EP3265609B1

EP3265609B1 - Pulp mixture for production of a paperboard product with high strength in z-direction

Info

Publication number: EP3265609B1
Application number: EP16710843.0A
Authority: EP
Inventors: Anna TUBEK LINDBLOM
Original assignee: Billerudkorsnas AB
Current assignee: Billerudkorsnas AB
Priority date: 2015-03-02
Filing date: 2016-02-25
Publication date: 2020-09-23
Anticipated expiration: 2036-02-25
Also published as: WO2016140609A1; SE1550239A1; SE539344C2; SE539344E; EP3265609A1

Description

TECHNICAL FIELD

The present invention relates to a pulp mixture for board production in order to provide high strength in Z-direction, i.e. out of plane.

BACKGROUND

Paperboard comprises a plurality of layers, also known as plies, of pulp and optional additives. The layers are purposively selected and arranged to achieve the desired properties of the paperboard as such. An essential property of the paperboard is the bending stiffness. The bending stiffness in paperboard is usually built up by having outer plies with high tensile stiffness and one or several bulky plies in between, so that the outer plies are placed at a desired distance from each other. The bulky ply/plies is/are often a middle layer/middle layers.
The middle layer in paperboard may comprise by a mechanical pulp, such as thermomechanical pulp TMP. However in recent years, the use of chemithermomechanical pulp, CTMP, has increased drastically. TMP and CTMP generally have a high bulk thereby enabling constructing paperboard with the desired high stiffness at low grammage, compared to for example chemical pulps. In the CTMP process, wood chips are impregnated with a lignin softening chemical prior to pressurized refining. This results in softening of lignin and the fiber rupture during refining will therefore be concentrated to the lignin rich middle lamella. This results in higher amounts of long fibers and lower amount of fines and shives at a certain energy input compared to TMP. A high concentration of long fibers is important for all products where high bulk is desired. Therefore, CTMP is more advantageous than TMP in paperboard.
The strength of paper is measured in three dimensions: the grain direction, also known as the X-direction; the cross-grain direction, also known as Y-direction; and the direction perpendicular to the paper surface plane, also known as the Z-direction. The force needed to delaminate a sample of a paper is recorded as its internal bond strength, or Z-directional tensile strength.
A high Z-strength in the middle layer of paperboard is desired in order to avoid delamination of the middle layer and hence delamination of the paperboard as such. Such a Z-strength must however be achieved without deteriorating the bending stiffness, that is without having to increase the paper web density.
Z-strength and density of a paperboard layer is usually optimized by altering the raw materials, by choosing different operation conditions in stock preparation and on the board machine and by addition of paper chemicals.
Like many other strength properties, strength in Z-direction increases with increased density and the effect comes from increase of bonded area between the fibers. The relationship between density and out-of-plane strength may vary depending on pulp type and densification method. Refining increases strength more than wet pressing.
The main purpose of refining is to improve the bonding properties of the fibers. Changes that improve fiber-to-fiber bonding are internal and external fibrillation together with fines creation. All three changes result in an increase of the water-holding capacity of the pulp, its density and strength properties such as tensile strength and stiffness, burst and compression strength, and also strength in Z-direction. Other effects that occur in industrial refining are changes in fiber curl. In LC refining, the fibers of bleached pulp usually become straighter while unbleached kraft fibers that are straight as unrefined might become more curlated during refining. With increased energy input in HC refining of chemical pulp, the decrease in shape factor counteracts the development of density, tensile stiffness and surface smoothness. LC refining as a post-stage after HC refining straightens pulp fibers, increases density and pulp properties related to tensile strength and improves surface properties. However, it is not previously elucidated how the HC refining and LC treatment of HC refined pulp affects out-of-plane properties.
WO 95/34711 A1 discloses a CTMP pulp for use in the manufacture of paper or paperboard products. The pulp is produced by impregnating chips with a lignin softening chemical, preheating the chips, and refining the chips to papermaking pulp. This results in a high temperature chemithermomechnical pulp (HTCTMP). The resulting pulp from the process is disclosed to have a sufficiently good Scott Bond value without high percentages of fine material.
Klinga et al., Energy efficient high quality CTMP for paperboard, Journal of Pulp and Paper Science, 34 (2008), 2, 98-106, discusses the relationship between bulk and internal bond strength in paper sheets and their dependency on fiber length. Furthermore, this article suggests manufacturing energy efficient high quality CTMP for paperboard. More specifically, Klinga et al. conclude that LC refining of spruce HTCTMP yields high quality pulp at low total energy input and thus is an interesting process concept for production of pulps intended for paperboard.
While CTMP provides a high bulk, the Z-strength of CTMP is comparatively low.
EP 1835075 discloses a method for forming at least one ply of a paperboard from a slurry of cellulose fibers comprising crosslinked fibers. The internal bond strength of the paperboard is improved by the addition of additives. Furthermore, it is shown that the addition of mechanically refined fiber increases the strength of the paperboard and increases the Scott Bond.
It is not only paperboard which requires high delamination resistance and bending stiffness. These properties are important in for example printing, in converting and in end-use situations. This means that producing paper and board with high strength in Z-direction at a given density is very important for many paper products.
Asikainen et al., Birch pulp fractions for fine paper and board, Nord. Pulp Paper Res. J. 2010, vol. 25(3), pp. 269-276, describe the use of birch pulp fractions for fine paper and board. It is shown that the Scott Bond of a board middle layer can be increased by mixing a fine fraction of mill birch kraft pulp, obtained by hydrocyclone and pressure screen fractionation, and mill CTMP.
Sjöberg and Höglund, High consistency refining of kraft pulp for reinforcing paper based on TMP furnishes, International Mechanical Pulping Conference 2007, TAPPI Press, vol. 2, pp. 943-953) discloses HC-refined kraft pulp (30%) mixed with TMP-pulp (70%) for base paper production of light weight coated (LWC) paper. The highest reinforcement effect is stated to be achieved for a kraft pulp that was HC refined and subsequently LC refined. Sjöberg and Höglund do however not discuss the strength in Z-direction.
WO99/02772 discloses a method for making kraft paper wherein a sulphate pulp is subjected to HC refining in combination with LC beating and addition of a strengthening agent. The kraft paper is intended to be used as sack paper.
Sack paper is often produced by subjecting a chemical pulp to HC refining followed by LC refining as this gives the excellent strength properties needed for such an application. However, such pulps are generally not used in other types of paper products as the energy needed for refining often is quite high.

SUMMARY

The object of the present invention is to be able to provide a paperboard product with a high Z-strength at the same time as enabling a low grammage.
The object is achieved by means of a method in accordance with independent claim 1. Embodiments are defined by the dependent claims.
The invention is focused on providing the desired properties in terms of Z-strength, bending stiffness and bulk to a layer, such as a middle layer, of paperboard.
It has surprisingly been found that a high Z-strength of a paperboard layer can be achieved at a low density, and thus a low grammage, by means of the pulp mixture in accordance with the present invention. This is due to the pulp mixture comprising a first pulp constituting a chemical softwood pulp, which has been subjected to high consistency refining followed by low consistency refining, and a second pulp constituting a bulk increasing pulp. By means of the pulp mixture according to the present invention it is possible to tailor the properties with regard to Z-strength, density, and bending stiffness as desired of a paperboard product obtained by means of said pulp mixture. Thus, by means of the pulp mixture it is for example possible to produce a paper web with high strength in Z-direction and high bulk.
High consistency refining of a chemical pulp results in higher Z-strength properties compared to low consistency refining for the same density. Thus, HC refining produces pulps with lower density at the same Z-tensile strength. However, HC refining does not result in sufficient tensile stiffness and surface smoothness. To straighten the HC refined fibers, the fibers are therefore subjected to subsequent LC refining. In contrast to what may be expected, it has been found that low consistency refining of a high consistency refined pulp (the resulting pulp hereinafter also denominated HC/LC refined pulp) does not reduce the Z-strength. If fact, the subsequent LC refining results in retained or even increased Z-strength. Thus, it has been found that HC/LC refined pulps may replace LC refined pulps in paperboard products to improve Z-strength. Furthermore, with maintained Z-strength compared to LC refined pulps, more CTMP can be included in a paperboard layer formed from a pulp mixture based on HC/LC refined pulp, which enables reduced grammage and thus reduced fibre consumption.
The excellent properties in Z-direction of HC/LC refined pulp extend the possibility to optimise paperboard density by combining such a pulp with bulk increasing pulps. Test results have shown that bleached and unbleached HC and LC refined chemical pulps can be mixed with for example LC refined pulps and/or CTMP. The results show that improved Z-strength versus density relation of the HC/LC pulp mixtures in comparison with LC refined pulps. That implies wider operating window for optimisation of paperboard properties in cross-direction (out-of-plane) and possible raw material savings for board producers.
Calculations for a model paper and board shows that, with preserved bending stiffness, grammage of paper made from never dried bleached or unbleached chemical softwood pulp can be decreased by approximately 1.2 % for each 10 units in density reduction by the pulp mixture according to the present invention. That implies for a middle size mill the raw material saving of the magnitude of several millions Euro per year.
The pulp mixture according to the present invention comprises a first pulp and a second pulp. Additional pulps or additives may be present if desired, but the pulp mixture may likewise exclusively consist of said first pulp and said second pulp. An example of an additional pulp is pulped broke, which is a pulp prepared by the defibration of broke from a board-making process. Pulped broke is often included in the pulp mixture forming a layer, such as a middle layer, of paperboard, When the pulp mixture comprises pulped broke, the pulped broke is normally present in an amount of 5-40 %, such as 10-35 % of the total dry weight of the pulp mixture. Additives (e.g. strength agents). are discussed below. The first pulp and the second pulp are preferably each never dries pulps. The first pulp is a chemical softwood pulp which has been subjected to HC refining followed by LC refining. The second pulp is a bulk increasing pulp selected from the group consisting of a chemi-thermomechanical pulp (CTMP), a thermomechanical pulp (TMP) or a chemical softwood pulp refined solely by low consistency refining.
To further improve the Z-direction properties of a paperboard comprising a plurality of layers, the pulp mixture may comprise at least one strength agent. The at least one strength agent can for example be selected from carboxymethyl cellulose ("CMC"), starch, microfibrillated cellulose ("MFC", see e.g. WO 2008/076056 ), polyacrylamide and polyvinylamine ("PVAm"), In addition to the at least one strength agent, the pulp may comprise a inorganic microparticle, such as silica or [other examples].
Preferred combinations of strengths chemicals comprise:

cationic starch + CMC (see e.g. WO 2006/041401 ); and
cationic starch + MFC (see e.g. WO 2011/ 068457 );

The first pulp is present in an amount of 20-90 %, such as 25-90 %, such as 30-90 %, such as 50-90 % by dry weight of the total dry weight of the first pulp and the second pulp in case the second pulp is CTMP or TMP.
When the amount of CTMP is relatively high, such as more than 50 % of the total dry weight of the first pulp and the second pulp, it is particularly beneficial to add the at least one strength agent.
The first pulp is present in an amount of 5-90 %, such as 10-70 %, such as 40-70 %, by dry weight of the total dry weight of the first pulp and the second pulp in case the second pulp is a chemical softwood pulp refined solely by low consistency refining.
According to one embodiment, the second pulp is CTMP and the first pulp is present in an amount of 60-90 %, preferably 75-88 %, by dry weight of the total dry weight of the first pulp and the second pulp. By means of such a pulp mixture it has for example been found to be possible to obtain a layer or paper having a density of less than 700 kg/m³ and a Z-tensile strength of more than 650 kPa (both in case of the first pulp being a bleached pulp and in the case of the first pulp being an unbleached pulp).
In one specific embodiment, the pulp mixture comprises:

a) the first pulp in an amount of 15-40 %, such as 20-35 %, by dry weight;
b) CTMP as the second pulp in an amount of 30-50 %, such as 40-50 %, by dry weight; and
c) pulped broke in an amount of 5-40 %, such as 10-35 %, by dry weight,

According to another embodiment, the second pulp is a chemical softwood pulp refined solely by low consistency refining and the first pulp is present in an amount of 8-85 %, preferably 10-60 %, by dry weight of the total dry weight of the first pulp and the second pulp. By means of such a pulp mixture it has for example been found to be possible to obtain a layer or paper having a density of less than 750 kg/m³ and a Z-tensile strength of at least 550 kPa, typically of at least 600 kPa. It has also been found to be possible to obtain a layer or paper of said pulp mixture having a density of less than 780 kg/m³ and a Z-tensile strength of at least 650 kPa at least for a bleached pulp mixture.
In one specific embodiment, the pulp mixture comprises:

a) the first pulp in a mount of 25-55 %, such as 35-50 %, by dry weigh;
b) chemical softwood pulp (refined solely by LC refining) as the second pulp in an amount of 10-40 %, such as 15-35 %, by dry weigh; and
c) pulped broke in an amount of 20-45 %, such as 25-40 %, by dry weigh,

The first pulp may suitably be a bleached chemical softwood pulp which has been subjected to high consistency refining followed by low consistency refining with a total refining energy of the high consistency refining and the low consistency refining of 70-250 kWh/t, preferably 130-220 kWh/t.
The first pulp may alternatively suitably be an unbleached chemical softwood pulp which has been subjected to high consistency refining followed by low consistency refining with a total refining energy of the high consistency refining and the low consistency refining of 150-400 kWh/t, preferably 200-360 kWh/t.
Low consistency refining of the high consistency refined chemical softwood pulp to obtain the first pulp may suitably be performed at a quite moderate refining energy, such as 15-40 kWh/t, preferably 15-30 kWh/t.
The first pulp may suitably have a Schopper Rielger (SR) number according to SO 5267-1:1999 of above 15.0, preferably 15.2-18.8, and/or a water retention value (WRV) according to SCAN-C 62:00 of above 1.55 g/g, preferably 1.58-1.90 g/g. In case the first pulp is an unbleached pulp, the WRV may optionally be equal to or above 1.80 g/g, whereas in the case of the first pulp is a bleached pulp, the WRV may optionally be less than 1.80 g/g.
The pulp mixture may suitably have a Schopper Rielger (SR) number according to SO 5267-1:1999 of 14.0-19.0, and/or a water retention value (WRV) according to SCAN-C 62:00 of 1.45-1.85 g/g.
The pulp mixture may comprise additional components, in particular one or more fillers, to thereby reduce the amount of fibrous fiber raw material at preserved strength in Z-direction.
The pulp mixture may be produced by firstly providing the first pulp and providing the second pulp. The first pulp is provided by subjecting a chemical softwood pulp to a high consistency refining step and a subsequent low consistency refining step. The first pulp is thereafter mixed with the second pulp and any optional additional component(s), such as the at least one strength agent and/or the inorganic microparticle, thereby providing the pulp mixture.
As understood from the discussion above, the preparation of the pulp mixture may also comprise mixing the first and/or the second pulp with pulped broke. At least part of the pulped broke is preferably obtained by pulping broke obtained from a board-making process in which the pulp mixture is used for a layer, such as a middle layer, of the board. The fibre composition of the pulped broke thus depend on the types of fibres that are used for the different layers of the board product.
The first pulp may suitably be provided by subjecting a bleached chemical softwood pulp to high consistency refining followed by low consistency refining, and wherein the total refining energy used during the high consistency refining and the low consistency refining thereof is 70-250 kWh/t, preferably 130-220 kWh/t.
The first pulp may alternatively suitably be provided by subjecting an unbleached chemical softwood pulp to high consistency refining followed by low consistency refining, and wherein the total refining energy used during the high consistency refining and the low consistency refining thereof is 150-400 kWh/t, preferably 200-360 kWh/t.
When providing the first pulp, the refining energy during the low consistency refining of the chemical softwood pulp subjected to high consistency refining followed by low consistency refining may suitably be quite moderate, such as 15-40 kWh/t, preferably 15-30 kWh/t.
In the case where the second pulp is a chemical softwood pulp refined solely by low consistency refining, the second pulp may suitably be provided by subjecting a chemical softwood pulp to a low consistency refining step at a refining energy of 20-60 kWh/t, preferably 25-55 kWh/t. Preferably, the refining energy during said low consistency refining step may be 20-40 kWh/t, more preferably 25-35 kWh/t, in case the chemical softwood pulp is a bleached chemical softwood pulp, or 35-60 kWh/t, more preferably 40-55 kWh/t, in case the chemical softwood pulp is an unbleached chemical softwood pulp.
By means of the pulp mixture and the method for producing said pulp mixture it is possible to tailor the properties of a paperboard layer, such as a middle layer, especially with respect to Z-strength and density.
In one embodiment, the chemical softwood pulp refined solely by low consistency refining and the first pulp are provided concurrently by co-refining at a low consistency an unrefined chemical softwood pulp and a chemical softwood pulp refined by high consistency refining.
The present invention relates to a method of producing a multi-layered board, such as paperboard, having a middle layer. The middle layer is formed from the pulp mixture as disclosed above, or formed from a pulp mixture produced as disclosed above.

BREIF DESCRIPTION OF DRAWINGS

Figure 1 illustrates experimental results of Z-tensile strength and Scott Bond in relation to density for HC- and LC-refined bleached pulp.
Figure 2 illustrates experimental results of Z-tensile strength and Scott Bond in relation to density for HC-and LC-refined unbleached pulp.
Figure 3 illustrates experimental results of density and Z-tensile strength in relation to refining energy for HC and HC/LC refined bleached pulp.
Figure 4 illustrates experimental results of density and Z-tensile strength in relation to refining energy for HC and HC/LC refined unbleached pulp.
Figure 5 illustrates experimental results of the relation between Z-tensile strength and density for mixtures of HC/LC pulps with low LC refined samples for bleached and unbleached pulp in comparison with LC refined pulps.
Figure 6 illustrates experimental results of the relation between density and Z-tensile strength for mixtures of bleached HC/LC pulp with low LC refined pulp in comparison with LC refined pulps.
Figure 7 illustrates experimental results of the density and strength in Z-direction for a mixture of 80% bleached HC/LC pulp with 20 % CTMP in comparison with LC refined bleached pulp.
Figure 8 illustrates experimental results of the density and strength in Z-direction for a mixture of 85% unbleached HC/LC pulp with 15 % CTMP in comparison with LC refined unbleached pulp.

DETAILED DESCRIPTION

Low consistency refining in the present disclosure is intended to mean refining at a consistency of about 2 % to 8 %, such as 2 % to 5 %, unless explicitly disclosed otherwise. High consistency refining is intended to mean refining at a consistency of about 25 % to 38 %, such as 28 % to 38 %, unless explicitly disclosed otherwise.
Low consistency refined pulp (or LC refined pulp) is herein intended to mean a pulp which has not been subjected to any preceding high consistency refining or medium consistency refining, but solely to low consistency refining, unless explicitly disclosed otherwise.
High consistency refined pulp (or HC refined pulp) is herein intended to mean a pulp which has not been subjected to any other type of refining, but solely to high consistency refining, unless explicitly disclosed otherwise.
HC/LC refined pulp is considered to mean a pulp which has been subjected to a high consistency refining followed by a low consistency refining.
It is well known to the skilled person that any of the refining steps high consistency refining and low consistency refining may be executed in a plurality of sub-steps. Such a procedure is intended to be encompassed by the scope of the invention, as well as a procedure wherein each refining step is performed in a single step.
In the present disclosure, percent by weight is considered to mean percent by weight of never dried pulps unless explicitly disclosed otherwise.
The invention is further illustrated below by means of the following examples, which do not limit the invention in any respect. The invention may be varied within the scope of the appended claims.
In accordance with the present invention, a pulp mixture comprising a first pulp and a second pulp is provided. The first pulp consists of a chemical softwood pulp which has been subjected to high consistency refining followed by low consistency refining (i.e. a HC/LC pulp). The second pulp is a bulk increasing bulk, meaning that it constitutes a pulp which compared to the first pulp increases the bulk, i.e. reduces the density, of the pulp mixture when added.
The first pulp is a chemical softwood pulp, inter alia in order to ensure that the cellulose fibers have an appropriate length. High consistency refining of said pulp is performed by conventional methods. Likewise, low consistency refining of said high consistency refined pulp is performed by conventional methods. Such a HC/LC refined softwood pulp is as such previously known for the purpose of making sack paper. It has however not been previously considered for purposes such as board.
In accordance with one embodiment, the second pulp may suitably be a chemi-thermomechanical pulp, CTMP. In the case of the second pulp being CTMP, the first pulp is present in an amount of 20-90 %, such as 25-90 %, such as 30-90 %, such as 50-90% by dry weight of the total dry weight if the first pulp and the second pulp. CTMP may be present in an amount of at least 60 % by dry weight of the total dry weight of the first and the second pulp, such as at least 75 % by dry weight of the total dry weight of the first and the second pulp. A too high addition of CTMP would reduce the Z-strength to such a degree that other pulps or pulp mixtures would be a better option from a cost perspective. Therefore, an upper limit for the amount of CTMP may be 50 % or 60 % by dry weight based on the total dry weight of the pulp mixture. Furthermore, in the case of the second pulp being CTMP, the first pulp is present in an amount of up to and including 90 % by dry weight, preferably up to and including 88 % by dry weight. A too low addition of CTMP would not result in the desired degree of bulk increase. The CTMP is suitably a softwood CTMP pulp but is not limited thereto.
It is also contemplated that the second pulp may be a thermomechanical pulp (TMP) as such pulp would also likely increase the bulk of a paper web obtained by such pulp mixture. Sjöberg and Höglund proposed a HC and LC refined Kraft pulp (30%) mixed with TMP-pulp (70%) for base paper production of light weight coated (LWC) paper, but does not discuss paperboard or Z-strength properties. In accordance with the present invention, in order to achieve the desired Z-strength, the first pulp should be present in an amount of 50-90 % by dry weight of the total dry weight of the first and the second pulp in case the second pulp is TMP. The embodiment wherein the second pulp is TMP is however less preferred than CTMP since TMP results in a lower amount of long fibers and a higher amount of shives at the same energy input compared to CTMP.
In accordance with an alternative embodiment, the second pulp is a chemical softwood pulp which has been subjected to a low consistency refining without preceding high consistency refining, i.e. a LC refined pulp. In such a case, the first pulp may preferably be present in an amount of at least 8 by dry weight of the total dry weight of the first pulp and the second pulp, more preferably at least 10% by dry weight of the total dry weight of the first pulp and the second pulp. Furthermore, the first pulp may is such a case preferably be present in an amount of up to and including 85 % by dry weight of the total dry weight of the first and the second pulp, more preferably up to and including 60 % or 70 % by dry weight of the total dry weight of the first and second pulp.
Irrespective of the nature of the second pulp, the pulp mixture may comprise one or more further components, more specifically fillers. The filler selected is not limiting the scope of the present invention and any previously known filler for the paper products concerned may be used. Examples of such fillers are calcium carbonate (PCC and GCC) and clay.
The pulp mixture according to the present invention may also exclusively consist of the first and the second pulp without any further additives or pulps. However, the pulp mixture normally comprises additives and/or at least one other pulp. Such as pulped broke.
As previously mentioned, high consistency refining and low consistency refining may be performed in accordance with conventional routes. Furthermore, for the embodiment of CTMP, CTMP may be produced in accordance with any previously known techniques without departing from the scope of the invention. Moreover, mixing of the first and the second pulp may be performed in accordance with any previously known technique.
A paperboard product may be produced by conventional means by the pulp mixture according to the present invention and thus includes production of a paper web of the pulp mixture.

Abbreviations

CSF: Canadian standard freeness
CTMP: chemithermomechanical pulp
HC: high consistency
LC: low consistency
SR: Schopper-Riegler number
WRV: water retention value

Experimental methods

All percentages referring to the amount of respective pulp in the pulp mixtures given in the Experimental tests below are given based on dry weight.
All studies described below were performed at Innventia and the evaluation of the trials was made in laboratory scale. All chemical pulps used were industrially produced and several sets of pulp samples from Iggesund mill and Gävle mill were delivered for experimental work during the project time. Never dried bleached and unbleached pulps were used as unrefined, LC-refined and HC-refined. Furthermore, industrially produced CTMP from Frövi was included in the study.
All the HC-refining trials of bleached and unbleached pulps were performed in Gävle mill. The refiner used for bleached pulp was a Sprout-Bauer 50-1B refiner equipped with refining fillings with an edge length of 182 km/rev. HC-refining of unbleached pulp was performed in a Sunds Defibrator RGP 254 refiner with refining fillings with an edge length of 105 km/rev. The pulp consistency during HC refining of both pulps was kept at 30 % to 35 %.
Bleached and unbleached LC refining curves were performed at Innventia in a Beloit 24" double disc refiner at a flow rate of 500 I/min. Refining segments used has an edge length of 8.38 km/rev and the refiner operated at 750 rpm. Refining consistency was 3.3% for both bleached and unbleached pulp. Refining was done as a single-stage refining and the energy input varied up to 145 kWh/t (six stages at 10, 30, 55, 80, 115 and 145 kWh/t) for bleached pulp and up to 140 kWh/t for unbleached pulp (six stages at 10, 30, 50, 80, 110 and 140 kWh/t). Individual mill LC-refined pulp samples were also delivered during the project time.
LC-refining of HC-refined pulp was performed in a Voith laboratory conical refiner (conical fillings 3-1, 0-60) at energy input of 20 kWh/t for all samples. The pulp consistency during refining was kept on the level of approximately 4 % and in all cases; the refiner speed was 1500 rpm.
All pulp samples were analysed with respect to SR, WRV-whole pulp and fiber dimensions using a L&W FiberTester Analyser. Standard laboratory sheets with a grammage of 60 g/m2 were tested with respect to density, tensile strength, Z-strength, Scott Bond and surface roughness.

Fibre and pulps were characterized in accordance to the following standards

SR number	SO 5267-1:1999
WRV	SCAN-C 62:00
Preparation of laboratory sheets	ISO 5269-1:2005
Test of laboratory sheets	ISO 5270:1999
Structural density	SCAN-P 88:01
Tensile properties	ISO1924-3:2005
Surface roughness Bendtsen 0.1 MPa	ISO 8791-2
Scott-Bond	T 833 pm-94
Z-tensile strength	ISO 15754

Experimental results 1

Experimental tests were performed in order to investigate how refining energy affects relationship between density and strength in Z-direction of HC-refined pulps compared to LC-refined pulps. The strength in Z-direction was measured both as Z-tensile strength and Scott Bond.
Figure 1 illustrates Z-tensile strength and Scott Bond in relation to density for HC- and LC-refined bleached pulp. HC-refining was done up to approximately 200 kWh/t and LC-refining up to 145 kWh/t. It should be noted that the Scott Bond was not determined for LC refining at 145 kWh/t (which corresponds to the density of about 840 kg/m³) because it was out of the measurable range and is therefore not shown in Figure 1.
Figure 2 illustrates Z-tensile strength and Scott Bond in relation to density for HC- and LC-refined unbleached pulp. HC-refining was done up to 350 kWh/t and LC-refining up to 140 kWh/t.
As can be seen in Figures 1 and 2, Z-tensile strength and Scott Bond follows density increase through the whole process for both refining techniques. The results show that at the same density HC-refining generates pulps with higher out-of-plane properties than LC-refining. The difference in Z-direction strength between LC- and HC-refining increases with increased density, i.e. with increased refining energy. Furthermore, at the same strength, the difference in density between the two refining processes increased with increased density.
To summarise, from the test results it can be concluded that at the same density, HC-refining generates pulp with higher Z-strength, measured as Z-tensile strength and Scott Bond, compared to LC-refined pulp. At the same Z-strength, HC-refining produced pulp with lower density compared to LC-refining.

Experimental results 2

HC-refining produces pulps with high strength in Z-direction but with a poorer tensile stiffness and lower surface smoothness than LC-refining. It is common general knowledge that LC-refining as a post-stage after HC-refining straightens the fibers, increases density and pulp properties related to tensile strength in-plane and improves surface properties. However, it is not previously known how the LC refining affects the out-of-plane properties of HC-refined pulp. Therefore, experimental tests of LC refining of HC-refined pulp was performed.
In order to limit pulp density increase, the HC-refined pulps were treated in a Voith laboratory refiner at low energy input of 20 kWh/t. After this stage, the pulp samples are noted HC/LC pulp. The effects of LC refining on density and Z-tensile strength in relation to refining energy are shown in Figure 3 for bleached pulp and in Figure 4 for unbleached pulp. The points shown in the Figures relating to HC/LC pulp correspond to the total refining energy of HC refining and LC refining.
As expected, LC refining of HC refined samples increased density and it was noted that this increase was greater for the HC samples refined at lower energy levels compared to HC samples refined at higher energy levels. The results show that Z-strength was principally retained or increased by the subsequent LC-refining, increased for HC samples refined at lower energy levels whereas retained for HC samples refined at higher energy levels.
The results show a high strength in Z-direction of HC/LC refined pulps and thus reveal the possibility of partly or fully replacing LC-refined pulps with HC/LC refined pulps in paper products to improve the Z-strength of such a paper product. High values of Z-strength of HC/LC refined pulps extend the possibility to optimize board density by combining this pulp with bulk increasing pulps.

Experimental results 3

In this experimental test, bleached and unbleached HC/LC refined pulps were mixed with low LC refined samples (not previously subjected to HC refining). No additional additives or pulps were added.

In this set of mixing trials only HC/LC samples refined at the high energy levels were mixed with low LC-pilot refined pulps. The total energy input was 195 kWh/t for HC/LC bleached pulp and 370 kWh/t for HC/LC unbleached pulp. The refining energy in LC-refining was 30 kWh/t and 50 kWh/t for bleached and unbleached pulp, respectively. The properties of the pulps used in the mixtures are given in Table 1.

Table 1

Pulp and total refining energy	SR / WRV, g/g	Density, kg/m³	Scott Bond, J/m²	Z-tensile strength, kPa
Bleached HC/LC195	15,9/1,69	755	510	965
Bleached LC30	14,5/1,55	690	210	450
Unbleached HC/LC370	15,4/1,85	705	500	940
Unbleached LC50	14,7/1,73	615	180	410

For bleached pulps, the amount of HC/LC-refined pulp in the mixtures was varied at five levels: 10%, 20%, 30%, 40% and 50% by weight, respectively. The SR for the bleached pulp mixtures ranged from 14.8 to 16.4, the highest value corresponding to the mixture comprising 50 % HC/LC-refined pulp. WRV was about 1.6 g/g for all of the bleached pulp mixtures.
For unbleached pulps, the amount of HC/LC-refined pulp was varied at 20%, 40% and 60% by weight, respectively. The SR for the mixtures ranged from 14.1 to 14.9, the highest value corresponding to the lowest amount of HC/LC refined pulp in the mixtures. WRV was about 1.7-1.8 g/g for the unbleached pulp mixtures.
Figure 5 shows the obtained relationships between Z-tensile strength and density for bleached pulp mixtures and for unbleached pulp mixtures in comparison with the LC-refined pulps.
It should be noted that the points plotted for the low LC-refined pulps in Figure 5 is a result of different refining energies which in turn results in different densities, whereas the points plotted for the HC/LC + LC mixtures corresponds to the different amounts of HC/LC pulp in the pulp mixtures and that the refining energy for the different pulps of the mixtures as given in Table 1 was used for said mixtures.
The results show that at the same density, Z-tensile strength was much higher for the HC/LC containing pulp mixtures compared to the LC-refined pulp, i.e. at the same Z-tensile strength, the density of HC/LC containing pulp mixtures was lower than for the LC-refined pulp. The relation between Z-strength and density slightly improved with increased content of HC/LC pulp in the mixture. The highest difference in density reached approximately 50-60 units.

Experimental results 4

In this experimental test, bleached HC/LC refined pulps were mixed with low LC refined samples (not previously subjected to HC refining). However, in this experimental test, mixing trials with low LC refined pulp, mill refining at 30 kWh/t, and HC/LC samples refined at different energy levels, 70 kWh/t, 145 kWh/t and 210 kWh/t, were used. The properties of the pulps used in the mixtures are given in Table 2.

Table 2

Pulp and total refining energy	SR / WRV, g/g	Density, kg/m³	Scott Bond, J/m²	Z-tensile strength, kPa
Bleached LC30	13,7 / 1,45	690	215	500
Bleached HC/LC70	15,3/1,58	770	360	740
Bleached HC/LC145	17,9/1,66	795	480	875
Bleached HC/LC215	18,7/1,69	785	560	930

The amount of HC/LC refined pulps in the mixtures was varied at 80 % and 60 % by weight, respectively. Figure 6 shows the relationship between Z-tensile strength and density for pulp mixtures in comparison with the LC refined pulp. It can be seen that the effect decreases with decreased HC/LC refining energy and that lower amount of HC/LC pulp in the mixtures gave more favourable relation than higher amount of HC/LC, although the difference is quite small.
From the Experimental results 3 and 4, where the mixture HC/LC + LC was tested, it can be concluded that the mixtures of HC/LC+LC pulps can be used instead of LC refined pulp in different paperboard products for three different purposes, namely

At the maintained density, Z-strength can be increased,
At the maintained strength in Z-direction, density can be decreased and thus bending stiffness increases,
At the maintained bending stiffness and at the same Z-strength, the grammage can be reduced.

Experimental results 5

Experimental test were performed with HC/LC samples mixed with bulk-giving CTMP pulp. CTMP pulp is presently often used in the middle layer of paperboard. The purpose of this test was to find out if the mixture could be used instead of LC refined pulp in production of board in order to decrease density with remaining Z-strength and preserved bending stiffness and thereby creating the possibility to decrease grammage.

In the test, bleached HC/LC refined pulp with a total energy input of 220 kWh/t was mixed with 20 % by weight of CTMP with CSF of 475 ml. Said mixture had SR 18.9 and WRV 1.5 g/g. The properties of the mixture were compared with the properties of LC mill pulp refined at 35 kWh/t. Table 3 present the properties of pulps used in the blends.

Table 3

Pulp (and total refining energy)	SR / WRV, g/g	Density, kg/m³	Scott Bond, J/m²	Z-tensile strength, kPa
Bleached HC/LC220	18,7/1,63	770	520	980
Bleached LC35	15,7/1,6	735	320	690
CTMP		475	90	160

The results are shown in Figure 7 illustrating the density and strength in Z-direction measured as Z-tensile strength and Scott Bond for the mixture of HC/LC pulp with 20 % CTMP in comparison with LC refined bleached pulp. The decrease in Z-strength which was expected by the addition of CTMP was balanced by the high Z-strength of HC/LC refined pulp. HC/LC pulp mixture with CTMP showed similar Z-strength measured as Scott Bond and Z-tensile strength at lower density that LC refined pulp.

Experimental results 6

Unbleached HC/LC refined pulp with total energy input of 360 kWh/t was mixed with 15 % by weight CTMP of the same quality as given above in Table 3. Said mixture had SR 17.9 and WRV 1.7 g/g. The properties of the mixture were compared to LC mill refined pulp at 50 kWh/t. The properties of the unbleached HC/LC and LC pulps used are given in Table 4. Table 4

Pulp (and total refining energy) SR /WRV, g/g Density, kg/m³ Scott Bond, J/m² Z-tensile strength, kPa

Unbleached HC/LC360 16,9/1,83 740 505 990

Unbleached LC50 14,5/1,65 670 275 650
The results are shown in Figure 8 illustrating the density and strength in Z-direction measured as Z-tensile strength and Scott Bond for the mixture of 85 % HC/LC with 15 % CTMP in comparison with LC refined unbleached pulp. Despite the same density, the mixtures of HC/LC refined pulp and CTMP showed much higher Z-strength measured as Scott Bond and Z-tensile strength than LC refined pulp. Scott Bond was improved by 25 % and Z-tensile strength was increased by 12 %. As seen from the results, the portion of CTMP can be greater with aim to further reduce density when keeping Z-strength at the same level as that of LC refined pulp.
From the Experimental results 5 and 6, where the mixture HC/LC + CTMP was tested, it can be concluded that the mixtures of HC/LC+CTMP pulps can be used in board production for three different purposes, namely

At the maintained density, Z-strength of paper can be increased,
At the maintained strength in Z-direction, density can be decreased and thus bending stiffness increased,
At the maintained bending stiffness and at the same Z-strength, the grammage can be reduced.

Full-scale trial

A three-layer board was produced in full-scale trials at the Gävle mill.
The pulp mixture for the top layer comprised bleached softwood chemical pulp (40 %) and bleached hardwood chemical pulp (60 %), The pulp mixture for the middle layer comprised unbleached softwood chemical pulp (about 75-80 %) and pulped broke (about 20-25 %). The bottom layer comprised unbleached chemical pulp and a small amount of pulped broke.
In reference trials, the unbleached softwood chemical pulp of the middle layer pulp was subjected to LC refining only. In the inventive trials, part of the unbleached softwood chemical pulp of the middle layer was subjected to HC refining before the LC refining was carried out. In the inventive trials, the HC refined pulp was thus co-refined with non-HC refined pulp at low consistency. Details of the preparations of middle layer pulps are presented in table 5 below.
In all trials, the board was coated with about 20 g/m² (dry) of a coating composition comprising calcium carbonate, clay and latex. Table 5

Trial Share of HC* (%) LC refining (kWh/t)

Reference 1 0 65

Inventive 1 10 65

Inventive 2 15 65

Inventive 3 15 55

Reference 2 0 55

* The percentage by dry weight of unbleached softwood chemical pulp subjected to HC refining and LC refining in the middle layer pulp, based on the total dry weight of the middle layer pulp.

Samples (A4-sized) obtained from the middle of the board produced in each trial were tested. The results of the testing is present in table 6 below.

Table 6

Trial	Structure density* (kg/m³)	Bending resistance** (mN)	Scott bond (J/m²)	Z tensile strength (kPa)	Surface roughness*** (ml/min)	Surface roughness PPS (µm)
Reference 1	720	150	235	285	175	2.3
Inventive 1	720	180	255	295	180	2.4
Inventive 2	720	170	275	315	170	2.5
Inventive 3	720	180	250	300	185	2.5
Reference 2	725	160	225	295	165	2.4

* Of the coated whole board
** Geometrical
*** Bendtsen

In table 6, it is shown that the Z tensile strength of a three layer board could be increased by as much as 10.5 % without increasing the density by subjecting a relatively small part of the chemical pulp of the middle layer pulp mixture to HC refining. The increase in Scott bond was even higher. Further, the HC refining increased the bending resistance by at least 12.5 %.
In contrast to what might have been expected, the surface roughness of the top layer was not significantly increased by the HC refining.

Claims

Method for producing a multi-layered board having a middle layer, wherein the middle layer is formed from a pulp mixture comprising a first pulp and a second pulp, the first pulp consisting of a chemical softwood pulp which has been subjected to high consistency refining followed by low consistency refining, the second pulp consisting of a bulk increasing pulp selected from the group consisting of a chemi-thermomechanical pulp (CTMP), a thermomechanical pulp (TMP) and a chemical softwood pulp which has been subjected to low consistency refining without preceding high consistency refining, and wherein:
in the case the second pulp is a chemi-thermomechanical pulp or a thermomechanical pulp, the first pulp is present in an amount of 20-90 %, such as 25-90 %, such as 50-90 % by dry weight of the total dry weight of the first pulp and the second pulp;

in the case the second pulp is a chemical softwood pulp which has been subjected to a low consistency refining without preceding high consistency refining, the first pulp is present in an amount of 5-90 % by dry weight of the total dry weight of the first pulp and the second pulp.
Method according to claim 1, wherein the second pulp is a chemi-thermomechanical pulp (CTMP) and wherein the first pulp is present in an amount of 60-90 % by dry weight of the total dry weight of the first pulp and the second pulp, preferably 75-88 % by dry weight of the total dry weight of the first pulp and the second pulp.
Method according to claim 1, wherein the second pulp is a chemical softwood pulp which has been subjected to low consistency refining without preceding high consistency refining, and the first pulp is present in an amount of 8-85 % by dry weight of the total dry weight of the first pulp and the second pulp, preferably 10-60 % by dry weight of the total dry weight of the first pulp and the second pulp.
Method according to any of the preceding claims, wherein the first pulp is a bleached chemical softwood pulp which has been subjected to high consistency refining followed by low consistency refining with a total refining energy of the high consistency refining and the low consistency refining of 70-250 kWh/t, preferably 130-220 kWh/t.
Method according to any of claims 1 to 3, wherein the first pulp is an unbleached chemical softwood pulp which has been subjected to high consistency refining followed by low consistency refining with a total refining energy of the high consistency refining and the low consistency refining of 150-400 kWh/t, preferably 200-360 kWh/t.
Method according to any of the preceding claims, wherein the first pulp is a bleached chemical softwood pulp which has been subjected to high consistency refining followed by low consistency refining, said low consistency refining performed with a refining energy of 15-40 kWh/t.
Method according to any of the preceding claims, wherein the first pulp has a SR number of above 15.0, preferably 15.2-18.8, and/or a WRV of above 1.55 g/g, preferably 1.58-1.90 g/g.
Method according to any of the preceding claims, wherein the pulp mixture has a SR number of 14.0-19.0 and/or a WRV of 1.45-1.85 g/g.
Method according to any of the preceding claims, wherein the pulp mixture further comprises pulped broke, preferably in an amount of 5-40 %, such as 10-35 % of the total dry weight of the pulp mixture.
Method according to any of the preceding claims, wherein the pulp mixture further comprises at least one strength agent, optionally selected from carboxymethyl cellulose (CMC), starch, microfibrillated cellulose (MFC), polyacrylamide and polyvinylamine (PVAm).
Method according to any of the preceding claims comprising subjecting a chemical softwood pulp to a high consistency refining step and a subsequent low consistency refining step thereby obtaining said first pulp, mixing said first pulp with at least said second pulp such as to achieve said pulp mixture.
Method according to claim 11, wherein the chemical softwood pulp is a bleached chemical softwood pulp and wherein the total refining energy used during the high consistency refining and the low consistency refining thereof so as to provide said first pulp is 70-250 kWh/t, preferably 130-220 kWh/t.
Method according to claim 11, wherein the chemical softwood pulp is an unbleached chemical softwood pulp and wherein the total refining energy used during the high consistency refining and the low consistency refining thereof so as to provide said first pulp is 150-400 kWh/t, preferably 200-360 kWh/t.
Method according to any of claims 11 to 13, wherein the refining energy during the low consistency refining of the chemical softwood pulp subjected to high consistency refining followed by low consistency refining is 15-40 kWh/t.
Method according to any of claims 11 to 13, wherein the second pulp is provided by subjecting a chemical softwood pulp to a low consistency refining step, without any preceding high consistency refining step, said low consistency refining step performed at a refining energy of 20-60 kWh/t, preferably 25-55 kWh/t.