WO2025012745A1

WO2025012745A1 - Multiply paperboard comprising pressure groundwood pulp and its method of manufacturing

Info

Publication number: WO2025012745A1
Application number: PCT/IB2024/056458
Authority: WO
Inventors: Frank Peng; Isto Heiskanen; Anders Moberg
Original assignee: Stora Enso Oyj
Priority date: 2023-07-11
Filing date: 2024-07-02
Publication date: 2025-01-16
Also published as: SE2330321A1; SE546912C2

Abstract

The invention discloses a paperboard comprising at least three plies: a first ply forming a top ply, a second ply forming at least one middle ply and a third ply forming a back ply, wherein the second ply is formed from a middle ply furnish comprising at least 50 % by weight of pressure groundwood (PGW) pulp, 0.1 - 1 % by weight of carboxymethyl cellulose (CMC) and 0.1 - 5 % by weight of cationic starch. The PGW pulp has a Freeness of at least 390 ml and comprises less than 17 % dry weight of a fine fraction that is able to pass through a 200 mesh screen.

Description

MULTIPLY PAPERBOARD COMPRISING PRESSURE GROUNDWOOD PULP AND ITS METHOD OF MANUFACTURING

Technical field

The present disclosure relates to a paperboard comprising high yield pulp and a method of producing such a paperboard.

Background

Paperboard is widely used in the packaging industry for its versatility, durability, and sustainability. The design and manufacture of paperboard involve combining multiple plies of fiber-based material to achieve the desired strength, stiffness, and printability.

Traditionally, paperboard is made by combining layers of wood pulp with different properties, such as chemical pulps, mechanical pulps, and recycled fibers. The resulting paperboard has a range of characteristics, such as brightness, opacity, strength, and surface properties, that are suited for various packaging applications. Folding boxboard, also known as FBB, is a type of multiply paperboard-based packaging material that is widely used in the production of folding cartons for various consumer goods such as food, cosmetics, pharmaceuticals and electronics.

One type of pulp that has gained attention in recent years for its potential use in paperboard and particularly in FBB is pressure groundwood pulp (PGW). PGW is produced by grinding wood logs under high pressure, which breaks down the wood fibers and produces a pulp with high yield and low energy consumption. PGW is attractive to paperboard manufacturers because it is cost-effective and capable of improving opacity and printability.

However, paperboard comprising a high amount of PGW pulp poses challenges in terms of strength, especially when it comes to mechanical strength properties.

Moreover, PGW has a higher variability in fiber quality compared to other pulping methods, which can result in inconsistent paperboard quality. This variability can lead to defects in the paperboard, such as weak spots, streaks, and holes.

To overcome these problems, there is a need for a multiply paperboard that exhibits high bulk while maintaining the required strength properties and consistency.

Description of the invention

It is an object of the present invention to provide a paperboard with a high bulk while maintaining excellent strength properties, particularly high mechanical properties such as internal bond strength.

Another objective of the present invention is to provide a paperboard that comprises high yield pulp while maintaining high strength properties.

Another objective of the present invention is to provide a method for manufacturing paperboard that is energy-efficient and yields consistently high- quality paperboard with exceptional strength properties.

The above-mentioned objects, as well as other objects as will be realized by the skilled person in the light of the present disclosure, are achieved by the various aspects of the present disclosure.

According to a first aspect illustrated herein, there is provided a paperboard comprising at least three plies: a first ply forming a top ply, a second ply forming at least one middle ply and a third ply forming a back ply, wherein the second ply is formed from a middle ply furnish comprising at least 50 %, preferably at least 60 %, or at least 65 %, or at least 70 % or at least 80 %, by weight of pressure groundwood (PGW) pulp, 0.1 - 1 % by weight, preferably 0.1 - 0.6 % by weight, of carboxymethyl cellulose (CMC) and 0.1 - 5 % by weight of cationic starch, all percentages calculated on the total dry weight of said middle ply furnish, wherein said PGW pulp has a Freeness of at least 390 ml as measured according to standard ISO 5267-2 and wherein said PGW pulp comprises less than 17 %, preferably less than 15% or less than 12% or less than 10% dry weight of a fine fraction that is able to pass through a 200 mesh screen as measured according to SCAN-CM 6:05.

Preferably, the said fine fraction is the fraction that is able to pass through a wire sieve cloth mesh no 200 with a wire diameter of 0.053 mm and a sieve opening of 0.074 mm as determined in the standard SCAN-CM 6:05.

In embodiments, the middle ply furnish comprises 50 - 99.8 wt% PGW pulp, or 60 - 99.8 wt% PGW pulp, or 80 - 99.8 wt% PGW pulp. The middle ply may comprise 50 - 98%, preferably 80 - 98 %, by weight of PGW pulp with the said freeness, 0.1 - 0.6 % by weight of CMC and 0.5 - 2 % by weight of cationic starch. The remaining pulp in the middle ply furnish can be or comprise chemical pulp, preferably originating from broke.

A paperboard comprising such high amounts of coarse PGW pulp (freeness at least 390 ml) with such low amount of fines and additives including CMC and cationic starch within the claimed ranges result in low density/high bulk structure. Despite the low density/high bulk, it possesses good mechanical strength properties, especially z-strength and Scott Bond internal bonding strength. The combination of said strength additives and the particular type of PGW pulp quality has been found to result in a paperboard with remarkable strength properties and high bulk. It has surprisingly been found that it is possible to use such a PGW quality and yet achieve high strength. Additionally, the invention offers energy savings in the production process because less energy is required to manufacture the PGW pulp with such high freeness compared to the production of a PGW pulp with lower freeness.

In embodiments, the PGW pulp comprises 0 - 17%, or 1 - 17% or 1 - 15% or 1 - 12 % or 1 - 10 % dry weight of the said fine fraction. The low amount of fines enhances the retention, whereby less retention aids are needed.. Moreover, the PGW pulp used in the invention is easier to wash, whereby fewer washing steps can be employed. In embodiments, the PGW pulp has a bulk of at least 2.9 dm³/kg, preferably at least 2.95 dm³/kg as measured according to ISO 354:2005. The bulk of the pulp is determined on a sheet made from the pulp, which sheet is made in accordance with the standard method ISO 5369-1.

In embodiments, the middle ply furnish further comprises 0.1 - 5 % by weight of highly refined cellulose, as calculated on the total dry weight of said middle ply furnish. The highly refined cellulose works both as a strength additive and as a retention aid for starch, allowing for a lower starch dosing.

The highly refined cellulose has preferably an Schopper-Riegler (SR) value in the range of 80 - 98, more preferably in the range of 85 - 95, as measured according to standard ISO 5267-1. In embodiments, the highly refined cellulose is microfibrillated cellulose (MFC).

Microfibrillated cellulose (MFC) shall in the context of the patent application be understood to mean a cellulose particle, fiber or fibril having a width or diameter of from 20 nm to 1000 nm.

Various methods exist to make MFC, such as single or multiple pass refining, pre-hydrolysis followed by refining or high shear disintegration or liberation of fibrils. One or several pre-treatment steps is usually required in order to make MFC manufacturing both energy efficient and sustainable. The cellulose fibers of the pulp used when producing MFC may thus be native or pre-treated enzymatically or chemically, for example to reduce the quantity of hemicellulose or lignin.

MFC can be produced from wood cellulose fibers, both from hardwood and softwood fibers. It can also be made from microbial sources, agricultural fibers such as wheat straw pulp, bamboo, bagasse, or other non-wood fiber sources. It can be made from pulp, including pulp from virgin fiber, e.g. mechanical, chemical and/or thermomechanical pulps. It can also be made from broke or recycled paper. There are different acronyms for MFC such as cellulose microfibrils, fibrillated cellulose, nanofibrillated cellulose, fibril aggregates, nanoscale cellulose fibrils, cellulose nanofibers, cellulose nanofibrils, cellulose microfibers, cellulose fibrils, microfibrillar cellulose, microfibril aggregates and cellulose microfibril aggregates. MFC can also be characterized by various physical or physical-chemical properties such as its large surface area or its ability to form a gel-like material at low solids (1-5 wt%) when dispersed in water.

The MFC is preferably from bleached chemical pulp, most preferably native bleached chemical pulp.

In embodiments, the PGW pulp is bleached pulp. In this way, the brightness of the paperboard is improved.

Preferably, the PGW pulp has been produced in a pressure groundwood manufacturing process using a grinding stone with a surface coated with grits partly made from diamonds. The utilization of the aforementioned grinding stone in PGW pulp manufacturing has been demonstrated to yield a pulp with a higher freeness that facilitates uniform paperboard quality when employed in conjunction with the strength additives as outlined in this application.

In embodiments, the top ply is formed from a top ply furnish comprising at least 80 wt% of chemical pulp, preferably between 80 - 100 wt% of chemical pulp, as calculated on the total dry weight of said to ply furnish. The back ply of the paperboard is preferably formed from a back ply pulp comprising 80 - 100 wt% chemical pulp. The back ply may, in an alternative embodiment, comprise 0 - 50 wt% of mechanical pulp.

In one embodiment, the top ply furnish comprises at least 80 wt% chemical pulp and 0-1 - 10 wt% highly refined cellulose, preferably MFC, as calculated on the total dry weight of said top ply furnish. MFC in the top ply allows for a higher dosing of pigments to the top ply, whereby the brightness can be enhanced. The addition of MFC to the top ply further improves the elastic modules which, in turn, increases the bending stiffness. In embodiments, the top ply furnish further comprises 1 - 10 wt%, preferably 5 - 10 wt% pigments, as calculated on the total dry weight of the top ply furnish.

In embodiments, the top ply furnish comprises composites comprising MFC and precipitated calcium carbonate (PCC), wherein the PCC is at least partly precipitated onto fibers or fibrils of the MFC. Such composites can be formed by performing carbonation of calcium hydroxide to form precipitated calcium carbonate in the presence of MFC. In this way, even more pigments can be added to the top ply furnish without losing strength properties. The composites may also be produced by co-fibrillation of MFC and PCC.

In embodiments, the paperboard further comprises microfibrillated cellulose applied between the first ply and the second ply and/or between the second ply and the third ply. Such microfibrillated cellulose may be added in an amount to form a layer of 0.1 - 5 gsm, preferably 1 - 2 gsm on the surface of the ply. A composition comprising MFC may be applied by curtain coating, which results in an even distribution of the MFC. The MFC composition may also be applied as a central layer using a multilayer headbox.

The inventive paperboard may be surface sized with surface sizing compositions, which may include starch, modified starch, polyvinyl alcohol (PVOH), or acrylic co-polymer. These surface sizing compositions improve the surface properties of the paperboard, such as its smoothness, strength, and printability.

In addition, the paperboard may be coated with or laminated with barrier coatings or layers, which provide further protection against moisture, oxygen, and other gases. These barrier coatings or layers may include dispersion- coated barrier layers or extrusion or laminated polymer layers, such as thermoplastic polymers comprising polyolefins. These coatings or layers are selected for their ability to provide a barrier to gases and liquids while maintaining the strength and durability of the paperboard. The paperboard may further comprise laminated cellulose-based barrier films or papers, which can optionally be metallized to provide even greater barrier properties. These laminated films or papers are also selected for their ability to provide a barrier to gases and liquids while maintaining the overall strength and durability of the paperboard.

According to a second aspect illustrated herein, there is provided a method for manufacturing a paperboard comprising the steps of:

- forming a first ply from a top ply furnish comprising 80 - 100 wt% chemical pulp,

- providing a first furnish comprising at least 50 wt%, preferably at least 60, or at least 65 wt% or at least 70 wt% pressure groundwood (PGW) pulp with a freeness as measured according to standard ISO 5267-2 of at least 390 ml, which PGW pulp comprises less than 17 % dry weight of a fine fraction that is able to pass through a 200 mesh screen as measured according to SCAN-CM 6:05,

- adding carboxymethyl cellulose (CMC) and cationic starch to said first furnish, thereby forming a middle ply furnish, wherein said CMC is added in an amount of 0.1 - 1 % by weight and said cationic starch is added in an amount of 0.1 -6% by weight, as calculated on the total dry weight of the middle ply furnish,

- forming at least one second ply from said middle ply furnish,

- forming a third ply from a back ply furnish comprising 80 - 100 wt% chemical pulp, wherein the first ply forms a top ply, the third ply forms a back ply and the second ply forms a middle ply arranged in-between said top ply and back ply in the thereof formed paperboard.

The CMC is preferably added to the first furnish in an amount of 0.1 - 0.6 wt%, and the cationic starch is preferably added in an amount of 0.5 - 2 wt%, as calculated on the dry weight of the middle ply furnish.

The first furnish preferably comprises 50 - 100 wt%, preferably 80 - 100 wt% pressure groundwood (PGW) pulp with a freeness and a fine fraction content as claimed. The remaining pulp in the furnish is preferably chemical pulp, most preferably originating from broke.

The amount of fines in the PGW pulp used in the method of the invention can be regulated in the PGW manufacturing process. In embodiments, the PGW pulp is manufactured in a process including a step of fractionating the pulp to provide the pulp with a fine fraction in an amount of less than the said percentage. In an alternative embodiment, the PGW pulp is manufactured in a process that does not include a step of fractionating. In this embodiment, the PGW pulp is refined in a process that is adapted to provide the said freeness and fine fraction content.

In embodiments, the method further comprises adding highly refined cellulose to the first furnish, wherein the middle ply is formed from the middle ply furnish comprising said added highly refined cellulose, cationic starch and CMC. The highly refined cellulose is preferably MFC and can be defined by the same characteristics as the highly refined cellulose included in the paperboard of the first aspect.

The starch is preferably added to the first furnish before the addition of highly refined cellulose and CMC. In this way, any remaining starch is absorbed by the highly refined cellulose.

In embodiments, the highly refined cellulose and the CMC are pre-mixed before addition to the first furnish. In this way, flocculation of the highly refined cellulose is prevented, since the anionic CMC works as a dispersion agent for the highly refined cellulose. The cationic starch is preferably added separately from the pre-mixture in order to avoid that the starch and the highly refined cellulose form flocculates.

In embodiments, the step of adding said CMC, cationic starch and said optional highly refined cellulose to the first furnish is performed by using an injection mixer. The injection mixer preferably injects the chemicals to the furnish by using an injection liquid for forcing a small amount of furnish to penetrate into the furnish flow. The injection liquid may be the same furnish to which the chemical is to be mixed. Such an injection liquid may be taken upstream of the mixer to be injected by means of the mixer together with the chemical to the furnish flow. Preferably, this is made by using TrumpJet system (available from Wetend Technologies Ltd.). Such an injection mixer ensures an efficient mixing of the additives in the furnish, whereby the amount of additives can be reduced while the beneficial effects still can be achieved.

In embodiments, the step of providing a middle ply furnish comprising 50 - 100 wt% pressure groundwood (PGW) pulp comprises the step of manufacturing PGW pulp in a pressure groundwood process using a grinding stone with a surface coated with grits partly made from diamonds. As outlined above, a PGW pulp produced in this way provides a consistent quality of the thereof formed paperboard. In addition, this process of producing PGW pulp is very energy- and cost-efficient and sharpening of the grinding stone is not needed as often as when using conventional grinding stones. The method further enables the use of a single PGW production line, as compared to conventional methods which oftentimes need several lines to ensure stable quality.

In embodiments, the pressure groundwood process is controlled by measuring the freeness of the produced pulp online and regulating the grinding process based on the measured freeness value. The freeness is preferably measured in accordance with the standard ISO 5267-2. The grinding process can be measured by regulating e.g., the grinding temperature, the grinding energy and/or the grinding stone rotation speed. In preferred embodiments, the groundwood process is controlled by means of a control system comprising a self-learning artificial intelligence unit.

In embodiments, the method further comprises the step of measuring at least one quality parameter on the produced board and regulating the dosing of the highly refined cellulose, the cationic starch and the optional anionic polymer based on said measured quality parameter. The quality parameter is preferably bulk and/or one or several strength properties including e.g. z- strength, bending stiffness and/or scott bond. The step of measuring the quality parameter and regulating the dosing may preferably be controlled by means of a self-learning artificial intelligence unit.

In embodiments, at least two of the first, second and third plies are formed by use of a multi-layer headbox and wherein a central layer, comprising at least 80 wt% highly refined cellulose calculated on the dry weight of said central layer, is formed in-between at least two of said first, second and third plies using the multi-layer headbox.

The multi-layer headbox preferably includes separate guide devices, such as guiding tubes, for guiding the furnishes and a composition forming the central layer to a nozzle, from which the furnishes and the central layer composition emerge via a gap onto a forming wire. The central layer composition comprises at least 80 wt% highly refined cellulose, such as between 80 - 100 wt% or 90 - 100 wt% or 100 wt% highly refined cellulose as calculated on the total solid content on the central layer composition. The central layer composition is preferably an aqueous suspension of highly refined cellulose and optional additives and has preferably a solid content of 10 - 50 wt%. In embodiments, the highly refined cellulose is microfibri Hated cellulose (MFC).

Methods and standards

The following methods and standards apply both to the definitions of the appended claims and to the measurements performed in the example below.

Freeness of the pulps refer to Canadian Standard Freeness (CSF) and is measured according to ISO 5267-2 (original funnel design).

Fines fraction content is measured by determining the dry weight percentage that is able to pass through a 200 mesh screen (wire sieve cloth mesh no 200) as measured according to SCAN-CM 60:05. In the below example also referred to as McNett < 200.

Bulk is measured in accordance with ISO 534:2005. The bulk of pulp is determined on a sheet made from the pulp, which sheet is made in accordance with the standard method ISO 5369-1. Tensile Index, Tensile energy absorption index and Tensile stiffness index are measured in accordance with IS01924-3.

Burst index is measured in accordance with ISO 2758.

Schopper-Riegler is measured in accordance with ISO 5267-1.

Scott Bond is measured in accordance with TAPPI UM-403.

Z-strength is measured in accordance with SCAN-P 80:98 Density is measured in accordance with ISO 534:2005. SOT is measured in accordance with ISO 9895.

Grammage is measured in accordance with ISO 536:2019.

Example

A test series was performed in which middle plies of 150 gsm comprising PGW with a freeness of 390 ml and a fines fraction content (McNett <200 (%)) of 16.6, and CMC, cationic starch and optionally MFC in different amounts were compared to a conventional middle ply comprising PGW with a freeness of 330 ml.

The properties of the PGW pulps used in the example are shown in Table 1.

Table 1.

All tests were performed according to the methods and standards as outlined above and all analysis were carried out according to available standards after conditioning at 23 °C, 50%RH unless otherwise indicated. The properties of the pulps as presented in table 1 are measured on the pulp as such, or on a sheet made by the pulp in accordance with the method ISO 5369-1 , as applicable.

Paperboard samples 1 - 9 were produced using a Dynamic Sheet Former (Formette Dynamique). The cationic starch used in said samples is Perlbond 930 and the CMC is Finnfix 30. Retention aids were added to the samples 1 - 9 and include 0.5 kg/ton of CPAM and 1.5 kg/ton of Bentonite. All additives are calculated based on 100 % dry weight of the furnish. The compositions and the properties of the paperboard samples 1-9 are shown in table 2.

Table 2

As can be seen in table 2, the strength properties of the samples made from PGW pulp 390 ml (sample 7 and 8) are comparable with or superior to the reference sample (sample 1).

In view of the above detailed description of the present invention, other modifications and variations will become apparent to those skilled in the art. However, it should be apparent that such other modifications and variations may be affected without departing from the spirit and scope of the invention.

Claims

1. A paperboard comprising at least three plies: a first ply forming a top ply, a second ply forming at least one middle ply and a third ply forming a back ply, wherein the second ply is formed from a middle ply furnish comprising at least 50 % by weight of pressure groundwood (PGW) pulp, 0.1 -1% by weight of carboxymethyl cellulose CMC and 0.1 - 6 % by weight of cationic starch, all percentages calculated on the total dry weight of said middle ply furnish, wherein said PGW pulp has a Freeness of at least 390 ml as measured according to standard ISO 5267-2 and wherein said PGW pulp comprises less than 17 % dry weight of a fine fraction that is able to pass through a 200 mesh screen as measured according to SCAN-CM 6:05.

2. The paperboard according to claim 1 , wherein the middle ply furnish comprises at least 60 %, preferably at least 65 %, by weight of pressure groundwood (PGW) pulp.

3. The paperboard according to anyone of the claims 1 - 2, wherein the PGW pulp has a bulk of at least 2.9 dm³/kg, preferably at least 2.95 dm³/kg as measured according to ISO 354:2005.

4. The paperboard according to any one of the preceding claims, wherein the middle ply furnish further comprises 0.1 - 5 % by weight of highly refined cellulose, as calculated on the total dry weight of said middle ply furnish.

5. The paperboard according to claim 4, wherein the highly refined cellulose has a Schopper-Riegler (SR) value in the range of 80 - 98, preferably in the range of 85 - 95, as measured according to standard ISO 5267-1.

6. The paperboard of any one of the claims 4 and 5, wherein the highly refined cellulose is microfibrillated cellulose (MFC).

7. The paperboard according to any one of the preceding claims, wherein the PGW pulp has been produced in a pressure groundwood manufacturing process using a grinding stone with a surface coated with grits partly made from diamonds.

8. The paperboard according to anyone of the preceding claims, wherein the top ply is formed from a top ply furnish comprising at least 80 wt% of chemical pulp, as calculated on the total dry weight of said top ply furnish.

9. The paperboard according to anyone of the preceding claims, which paperboard further comprises microfibrillated cellulose applied between the first ply and the second ply and/or between the second ply and the third ply.

10. A method for manufacturing a paperboard comprising the steps of:

- providing a first furnish comprising at least 50 wt% pressure groundwood (PGW) pulp with a freeness of at least 390 ml, as measured according to standard ISO 5267-2, which PGW pulp comprises less than 17 % dry weight of a fine fraction that is able to pass through a 200 mesh screen as measured according to SCAN-CM 6:05,

- adding carboxymethyl cellulose (CMC) and cationic starch to said first furnish thereby forming a middle ply furnish, wherein said CMC is added in an amount of 0.1 - 1 % by weight and said cationic starch is added in an amount of 0.1 - 6 % by weight, as calculated on the total dry weight of the middle ply furnish,

- forming at least one second ply from said middle ply furnish, - forming a third ply from a back ply furnish comprising 80 - 100 wt% chemical pulp, wherein the first ply forms a top ply, the third ply forms a back ply and the second ply forms a middle ply arranged in-between said top ply and back ply in the thereof formed paperboard.

11. The method according to claim 10, wherein the PGW pulp is manufactured in a process including a step of fractionating the pulp to provide the pulp with a fine fraction content of less than 17%.

12. The method according to any one of claims 10 or 11 , wherein the PGW pulp has a bulk of at least 2.9 dm³/kg, preferably at least 2.95 dm³/kg as measured according to ISO 354:2005.

13. The method according to any one of claims 10 - 12, further comprising adding highly refined cellulose to the first furnish, wherein the middle ply is formed from the middle ply furnish comprising said added highly refined cellulose, cationic starch and CMC.

14. The method according to claim 13, wherein the step of providing a first furnish comprising at least 50 wt% pressure groundwood (PGW) pulp comprises the step of manufacturing PGW pulp in a pressure groundwood process using a grinding stone with a surface coated with grits partly made from diamonds.

15. The method according to any one of the claims 10 - 14, wherein at least two of the first, second and third plies are formed by use of a multi-layer headbox and wherein a central layer, comprising at least 80 wt% highly refined cellulose calculated on the dry weight of said central layer, is formed in-between at least two of said first, second and third plies using the multi-layer headbox.