bandknife-Splitting Machine (Type AB <NUM> E/P) can be used. Such a machine that has been customized for paperboard splitting is commercially available and is used by several major companies in the paperboard field.

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

An additive selected from the group consisting of starch, fines, MFC, CMC and PVOH is added to the outer surface of the second layer. No further cellulose fibre-based layer is provided on the outer surface of the second layer.

The grammage of the paperboard of the first aspect is typically in the range of <NUM>-<NUM>/m<NUM>. In an embodiment, it is in the range of <NUM>-<NUM>/m<NUM>. In another embodiment, it is above <NUM>/m<NUM>, but not higher than <NUM>/m<NUM>.

In the context of the present disclosure, grammage is preferably measured according to ISO <NUM>:<NUM>.

The density of the second layer is below <NUM>/m<NUM>, preferably below <NUM>/m<NUM>, such as below <NUM>/m<NUM>. In contrast, the density of the first layer is typically above <NUM>/m<NUM>, such as above <NUM>/m<NUM>.

In an alternative of complimentary embodiment, the second layer is formed from a second furnish comprising a higher relative amount of mechanical pulp (preferably CTMP) than a first furnish from which the first layer is formed. As an example, the second furnish may comprise at least <NUM>% by dry weight of mechanical pulp (preferably CTMP), whereas the first furnish comprises less than <NUM>%, such as <NUM>%, by dry weight of mechanical pulp.

Independent of the composition of the first layer, the furnish from which the second layer is formed (the "second furnish") preferably comprises at least <NUM>% mechanical pulp (preferably CTMP), such as at least <NUM>% mechanical pulp (preferably CTMP), such as at least <NUM>% mechanical pulp (preferably CTMP). Generally, the density decreases with a higher proportion of CTMP.

In a preferred embodiment, the second furnish broke pulp, such as at least <NUM>% by dry weight broke pulp, such as at least <NUM>% by dry weight broke pulp, such as at least <NUM>% by dry weight broke pulp.

Independent of the composition of the second layer, the furnish from which the first layer is formed (the "first furnish") preferably comprises chemical pulp, such as kraft pulp. In one embodiment, the first furnish comprises at least <NUM>% by dry weight kraft pulp, such as at least <NUM>% by dry weight kraft pulp.

In one embodiment, the first furnish comprises at least <NUM>% by dry weight hardwood pulp, such as at least <NUM>% by dry weight hardwood pulp. In another embodiment, first furnish comprises at least <NUM>% by dry weight softwood pulp, such as at least <NUM>% by dry weight softwood pulp.

The paperboard of the first aspect may further comprise a third cellulose fibre-based layer ("third layer") arranged between the first and the second layer. The density of the third layer is lower than the density of the first layer. Preferably, the density of the third layer is below <NUM>/m<NUM>, such as below <NUM>/m<NUM>, such as below <NUM>/m<NUM>. A typical lower limit may be <NUM>/m<NUM>.

The third layer is preferably formed from a furnish ("third furnish") comprising mechanical pulp, such as chemitermomechanical pulp (CTMP).

The third furnish may comprise at least <NUM>% by dry weight mechanical pulp (preferably CTMP), such as at least <NUM>% by dry weight mechanical pulp (preferably CTMP), such as at least <NUM>% by dry weight mechanical pulp (preferably CTMP).

In one embodiment, the third furnish comprises broke pulp, such as at least <NUM>% by dry weight broke pulp, such as at least <NUM>% by dry weight broke pulp, such as at least <NUM>% by dry weight broke pulp.

In an embodiment, the paperboard of the first aspect also comprises a fourth cellulose fibre-based layer arranged between the first and the third layer. The density of the fourth layer is lower than the density of the first layer. Preferably, the density of the fourth layer is below <NUM>/m<NUM>, such as below <NUM>/m<NUM>, such as below <NUM>/m<NUM>. A typical lower limit may be <NUM>/m<NUM>.

In an embodiment, the inner surface of the second layer is in direct contact with the first layer, which means that neither the third nor the fourth layer is included.

In an embodiment, the third layer is in direct contact with the first and the second layer, which means that the fourth layer is not included.

As explained above, an additive selected from the group consisting of starch, fines, MFC, CMC and PVOH is added to the outer surface of the second layer.

Preferably, the additive is selected from the group consisting of starch, fines, MFC and PVOH.

"Fines" refers to a collection of fibres in which in length-weighted proportion of fibres having a length below <NUM> is above <NUM>%. Such a proportion may be measured according to TAPPI T271, preferably using the equipment kajaaniFS300.

The term "MFC" is known to the skilled person. In the context of the present disclosure, MFC covers highly refined pulp produced according to <CIT> or <CIT>.

In an embodiment, the additive is starch or PVOH.

The amount of the additive is relatively small, <NUM>-<NUM>/m<NUM>, such as <NUM>-<NUM>/m<NUM>, such as <NUM>-<NUM>/m<NUM>. When the additive is fines or MFC, the amount is preferably <NUM>-<NUM>/m<NUM>. When the additive is starch or PVOH, the amount is preferably <NUM>-<NUM>/m<NUM>. Means for applying the additive are discussed below.

As a second aspect of the present disclosure, there is provided a method of forming, in a full-scale paperboard machine, a multi-layered paperboard comprising a first cellulose fibre-based layer ("first layer") and a second cellulose fibre-based layer ("second layer").

A full-scale paperboard machine comprises a forming section, a pressing section and a drying section. The full-scale paperboard machine may further comprise a coating section arranged downstream the drying section. In one embodiment, the full-scale paperboard machine comprises at least on calender arranged downstream the drying section or in a downstream region of the drying section. When the coating section is included, the calender is typically arranged upstream thereof. A full-scale paperboard machine typically produces at least <NUM>,<NUM> tonnes of paperboard per year.

As understood by the skilled person, the first web ultimately becomes the first layer and the second web ultimately becomes the second layer.

In the second aspect, the density of the second layer is lower than the density of the first layer. The density of the second layer is below <NUM>/m<NUM>, such as below <NUM>/m<NUM>, such as below <NUM>/m<NUM>. In contrast, the density of the first layer is preferably above <NUM>/m<NUM>, such as above <NUM>/m<NUM>.

The second furnish preferably comprises a higher relative amount of mechanical pulp (preferably CTMP) than the first furnish. As an example, the second furnish may comprise at least <NUM>% by dry weight of mechanical pulp (preferably CTMP), whereas the first furnish comprises less than <NUM>%, such as <NUM>%, by dry weight of mechanical pulp.

In an embodiment of the second aspect, the multi-layered paperboard further comprises a third cellulose fibre-based layer ("third layer") arranged between the first and the second layer and the method further comprises the step of forming a third fibre web from a third furnish. As understood by the skilled person, the third fibre web ultimately becomes the third layer.

The density of the third layer is preferably lower than the density of the first layer. In one embodiment, the density of the third layer is below <NUM>/m<NUM>, such as below <NUM>/m<NUM>, such as below <NUM>/m<NUM>.

The third furnish preferably comprises a higher relative amount of mechanical pulp (preferably CTMP) than the first furnish. As an example, the third furnish may comprise at least <NUM>% by dry weight of mechanical pulp (preferably CTMP), whereas the first furnish comprises less than <NUM>%, such as <NUM>%, by dry weight of mechanical pulp.

In an embodiment of the second aspect, the multi-layered paperboard further comprises a fourth cellulose fibre-based layer ("fourth layer") arranged between the first and the third layer and the method further comprises the step of forming a fourth fibre web from a fourth furnish. As understood by the skilled person, the fourth fibre web ultimately becomes the fourth layer.

The density of the fourth layer is preferably lower than the density of the first layer. In one embodiment, the density of the third layer is below <NUM>/m<NUM>, such as below <NUM>/m<NUM>, such as below <NUM>/m<NUM>.

The fourth furnish preferably comprises a higher relative amount of mechanical pulp (preferably CTMP) than the first furnish. As an example, the fourth furnish may comprise at least <NUM>% by dry weight of mechanical pulp (preferably CTMP), whereas the first furnish comprises less than <NUM>%, such as <NUM>%, by dry weight of mechanical pulp.

The additive is added in an amount of <NUM>-<NUM>/m<NUM>, such as <NUM>-<NUM>/m<NUM>, such as <NUM>-<NUM>/m<NUM>. When the additive is fines or MFC, the amount is preferably <NUM>-<NUM>/m<NUM>. When the additive is starch or PVOH, the amount is preferably <NUM>-<NUM>/m<NUM>.

In an embodiment, the additive is added to the second fibre web upstream the pressing section of the paperboard machine, e.g. by spraying or by using a headbox, such as a "fines headbox" or a "plybond headbox" (i.e. a headbox traditionally used to apply glue pulp between webs in the forming section of multi-layered paperboard production). In this embodiment, the dry solids content of the second fibre web is typically the range of <NUM>%-<NUM>%, such as <NUM>%-<NUM>%, such as <NUM>%-<NUM>%, when the additive is added. This embodiment is particularly advantageous when the additive is fines or MFC.

In another embodiment, the additive is added in the pressing section or between the pressing section and the drying section, e.g. by spraying or by using a size press. In this embodiment, the dry solids content of the second fibre web is typically the range of <NUM>-<NUM>%, such as <NUM>%-<NUM>%, when the additive is added. This embodiment is particularly applicable when the additive is starch or PVOH.

It is often beneficial to orient the fibres of the paperboard layers in the machine direction (MD) to reduce the cracking tendency of the printing layer in packaging applications. However, such MD orientation may increase the cracking tendency of the reverse layer. In case of the paperboard of the present disclosure, any cracking of the second layer is however less of a concern since this layer is intended to be covered by a barrier paper.

One way of orienting the fibres in the MD is to have a difference between the jet speed and the wire speed. In one embodiment of the second aspect, the jet-wire speed difference when forming the layers is +/- <NUM>-<NUM>/min. For the avoidance of doubt, this means that the speed of the wire is <NUM>-<NUM>/min faster or <NUM>-<NUM>/min slower than the speed of the jet from the headbox.

To improve the adhesion between the layers of the paperboard, in particular when using a multi wire forming section as in <FIG>, starch may be applied between the layers in the forming section before they are merged.

What is described above in connection to the first aspect applies to the second aspect mutatis mutandis.

<FIG> shows an example of a wire section <NUM> of a paper machine that can be used for forming a multi-layered paperboard according to the present disclosure.

The wire section <NUM> was originally designed to produce a conventional paperboard having three fibre-based layers, namely:.

In the production of such a conventional paperboard, the third web is the uppermost web after it has been combined with the first and the second web in the wire section <NUM>. The point on the first wire <NUM> in which the first web is intended to be merged with the second and the third web is herein referred to as the merging point <NUM>.

However, the wire section <NUM> has been adapted to the multi-layered paperboard of the present disclosure by arranging a device <NUM> for applying starch, fines, MFC, CMC or PVOH above the first wire <NUM> downstream the merging point <NUM>.

In case a), the resulting multi-layered paperboard <NUM> contains: a print layer <NUM>; a bulky layer <NUM>; and MFC, fines, CMC, starch or PVOH <NUM>. One side of the print layer <NUM> is intended for printing and may be covered by a pigment coating (not shown). The other side of the print layer <NUM> is merged with one side of the bulky layer <NUM>. The MFC, starch or PVOH <NUM> has been added the other side of the bulky layer <NUM> (see <FIG>). In case a), the grammage of the bulky layer <NUM> is preferably increased compared to the grammage of the grammage of the conventional paperboard.

In case b), the resulting multi-layered paperboard <NUM> contains: a print layer <NUM>; a first bulky layer <NUM>; a second bulky layer <NUM>; and MFC, fines, CMC, starch or PVOH <NUM>. One side of the print layer <NUM> is intended for printing and may be covered by a pigment coating (not shown). The other side of the print layer <NUM> is merged with one side of the second bulky layer <NUM>. The other side of the second bulky layer <NUM> is merged with one side of the first bulky layer <NUM>. The MFC, fines, CMC, starch or PVOH <NUM> has been added to the other side of the first bulky layer <NUM> (see <FIG>).

Case b) may be more preferred since it utilizes the dewatering capacity of the third wire.

In one embodiment, the same CTMP-containing furnish is used for the first bulky layer <NUM> and the second bulky layer <NUM>. In another embodiment, different furnishes are used for the layers <NUM>, <NUM>.

A typical middle layer furnish comprises broke pulp and chemical pulp in addition to CTMP.

The effects of an embodiment of the paperboard according to <FIG> were calculated. In these calculations, the density and tensile stiffness values were set according to table <NUM>. Any impact of the MFC/fines/CMC/starch/PVOH added to the reverse side of the middle layer was disregarded.

A "traditional" paperboard design having a <NUM>/m<NUM> print layer, a <NUM>/m<NUM> middle layer and <NUM>/m<NUM> reverse layer was used as a reference.

In the inventive paperboard, the reverse layer was omitted and the grammage of the middle layer was increased by <NUM>/m<NUM>.

In addition to paperboard, the structures for which the calculations were made contained a barrier paper provided on the reverse side. Any impact of the adhesive used for attaching the barrier paper to the reverse side of the paperboard has been disregarded in the calculations. The density and tensile stiffness index of this barrier paper was set to <NUM>/m<NUM> and <NUM> kNm/g, respectively.

In example <NUM>, it is assumed that the grammage of the print layer of the "traditional" paperboard is optimized to <NUM>/m<NUM> (based on i. printing properties).

In the inventive paperboard (grammage = <NUM>/m<NUM>), the grammage is reduced by <NUM>/m<NUM> when compared to the reference paperboard (grammage = <NUM>/m<NUM>). This <NUM>% reduction of may appear small, but in a full-scale machine producing tens or hundreds of thousands of tonnes of paperboard each year, such a reduction has a significant effect on the economics and environmental impact of the paperboard production.

The results of the calculations are presented in table <NUM> below.

Notably, the inventive structure has a slightly higher bending resistance than the reference structure despite the significantly lower grammage of the inventive paperboard.

The effects of another embodiment of the paperboard according to <FIG> were calculated. In these calculations, the density and tensile stiffness values were set according to table <NUM> above. Again, any impact of the MFC/fines/CMC/starch/PVOH added to the reverse side of the middle layer was disregarded.

Notably, the inventive structure has a significantly higher bending resistance than the reference structure despite that the grammage of the inventive paperboard is lower.

The effects of yet another embodiment of the paperboard according to <FIG> was calculated. In these calculations, the density and tensile stiffness values were set according to table <NUM> above. Again, any impact of the MFC/fines/CMC/starch/PVOH added to the reverse side of the middle layer was disregarded.

In the inventive paperboard (grammage = <NUM>/m<NUM>), the grammage is reduced by <NUM>/m<NUM> when compared to the reference paperboard (grammage = <NUM>/m<NUM>). This <NUM>% reduction has a great effect on the economics and environmental impact of the paperboard production.

Claim 1:
A multi-layered paperboard (<NUM>, <NUM>) comprising a first cellulose fibre-based layer (<NUM>, <NUM>) and a second cellulose fibre-based layer (<NUM>, <NUM>), which second layer (<NUM>, <NUM>) has an inner surface facing in the direction of the first layer (<NUM>, <NUM>) and an outer surface facing in the opposite direction, wherein the density of the second layer (<NUM>, <NUM>) is below <NUM>/m<NUM> and lower than the density of the first layer (<NUM>, <NUM>), wherein said second layer is formed from a furnish comprising mechanical pulp and wherein an additive (<NUM>, <NUM>) selected from the group consisting of starch, fines, MFC, CMC and PVOH is added to the outer surface of the second layer (<NUM>, <NUM>) in an amount of <NUM>-<NUM>/m<NUM>, with the proviso that no further cellulose fibre-based layer is provided on the outer surface of the second layer (<NUM>, <NUM>).