SE1550239A1 - Pulp mixture for production of a paper product with high strength in z-direction - Google Patents

Abstract

23 ABSTRACT A pulp mixture comprising a first pulp and a second pulp is disclosed. The first pulp is a chemicalsoftwood pulp which has been subjected to HC and LC refining. The second pulp may be a CTMP,TMP or a LC refined chemical softwood pulp. By means of the pulp mixture it is possible to produce a paper web with high strength in Z-direction and high bulk.

Description

PULP MIXTURE FOR PRODUCTION OF A PAPER PRODUCT WITH HIGH STRENGTH IN Z-DIRECTION TECHNICAL FIELD The present invention relates to a pulp mixture for paper or board production in order to providehigh strength in Z-direction, i.e. out of plane. lt also relates to a method for producing such a pulp and to products obtained by means of said mixed pulp.

BACKGROUND Paperboard comprises a plurality of layers, also known as plies, of pulp and optiona| additives. Thelayers are purposively selected and arranged to achieve the desired properties of the paperboard assuch. An essential property of the paperboard is the bending stiffness. The bending stiffness inpaperboard is usually built up by having outer plies with high tensile stiffness and one or severalbulky plies in between, so that the outer plies are placed at a desired distance from each other. The bulky plies are often referred to as middle layer or middle layers.

The middle layer in paperboard may be made by mechanical pulps, such as thermomechanical pulpTMP. However in recent years, the use of chemithermomechanical pulp, CTMP, has increaseddrastically. TMP and CTMP generally have a high bulk thereby enabling constructing paperboard withthe desired high stiffness at low grammage, compared to for example chemical pulps. ln the CTMPprocess, 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 concentratedto the lignin rich middle lamella. This results in higher amounts of long fibers and lower amount offines and shives at a certain energy input compared to TMP. A high concentration of long fibers isimportant 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 thepaper 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 paper is usually optimized by altering the raw materials, by choosingdifferent operation conditions in stock preparation and on the paper/board machine and by addition of paper chemicals.

Like many other strength properties, paper strength in Z-direction increases with increased densityand the effect comes from increase of bonded area between the fibers. The relationship betweendensity 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 thatimprove fiber-to-fiber bonding are internal and external fibrillation together with fines creation. Allthree changes result in an increase of the water-holding capacity of the pulp, its density and strengthproperties such as tensile strength and stiffness, burst and compression strength, and also strengthin Z-direction. Other effects that occur in industrial refining are changes in fiber curl. ln LC refining,the fibers of bleached pulp usually become straighter while unbleached kraft fibers that are straightas unrefined might become more curlated during refining. With increased energy input in HC refiningof chemical pulp, the decrease in shape factor counteracts the development of density, tensilestiffness 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 temperaturechemithermomechnical 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 CTMPfor paperboard, Journal of Pulp and Paper Science, 34(2008), 2, 98-106, discusses the relationship between bulk and internal bond strength in paper sheetsand 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 pu|ps 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 cellulosefibers comprising crosslinked fibers. The internal bond strength of the paperboard is improved by theaddition of additives. Furthermore, it is shown that the addition of mechanically refined fiber increases the strength of the paperboard and increases the Scott Bond. lt is not only paperboard which requires high delamination resistance and bending stiffness. Theseproperties are important in for example printing, in converting and in end-use situations. This meansthat 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 fractionsforfine 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. lt is shown thatthe 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 TMPfurnishes, 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 lightweight coated (LWC) paper. The highest reinforcement effect is stated to be achieved for a kraft pulpthat 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 HCrefining 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 asthis gives the excellent strength properties needed for such an application. However, such pu|ps aregenerally 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 paper product with a high Z-strength at the same time as enabling a low grammage.

The object is achieved by means of a pulp mixture in accordance with independent c|aim 1 and amethod in accordance with independent c|aim 10. Embodiments are defined by the dependent claims.

The invention is mainly focused on providing the desired properties in terms of Z-strength, bendingstiffness and bulk to a middle layer of paperboard. However, the pulp mixture according to theinvention may also be used in production of other paper products where Z-strength is important, especially fine paper. lt has surprisingly been found that a high Z-strength of a paper product can be achieved at a lowdensity, and thus a low grammage, by means of the pulp mixture in accordance with the presentinvention. This is due to the pulp mixture comprising a first pulp constituting a chemical softwoodpulp, which has been subjected to high consistency refining followed by low consistency refining, anda second pulp constituting a bulk increasing pulp. By means of the pulp mixture according to thepresent invention it is possible to tailor the properties with regard to Z-strength, density, andbending stiffness as desired of a paper product obtained by means of said pulp mixture. Thus, bymeans 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 lowconsistency refining for the same density. Thus, HC refining produces pulps with lower density at thesame Z-tensile strength. However, HC refining does not result in sufficient tensile stiffness andsurface smoothness. To straighten the HC refined fibers, the fibers are therefore subjected tosubsequent LC refining. ln contrast to what may be expected, it has been found that low consistencyrefining of a high consistency refined pulp (the resulting pulp hereinafter also denominated HC/LCrefined pulp) does not reduce the Z-strength. lf fact, the subsequent LC refining results in retained oreven increased Z-strength. Thus, it has been found that HC/LC refined pulps may replace LC refined pulps in paper products to improve Z-strength. Furthermore, with maintained Z-strength compared to LC refined pulps, more filler could be added to a paper produced by HC/LC refined pulp to thereby enable to reduce the grammage and thus reduce the amount of fibrous fibre raw material.

The excellent properties in Z-direction of HC/LC refined pulp extend the possibility to optimise paperdensity by combining such a pulp with bulk increasing pulps. Test results have shown that bleachedand unbleached HC and LC refined chemical pulps can be mixed with for example LC refined pulpsand/or CTMP. The results show that improved Z-strength versus density relation of the HC/LC pulpmixtures in comparison with LC refined pulps. That implies wider operating window for optimisationof paper properties in cross-direction (out-of-plane) and possible raw material savings for paper and board producers.

Calculations for a model paper and board shows that, with preserved bending stiffness, grammage ofpaper made from never dried bleached or unbleached chemical softwood pulp can be decreased byapproximately 1.2 % for each 10 units in density reduction by the pulp mixture according to thepresent 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 exclusivelyconsist of said first pulp and said second pulp. The first pulp and the second pulp are preferably eachnever dries pulps. The first pulp is a chemical softwood pulp which has been subjected to HC refiningfollowed by LC refining. The second pulp is a bulk increasing pulp selected from the group consistingof a chemi-thermomechanical pulp (CTMP), a thermomechanical pulp (TMP) or a chemical softwood pulp refined solely by low consistency refining.

The first pulp is present in an amount of 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.

The first pulp is present in an amount of 5-90 % by dry weight of the total dry weight of the first pulpand 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 of60-90 %, 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).

According to another embodiment, the second pulp is a chemical softwood pulp refined solely by lowconsistency refining and the first pulp is present in an amount of 8-85 %, preferably 10-60 %, by dryweight of the total dry weight of the first pulp and the second pulp. By means of such a pulp mixtureit has for example been found to be possible to obtain a layer or paper having a density of less than750 kg/m3 and a Z-tensile strength of at least 550 kPa, typically of at least 600 kPa. lt has also beenfound 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.

The first pulp may suitably be a bleached chemical softwood pulp which has been subjected to highconsistency 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 beensubjected to high consistency refining followed by low consistency refining with a total refiningenergy of the high consistency refining and the low consistency refining of 150-400 kWh/t, preferably200-360 kWh/t.

Low consistency refining of the high consistency refined chemical softwood pulp to obtain the firstpulp may suitably be performed at a quite moderate refining energy, such as 15-40 kWh/t, preferably15-30 kWh/t.

The first pulp may suitably have a Schopper Rielger (SR) number according to SO 5267-1:1999 ofabove 15.0, preferably 15.2-18.8, and/or a water retention value (WRV) according to SCAN-C 62:00of above 1.55 g/g, preferably 1.58-1.90 g/g. ln case the first pulp is an unbleached pulp, the WRVmay 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 of14.0-19.0, 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 stepand a subsequent low consistency refining step. The first pulp is thereafter mixed with the second pulp and any optional additional component, such as a filler, thereby providing the pulp mixture.

The first pulp may suitably be provided by subjecting a bleached chemical softwood pulp to highconsistency refining followed by low consistency refining, and wherein the total refining energy usedduring 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 softwoodpulp to high consistency refining followed by low consistency refining, and wherein the total refiningenergy 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 chemicalsoftwood 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. ln the case where the second pulp is a chemical softwood pulp refined solely by low consistencyrefining, the second pulp may suitably be provided by subjecting a chemical softwood pulp to a lowconsistency refining step at a refining energy of 20-60 kWh/t, preferably 25-55 kWh/t. Preferably, therefining energy during said low consistency refining step may be 20-40 kWh/t, more preferably 25-35kWh/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 tailorthe properties of a paper product, especially with respect to Z-strength and density. Thus, it ispossible to obtain a paper product having a density of less than 750 kg/m3 and a Z-tensile strength ofat least 550 kPa. The paper product may for example, but not limited to, be a layer of a paperboard (suitably a middle layer) or a fine paper.

The present invention also relates to a paperboard comprising a layer made of the pulp mixturedescribed above and optionally at least one additional layer, suitably at least two additional layers.Said layer made of the pulp mixture according to the present invention is suitably a middle layer of the paperboard and interposed between two outer layers.

The present invention also relates to a method of producing a multi-layered board, such aspaperboard, 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.

The present invention also relates to fine paper, such as printing and writing paper grades, obtained by the pulp mixture described 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 formixtures 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 ofabout 2 % to 8 % unless explicitly disclosed otherwise. High consistency refining is intended to mean refining at a consistency of about 25 % to 38 % unless explicitly disclosed otherwise.

Low consistency refined pulp (or LC refined pulp) is herein intended to mean a pulp which has notbeen 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 notbeen 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. lt is well known to the skilled person that any of the refining steps high consistency refining and lowconsistency refining may be executed in a plurality of sub-steps. Such a procedure is intended to beencompassed by the scope of the invention, as well as a procedure wherein each refining step is performed in a single step. ln 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. ln accordance with the present invention, a pulp mixture comprising a first pulp and a second pulp isprovided. The first pulp consists of a chemical softwood pulp which has been subjected to highconsistency refining followed by low consistency refining (i.e. a HC/LC pulp). The second pulp is abulk 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 conventionalmethods. Such a HC/LC refined softwood pulp is as such previously known for the purpose of making sack paper. lt has however not been previously considered for purposes such as board or fine paper. ln accordance with one embodiment, the second pulp may suitably be a chemi-thermomechanicalpulp, CTMP. ln the case of the second pulp being CTMP, the first pulp is present in an amount of 50-90% by weight of the total weight if the first pulp and the second pulp. Suitably, CTMP is present inan amount of at least 60 % by weight of the total weight of the first and the second pulp, preferablyat least 75 % by weight of the total weight of the first and the second pulp. A too high addition ofCTMP would reduce the Z-strength to such a degree that other pulps or pulp mixtures would be abetter option from a cost perspective. Furthermore, in the case of the second pulp being CTMP, thefirst pulp is present in an amount of up to and including 90 % by weight, preferably up to andincluding 88 % by 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. lt is also contemplated that the second pulp may be a thermomechanical pulp (TMP) as such pulpwould also likely increase the bulk of a paper web obtained by such pulp mixture. Sjöberg andHöglund proposed a HC and LC refined Kraft pulp (30%) mixed with TMP-pulp (70%) for base paperproduction of light weight coated (LWC) paper, but does not discuss Z-strength properties. lnaccordance with the present invention, in order to achieve the desired Z-strength, the first pulpshould be present in an amount of 50-90 % by weight of the total weight of the first and the secondpulp in case the second pulp is TMP. The embodiment wherein the second pulp is TMP is howeverless 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. ln 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 11 refined pulp. ln such a case, the first pulp may preferably be present in an amount of at least 8 byweight of the total weight of the first pulp and the second pulp, more preferably at least 10% byweight of the total weight of the first pulp and the second pulp. Furthermore, the first pulp may issuch a case preferably be present in an amount of up to and including 85 % by weight of the totalweight of the first and the second pulp, more preferably up to and including 60 % by weight of the total weight of the first and second pulp. lrrespective of the nature of the second pulp, the pulp mixture may comprise one or more furthercomponents, more specifically fillers. The filler selected is not limiting the scope of the presentinvention 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 amount of filler, if present, may be selected in order to tailor to the desired properties of thepaper product to be obtained, but could for example be in the range of 5-35 % by weight of the total weight of the pulp mixture.

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.

As previously mentioned, high consistency refining and low consistency refining may be performed inaccordance with conventional routes. Furthermore, for the embodiment of CTMP, CTMP may beproduced in accordance with any previously known techniques without departing from the scope ofthe invention. Moreover, mixing of the first and the second pulp may be performed in accordance with any previously known technique.

A paper 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 freenessCTMP chemithermomechanical pulpHC high consistency LC low consistency SR Schopper-Riegler number 12 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 |nnventia and the evaluation of the trials was made inlaboratory scale. All chemical pulps used were industrially produced and several sets of pulp samplesfrom 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 refinerused for bleached pulp was a Sprout-Bauer 50-1B refiner equipped with refining fillings with an edgelength 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 |nnventia in a Beloit 24” double discrefiner at a flow rate of 500 l/min. Refining segments used has an edge length of 8.38 km/rev and therefiner 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 stagesat 10, 30, 55, 80, 115 and 145 kWh/t) for bleached pulp and up to 140 kWh/t for unbleached pulp (sixstages 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 aL&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. 13 Fibre and pulps were characterized in accordance to the following standards SR number SO 5267-121999 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 8791-2Scott-Bond T 833 pm-94Z-tensile strength ISO 15754 Experimental results 1 Experimental tests were performed in order to investigate how refining energy affects relationshipbetween 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-refinedbleached pulp. HC-refining was done up to approximately 200 kWh/t and LC-refining up to 145kWh/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 throughthe 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 increasedrefining energy. Furthermore, at the same strength, the difference in density between the two refining processes increased with increased density. 14 To summarise, from the test results it can be concluded that at the same density, HC-refininggenerates pulp with higher Z-strength, measured as Z-tensile strength and Scott Bond, compared toLC-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 andlower surface smoothness than LC-refining. lt is common general knowledge that LC-refining as apost-stage after HC-refining straightens the fibers, increases density and pulp properties related totensile strength in-plane and improves surface properties. However, it is not previously known howthe LC refining affects the out-of-plane properties of HC-refined pulp. Therefore, experimental tests of LC refining of HC-refined pulp was performed. ln order to limit pulp density increase, the HC-refined pulps were treated in a Voith laboratory refinerat low energy input of 20 kWh/t. After this stage, the pulp samples are noted HC/LC pulp. The effectsof LC refining on density and Z-tensile strength in relation to refining energy are shown in Figure 3 forbleached 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 increasewas greater for the HC samples refined at lower energy levels compared to HC samples refined athigher energy levels. The results show that Z-strength was principally retained or increased by thesubsequent 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 possibilityof partly or fully replacing LC-refined pulps with HC/LC refined pulps in paper products to improvethe Z-strength of such a paper product. High values of Z-strength of HC/LC refined pulps extend the possibility to optimize paper and board density by combining this pulp with bulk increasing pulps.

Experimental results 3 ln this experimental test, bleached and unbleached HC/LC refined pulps were mixed with low LCrefined samples (not previously subjected to HC refining). No additional additives or pulps were added. ln this set of mixing trials only HC/LC samples refined at the high energy levels were mixed with lowLC-pilot refined pulps. The total energy input was 195 kWh/t for HC/LC bleached pulp and 370 kWh/tfor HC/LC unbleached pulp. The refining energy in LC-refining was 30 kWh/t and 50 kWh/t forbleached and unbleached pulp, respectively. The properties of the pulps used in the mixtures are given in Table 1.

Table 1 Pulp and total SR / WRV, g/g Density, kg/m3 Scott Bond, J/mz Z-tensile strength,refining energy kPa Bleached 15,9/ 1,69 755 510 965 HC/LC195 Bleached LC30 14,5/ 1,55 690 210 450 Unbleached 15,4/ 1,85 705 500 940 HC/LC370 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 from14.8 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 tothe 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. 16 lt should be noted that the points plotted for the low LC-refined pulps in Figure 5 is a result ofdifferent refining energies which in turn results in different densities, whereas the points plotted forthe HC/LC + LC mixtures corresponds to the different amounts of HC/LC pulp in the pulp mixturesand 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/LCcontaining pulp mixtures compared to the LC-refined pulp, i.e. at the same Z-tensile strength, thedensity of HC/LC containing pulp mixtures was lower than for the LC-refined pulp. The relationbetween 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 ln this experimental test, bleached HC/LC refined pulps were mixed with low LC refined samples (notpreviously subjected to HC refining). However, in this experimental test, mixing trials with low LCrefined pulp, mill refining at 30 kWh/t, and HC/LC samples refined at different energy levels, 70kWh/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 SR / WRV, g/g Density, kg/m3 Scott Bond, J/mz Z-tensile strength, kParefining energy Bleached LC30 13,7/ 1,45 690 215 500 Bleached HC/LC70 15,3/ 1,58 770 360 740 Bleached 17,9/ 1,66 795 480 875 HC/LC145 Bleached 18,7/ 1,69 785 560 930 HC/LC215 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. lt can be seen that the effect decreases with 17 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 concludedthat the mixtures of HC/LC+LC pulps can be used instead of LC refined pulp in different paper andpaperboard products for three different purposes, namely - At the maintained density, Z-strength can be increased, - At the maintained strength in Z-direction, paper density can be decreased and thus bending stiffness increases,- At the maintained bending stiffness and at the same Z-strength, the paper grammage can be reduced.

Experimental results 5 Experimental test were performed with HC/LC samples mixed with bulk-giving CTMP pulp. CTMPpulp is presently often used in the middle layer of paper board. The purpose of this test was to findout if the mixture could be used instead of LC refined pulp in production of paper and board in orderto decrease paper density with remaining Z-strength and preserved bending stiffness and thereby creating the possibility to decrease paper grammage. ln the test, bleached HC/LC refined pulp with a total energy input of 220 kWt/h 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 ofthe 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 SR / WRV, g/g Density, kg/m3 Scott Bond, J/mz Z-tensilerefining energy) strength, kPaBleached 18,7/ 1,63 770 520 980HC/LC220 Bleached LC35 15,7/ 1,6 735 320 690 CTMP 475 90 160 18 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 LCrefined bleached pulp. The decrease in Z-strength which was expected by the addition of CTMP wasbalanced 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 weightCTMP of the same quality as given above in Table 3. Said mixture had SR 17.9 and WRV 1.7 g/g. Theproperties 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 SR /WRV, g/g Density, kg/m3 Scott Bond, J/mz Z-tensilerefining energy) strength, kPaUnbleached 16,9/ 1,83 740 505 990HC/LC360 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 LCrefined unbleached pulp. Despite the same density, the mixtures of HC/LC refined pulp and CTMPshowed 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 theresults, 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 beconcluded that the mixtures of HC/LC+CTMP pulps can be used in paper or board production forthree different purposes, namely - At the maintained density, Z-strength of paper can be increased, 19 At the maintained strength in Z-direction, paper density can be decreased and thus bending stiffness increased, At the maintained bending stiffness and at the same Z-strength, the paper grammage can be reduced.

Claims (20)

1. Pulp mixture comprising a first pulp and a second pulp, the first pulp consisting of a chemicalsoftwood pulp which has been subjected to high consistency refining followed by lowconsistency refining, the second pulp consisting of a bulk increasing pulp selected from thegroup consisting of a chemi-thermomechanical pulp (CTMP), a thermomechanical pulp (TMP)and a chemical softwood pulp which has been subjected to low consistency refining withoutpreceding high consistency refining, and wherein: in the case the second pulp is a chemi-thermomechanical pulp or athermomechanical pulp, the first pulp is present in an amount of 50-90 % by dry weight ofthe 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 toa low consistency refining without preceding high consistency refining, the first pulp ispresent in an amount of 5-90 % by dry weight of the total dry weight of the first pulp and the second pulp.

2. Pulp mixture according to claim 1, wherein the second pulp is a chemi-thermomechanicalpulp (CTMP) and wherein the first pulp is present in an amount of 60-90 % by dry weight ofthe 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.

3. Pulp mixture according to claim 1, wherein the second pulp is a chemical softwood pulpwhich has been subjected to low consistency refining without preceding high consistencyrefining, and the first pulp is present in an amount of 8-85 % by dry weight of the total dryweight 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.

4. Pulp mixture according to any of the preceding claims, wherein the first pulp is a bleachedchemical softwood pulp which has been subjected to high consistency refining followed bylow 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.

5. Pulp mixture 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 10. 11. 12. 13. 21 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.

6. Pulp mixture according to any of the preceding claims, wherein the first pulp is a bleachedchemical softwood pulp which has been subjected to high consistency refining followed bylow consistency refining, said low consistency refining performed with a refining energy of 15-40 kWh/t.

7. Pulp mixture according to any of the preceding claims, wherein the first pulp has a SRnumber of above 15.0, preferably 15.2-18.

8. , and/or a WRV of above 1.55 g/g, preferably1.58-1.90 g/g. Pulp mixture 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. Pulp mixture according to any of the preceding claims further comprising at least one additional component, preferably a filler. Method of producing a pulp mixture according to any of the preceding claims, the methodcomprising subjecting a chemical softwood pulp to a high consistency refining step and asubsequent 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 10, wherein the chemical softwood pulp is a bleached chemicalsoftwood pulp and wherein the total refining energy used during the high consistencyrefining 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 10, wherein the chemical softwood pulp is an unbleachedchemical softwood pulp and wherein the total refining energy used during the highconsistency 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 10 to 12, wherein the refining energy during the lowconsistency refining of the chemical softwood pulp subjected to high consistency refining followed by low consistency refining is 15-40 kWh/t. 14. 15. 16. 17. 18. 19. 20. 22 Method according to any of claims 10 to 12, wherein the second pulp is provided bysubjecting a chemical softwood pulp to a low consistency refining step, without anypreceding high consistency refining step, said low consistency refining step performed at a refining energy of 20-60 kWh/t, preferably 25-55 kWh/t. Paper product obtained by the pulp mixture according to any of claims 1 to 9, having a density of less than 750 kg/m3 and a Z-tensile strength of at least 550 kPa. Paperboard comprising a layer formed from the pulp mixture according to any of claims 1 to 9. Paperboard according to claim 16, comprising a plurality of layers and wherein the layermade of the pulp mixture according to any of claims 1 to 9 constitutes a middle layer of said paperboard. Method for producing a multi-layered board having a middle layer, wherein the middle layer is formed from a pulp mixture according to any of the claims 1 to

9. Method for producing a multi-layered board having a middle layer, the method comprisingproducing a pulp mixture in accordance with the method of any of claim 10 to 14 and forming the middle layer from said pulp mixture. Fine paper obtained by the pulp mixture according to any of claims 1 to 9.