Optimization of tear strength in a multi-ply paperboard

Optimization of tear strength in a multi-ply paperboard

Evaluation of the potential improvement in tear strength without a negative effect on other mechanical properties

Optimering av rivstyrka i flerskiktskartong

Utvärdering av potentiella förbättringar i rivstyrka utan negativ påverkan på andra mekaniska egenskaper

Sofie Erhage

Faculty of Health, Science and Technology

Degree project for Master of Science in mechanical engineering 30 hp

Supervisor: Mahmoud Mousavi, Karlstads University Examiner: Jens Bergström

2017-05-30

Abstract

The thesis has been carried out as a part of the Master of Science in mechanical engineering at Karlstads University.

The purpose of the study was to evaluate how different variables affect the tearing resistance in a multi-ply paperboard. The paperboard’s final quality is dependent on several different process – and recipe variables and has, in this study, been analyzed with respect to ply grammage, ply composition and refining energies.

This was investigated by manufacturing of anisotropic sheets consisting of four plies and isotropic sheets consisting of single-ply sheets. The multi-ply sheets were manufactured where one variable at a time were tested at different values. The single-ply sheets were manufactured of pure pulps with the refining energy being varied.

A total of nineteen unique multi-ply sheets and twelve unique single-ply sheets were manufactured.

All sheets were tested with respect to the properties including tensile strength, bending stiffness, tear strength and delamination strength. The study shows that an improvement in tearing resistance is obtained when the amount of CTMP-pulp in the center plies 2 and 3 decreases simultaneously as the amount of broke-pulp and unbleached chemical pulp in the same plies increases. The consequence of this improvement is reduced thickness and thereby reduced bending stiffness. The study also shows that several pulps has an optimal degree of refining for maximizing tear strength in paperboard.

The major source of error is believed to be the method of manufacturing. It is also possible that the amount of test points has been too few to be able to see any clear trends for several variables.

Sammanfattning

Examensarbetet har utförts som en del av civilingenjörsprogrammet i maskinteknik på Karlstads Universitet.

Syftet med studien var att undersöka hur olika variabler påverkar rivstyrkan hos flerskiktskartong. Kartongens slutliga kvalitet påverkas av en rad olika process – och receptvariabler och har, i denna studie, undersökts med avseende på skiktytvikt, skiktsammansättning samt malenergier.

Detta undersöktes genom att tillverka anisotropa ark bestående av fyra skikt samt isotropa ark bestående av ett skikt. Flerskiktsarken tillverkades där en variabel i taget testades vid olika värden. Enskiktsarken tillverkades av rena massor där malenergierna varierades.

Totalt konstruerades nitton unika flerskiktsark och tolv unika enskiktsark.

Samtliga ark testades med avseende på egenskaperna dragstyrka, böjstyvhet, rivstyrka och delamineringsstyrka.

Studien visar att förbättrad rivstyrka fås då andelen CTMP-massa i centerskikt 2 och 3 minskar samtidigt som andelen utskott och kemisk massa ökar i samma skikt. Detta med bekostnad på en minskad tjocklek för kartongen och därmed minskad böjstyvhet. Studien visar också att flertalet massor har en optimal malgrad för att maximera rivstyrkan i kartong.

Den största felkällan tros vara tillverkningsmetoden. För flera variabler är det även möjligt att antalet testpunkter för varje variabel har varit för få för att se tydliga trender.

Acknowledgements

The thesis was performed at the Stora Enso Skoghall Mill and the Stora Enso Research Center between January and May 2017.

First of all, I would like to thank Stora Enso for the opportunity to do my thesis work at the company and the ability to use all their facilities and equipment.

I would like to send a special thanks to Kristian Goldszer, supervisor at Stora Enso, and Mahmoud Mousavi, supervisor at Karlstads University, for guidance and advice during the work.

Thanks to all personnel at the company for your help and support. A special thanks goes to Lisa Edqvist Holt, Peter Sjönneby, Klas Norborg and IngMarie Bernskiöld, who all have been valuable for practical help and discussions.

I would also like to thank my fellow students Jessica, Magdalena and Nadine for all we have shared during these five years. Tough periods of exams have been so much easier thanks to you. We made it!

Finally, I would like to thank my family, especially my dearest Anton, who has been supporting me during this semester as well as during my education. Thank you for always being there for me.

Sofie Erhage

17-05-30

List of abbreviations

Abbreviation Meaning

FBB Folding BoxBoard

SBS Solid Bleached Sulphate

SUB Solid Unbleached Board

WLC White Lined Chipboard

LPB Liquid Packaging Board

BM7 Board Machine 7

BM8 Board Machine 8

CKB Coated Kraft Back

SEC Specific Energy Consumption

YS Top ply

CS1 Center ply 1

CS2 Center ply 2

CS3 Center ply 3

BS Bottom ply

CTMP Chemi-ThermoMechanical Pulp

SR Shopper riegler

CSF Canadian Standard Freeness

MD Machine Direction

CD Cross Direction

ZD Thickness Direction

GM Geometric Mean

List of symbols

Symbol Meaning

A Area

b Width

F Force

g gram

k kilo

m mass or meter

mm millimeter

N Newton

Pa Pascal

t Thickness

w Grammage

Å Ångström

ρ Density

σ Stress

Table of contents

1 INTRODUCTION ... 1

1.1 Stora Enso ... 1

1.2 Paper and paperboard ... 1

1.2.1 Production ... 1

1.2.2 Paperboard grades ... 2

1.3 Problem formulation ... 2

1.3.1 Background ... 2

1.3.2 Purpose and objective ... 3

1.3.3 Delimitations ... 4

2 THEORY ... 5

2.1 Introduction ... 5

2.2 Source of fibers ... 5

2.3 Pulping ... 5

2.3.1 Mechanical pulp ... 5

2.3.2 Chemical pulp ... 5

2.3.3 Broke ... 6

2.3.4 Pulp composition ... 6

2.4 Preparation of fibers ... 6

2.5 Fiber properties ... 7

2.5.1 Fiber length ... 7

2.5.2 Fiber width ... 7

2.5.3 Fiber shape ... 7

2.5.4 Fibril area ... 7

2.5.5 Fines ... 7

2.5.6 Drainage resistance ... 7

2.6 Structure of paper ... 7

2.6.1 Modeling of paper ... 8

2.7 Fracture mechanics of paperboard ... 9

2.8 Paperboard properties ... 10

2.8.1 Basic properties ... 10

2.8.2 Strength properties ... 11

3 METHOD ... 15

3.1 Introduction ... 15

3.2 Experimental method ... 15

3.2.1 Test plan ... 15

3.2.2 Stock preparation ... 17

3.2.3 Dynamic sheet forming ... 17

3.2.4 SCAN sheet forming ... 19

3.2.5 Testing of sheets ... 20

4 RESULTS ... 25

4.1 Fiber properties ... 25

4.2 Basic properties ... 28

4.3 Strength properties ... 30

4.3.1 Tear test ... 30

4.3.2 Tensile test ... 33

4.3.3 Bending test ... 35

4.3.4 Z-strength test ... 36

4.3.5 Scott Bond test ... 38

5 DISCUSSION ... 40

5.1 Experimental method ... 40

5.2 Fiber properties ... 40

5.2.1 Fiber length ... 40

5.2.2 Fiber width ... 40

5.2.3 Fiber shape ... 40

5.2.4 Fibril area ... 40

5.2.5 Fines ... 41

5.3 SCAN sheets ... 41

5.3.1 Bleached chemical pulp ... 41

5.3.2 Unbleached chemical pulp ... 41

5.3.3 Broke ... 41

5.3.4 CTMP ... 41

5.4 Formette sheets ... 41

5.4.1 Pulp composition in CS2+3 ... 42

5.4.2 Grammage CS1 ... 42

5.4.3 Refining energy of CTMP in CS2+3 ... 42

5.4.4 Refining energy of bleached chemical pulp ... 42

5.4.5 Refining energy of unbleached chemical pulp ... 42

5.4.6 Refining energy of broke ... 43

5.5 Future work ... 43

6 CONCLUSIONS ... 44

7 REFERENCES ... 45

8 APPENDIX ... 47

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1 INTRODUCTION

1.1 Stora Enso

Stora Enso was formed in 1998 through a unification of two companies, Enso Oyj (Finnish) and Stora (Swedish), and came to be a leading provider of renewable solutions. The company consists of the following five divisions:

Consumer Board, Packaging Solutions, Biomaterials, Wood Products and Paper, with the head office located in Helsinki, Finland.

Stora Enso’s focus is on fiber-based packaging, plantation-based pulp, innovation in biomaterials and sustainable building solutions with the aim to replace fossil based materials. The products provide an environmentally friendly alternative to many products usually made from non-renewable materials. With those alternatives, the company is able to meet the consumers increasing demands for sustainability.

The group is spread out in more than 35 countries with about 26 000 employees in total [1].

1.2 Paper and paperboard

In everyday life, people use a variety of paper products. It starts early in the morning, using filters for coffee, reading the newspaper and making breakfast from the content in several kinds of food packages. The wide use of paper continues during the day with everything from copy paper at work to the use of tissues at dinner in the evening [2].

The craft of making paper has its roots in China where the first attempt of making paper was done in the beginning of the first millennium A.D. The papermaking remained a relatively small-scale, artisan activity until the paper production became industrialized during the 19^th century. Paper was initially intended for writing and printing but has, during the years, evolved for several applications [3].

There is no clear definition of how to distinguish between paper and paperboard, although mostly two criteria apply. Paperboard usually consist of several different layers and has a basis weight, usually referred to as grammage, higher than 150 g/m² [4].

In relation to its weight and thickness, paperboard is considered to be a very strong material. Different kinds of paperboard are often separated by different grades, which is a classification on the basis of their content, appearance, manufacturing history and end use.

1.2.1 Production

The basic principles of making paper goes back to more than over 2000 years. Briefly explained, the production starts with a headbox squirting a mixture of fibers and water onto an endless moving wire mesh. This is followed by a sheet formation, where the water is removed and the fibers starts to spread and consolidate into a thin mat.

The next step in the process is the press section which squeezes the web of wet papers and lowers the water content.

The press section is followed by drying, which is done by letting the web of sheets pass through a series of heated cylinders. The process is then finished with coating and calendering which is done in order to improve the printing properties and give the paper its smooth and glossy structure. Figure 1 shows a schematic overview of the process [5].

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Figure 1. Schematic overview of the process [5].

Several parameters will affect the final paperboard: choices in source of fibers, method of fiber separation (pulping), preparation of the fibers and whether the fiber is bleached or not [2].

1.2.2 Paperboard grades

There are five different main types of paperboards used: Folding Boxboard (FBB), Solid Bleached Sulphate (SBS), Solid Unbleached Board (SUB), White Lined Chipboard (WLC) and Liquid Packaging Board (LPB).

The Skoghall mill produces the Solid Unbleached Board and the Liquid Packaging Board on two board machines, BM7 and BM8 [6].

1.3 Problem formulation 1.3.1 Background

A paperboard is usually subjected to tearing forces both during the conversion of the paperboard as well as in the end use of the final product. For example, a carrier board intended for beer cans usually requires holes on the top of the package to work as handles, which is an initiation point for fracture. The tearing resistance is thus of great importance [7].

The customer demand for this kind of products is growing and so does their requirements on tearing resistance.

Since about two years back, a downward trend has been identified regarding the values of tear strength for the paperboard quality CKB 350 g/m², which is the main driving force for this investigation, see figure 2.

Figure 2. Tear strength GM of CKB 350 G/m². 5000

5500 6000 6500 7000

2015-01-11 2015-03-20 2015-04-28 2015-08-18 2015-10-07 2015-12-07 2016-02-27 2016-04-19 2016-06-28 2016-09-16 2016-12-03

Tear strength GM [mN]

Date

Tear strength, Geometric Mean CKB 350 g/m² 2015-01-11 - 2016-12-03

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Stora Enso’s CKB stands for Coated Kraft Back board and is a type of SUB which is an excellent choice of paperboard when it comes to folding cartons. The fact that CKB is light and strong, easy to buy, carry and open makes it very suitable for beverage multipacks, see figure 3. The paperboard is built up by five plies, see table 1.

The table shows the pulp compositions, the refining energies, sometimes referred to as specific energy consumption or SEC, and the grammage of each individual ply. YS refers to the top ply, CS1, CS2 and CS3 to the center plies 1-3 and BS to the bottom ply. Observe that center plies CS2 and CS3 are identical.

Figure 3. Example of a multipack product [8].

Table 1. Structure of CKB 350 g/m² paperboard.

Ply Ply composition SEC [kWh/t] Grammage [g/m2]

YS 100% bl. chem. pulp 175 48

CS1 55% CTMP 30

45% bl. chem. pulp 175 51

CS2 70% CTMP 30

30% broke 220 181

CS3 70% CTMP 30

30% broke 220

BS unbl. chem. pulp 130 48

1.3.2 Purpose and objective

The purpose of the thesis is to investigate and identify process – and recipe variables affecting the tearing resistance in the CKB multi-ply paperboard. The objective is to increase the tearing resistance while the compromise on other product specification properties are kept as low as possible. Since most of the previous studies regarding tear strength have been done on paper there is limited knowledge about tear strength in paperboard, which is the driving force for this thesis.

The study will try to answer the following questions:

- Which variables will have a positive effect on the tearing resistance when being changed?

- How will an improvement in tearing resistance affect other mechanical properties?

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1.3.3 Delimitations

The study will have the following delimitations:

- The study will only include analysis on the quality CKB 350 g/m² produced in BM7 at the Stora Enso Skoghall mill.

- The potential of increasing the tearing resistance by changing the design of the final product will not be evaluated.

- The potential of increasing the tearing resistance by changing the type or amount of fillers, chemicals or coatings of the product will not be evaluated.

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2 THEORY

2.1 Introduction

Every possible composition to build up the multi-ply paperboard will affect the basic properties, strength properties and stiffness properties. Despite the properties of the different ingoing pulps, it will be affected by the grammage and pulp mixture in each individual ply.

2.2 Source of fibers

The properties of the fibers depend on the tree from which the fibers are derived. The paperboard maker has a choice between hardwood and softwood trees. Softwood, like spruce and pine, have longer fibers, providing strength, toughness and structure. Hardwood, like birch and eucalyptus, have shorter fibers, providing low density and smoothness. The longer fibers are about 3-4 mm whereas the shorter fibers are about 1-1.5 mm [2].

Sometimes, different lengths of the fibers are referred to as fiber fraction. See table 2 for different lengths and their corresponding fractions.

Table 2. Fiber fraction parameters.

Fiber fraction parameters

Fiber fraction Fiber length [mm]

0 0.0

1 0.2

2 0.5

3 1.0

4 3.0

5 7.0

6 7.6

2.3 Pulping

In the tree, the fibers are locked together by lignin, serving as a glue between the fibers. Pulping is the process describing the separation of fibers from wood and can be divided into mechanical and chemical methods [2].

2.3.1 Mechanical pulp

In mechanical pulping, the fibers are separated from the wood by mechanical forces. This grinding action generates heat and softens the lignin and separates the individual fibers. The yield of pulp from this process is very high (about 90%) as it does not remove the lignin. Sheets made from mechanical pulp will have a high bulk and thereby a relatively low grammage for a given thickness. If the wood chips are heated prior to pulping and is accompanied by a limited chemical treatment to remove some of the lignin, it is called chemi-thermomechanical pulp, CTMP [2].

CTMP can be characterized as an intermediate pulp between pure mechanical and chemical pulp. Relatively low chemical doses are applied and the pulp is produced with pressurized refining.

Board-grade CTMP pulps are frequently used in the center ply or plies in paperboards. CTMP with its properties introduces the possibility of lowering the grammage compared with the use of chemical pulp [9].

2.3.2 Chemical pulp

In chemical pulping, the fibers are separated from the wood by using chemicals. This method can be divided into the sulphate process and the sulphite process with the sulphate process being the most common. The yield of pulp from this process is lower (about 50%) than that of mechanical pulp, due to the lignin being removed. This will result in a higher degree of interfiber bonding and sheets made from chemical pulp will be stronger and more

6

flexible compared to sheets made from mechanical pulp. Chemical pulp can be used as bleached or unbleached [2].

2.3.3 Broke

Paperboard that is discarded at any point in the manufacturing process is repulped and used as broke. Broke occurs both in a continuous way, as trims from the wire and winders, and occasionally, during breaks [10].

2.3.4 Pulp composition

As pure furnishes, chemical pulps usually provide higher tensile strength and elastic modulus than mechanical pulps. A corresponding improvement is also seen when adding chemical pulp to a mechanical pulp. As a function of the mixing ratio, the properties usually change in a nonlinear manner [11].

2.4 Preparation of fibers

The properties of the fibers can be modified prior to manufacturing. This step is usually referred to as stock preparation.

Refining is one of the most important operations when preparing fibers for papermaking. The term refining, in connection with papermaking, means mechanical treatment of the fibers in order to make them more suitable for the papermaking. It can be defined as the process of creating desirable structural changes in the cell wall of the fibers at the consumption of mechanical energy. The refining makes the fibers flexible and prepares them to form fiber to fiber bonds in the paper.

The specific refining energy is used for calculating how much energy the fibers receives from the refiner. The higher the energy, the greater the refining. This process is a major energy consumer in the mill.

Two primary refining effects are:

- Cutting of the fibers

- Internal and external fibrillation

Cutting of the fibers to achieve shorter fibers and fibrillation to achieve increased bonding and thereby increased strength [12].

When the refining takes place, fibers are treated between two parallel grooved plates, stator and rotor, see figure 4. First of all, the fibers are collected and trapped between the edges of the bar. This is followed by compression of the surfaces of the stator and rotor, resulting in normal forces acting on the fibers. This is followed by the movement of bar surface, resulting in shear forces and finally corner forces [13].

Figure 4. Schematic overview of refining [14].

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2.5 Fiber properties

Fiber properties are important as they affect the formation and consolidation of the structure and are responsible for the properties of the final product [11].

2.5.1 Fiber length

One of the most important properties is the fiber length. A long fiber can be more strongly held in the network than a short fiber, due to its ability to have more bonds with other fibers. Paper properties such as tensile strength, tensile stiffness, stretch at break and tensile energy absorption usually increases as the fiber length increases. The fiber length is measured in the unit of mm.

2.5.2 Fiber width

The fiber width is usually not mentioned in the same extent as the fiber length but is still a fiber property that sometimes is measured, in the unit of mm.

2.5.3 Fiber shape

The fiber shape is defined as the projected length of the fiber divided by the actual length. It is a measure of the fibers straightness in the unit of percentage where a value of 100% corresponds to a completely straight fiber.

2.5.4 Fibril area

One way to evaluate how fibrillated the fibers are is to measure the fibril area. The fibril area is a measure of the amount of fiber fibrils. Fibril area is measured in the unit of percentage.

2.5.5 Fines

The pulping and refining processes generates fiber fragments and other small particles called fines. Fines are defined as material shorter than 0.2 mm. Because of the small particle size and large specific surface area, fines can bind more water than fibers. Chemical pulp contains less fines than mechanical pulps. Fines are measured in the unit of percentage.

2.5.6 Drainage resistance

Dewatering tests assess the degree of external fibrillation and amount of fines. The tests measures how fast water drains from the pulp. The two most common tests are the Shopper-Riegler number, SR, and Canadian Standard Freeness, CSF. The SR number increases as the refining increases while the CSF decreases [15].

2.6 Structure of paper

As a consequence of the forming process and the fact that fibers are much longer than thick, paper becomes a layered structure, with the fibers oriented more or less in the plane of the paper. Paper will show an anisotropic behavior and is usually described by three different directions. The machine direction, MD, the cross-machine direction, CD and the thickness direction, ZD, see figure 5. Paper shows a higher strength in the plane than in the thickness direction since the fibers are much stronger in the longitudinal direction compared to the transverse direction.

The fibers will preferentially be oriented in the MD direction, due to the shear stresses in the flow caused by the acceleration of the stock suspension towards the slice on the paperboard machine [16].

8

Figure 5. Principal directions of paper, related to the paper roll [17].

About twice as many fibers will be oriented in the MD direction compared to the CD direction, see eq. 1.

!"#$%&_'(

!"#$%&₎₍ =^+,

- = 2 (1)

2.6.1 Modeling of paper

To model the behavior of paper, different models must be used regarding the loading situation, the time scale and the environmental conditions. The more complicated the loading situation, the more complex the model [18].

The mechanical properties of paper are of large importance in several applications. Regarding packaging, there is always an aim to minimize the paper usage which makes the stiffness of the paper playing an important role.

Experiments on paper have shown that it has a certain time dependent behavior and that it shows non-linearities at large deformations. Linear elasticity could though be a good approximation for small deformations. Due to the stiffness differences in the MD and CD, orthotropic elasticity is required to describe stresses and deformations in paper.

Since paper shows a higher strength in the plane than in the thickness direction, only the plane stress stiffness properties of paper are of interest in many applications. The stress-strain relationship could be described according to eq. 2.

𝜀₀= 𝜀₁ 𝜀₂ 𝛾₁₂ =

+ 4₅

67₈₅

4₈ 0

6:₅₈ 4₅

+

4₈ 0

0 0 ⁺

;₅₈

𝜎₁ 𝜎₂

𝜏₁₂ = 𝑆_0&𝜎₀ (2)

where

S_ps denotes the plane stress compliance matrix.

The matrix is symmetrical, resulting in eq. 3 [19].

67₈₅ 4₈ =^6?₄⁵⁸

5 (3)

2.6.1.1 Laminate theory

Paperboard, in contrast to paper, is a laminate. A laminated structure is composed of several thin layers, bonded to each other, see figure 6. The objective with lamination of plies is usually to improve mechanical properties of the application, such as stiffness and strength.

A coordinate system is defined with the z-axis perpendicular to the plate. The middle surface is defined by z = 0.

It can be exposed to in-plane or bending deformations. For paperboard, bending deformations are usually of primary importance. The mechanical properties of a laminate can theoretically be determined based on the laminate layup and the properties of the individual layers [19].

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Figure 6. A laminated plate [20].

2.6.1.2 Layer adhesion

A unique characteristic of cellulosic fibers laminates is that they bond together naturally, without any addition of paste or glue [21]. Adhesion between the different layers in the laminate occurs when the layers dries and form hydrogen bonding between the fibers in the contact area. The process requires that the cellulose chains come as close as 2.5 to 3.5 Å. To make the hydrogen bonding possible, a certain percentage of wetness is required and the fiber surfaces must obviously be able to form hydrogen bonding [22].

2.6.1.3 Beam theory

The multi-ply technique is used to optimize stiffness and to achieve the desired surface properties with a minimum use of fibers. High stiffness can be obtained with high thickness and a high modulus of elasticity concentrated in the outer layers of a multi-ply sheet. In the paperboard quality CKB, this is done by the combination of inner layers of mechanical pulp, providing high bulk, and outer layers of chemical pulp, providing good bonding and hence high modulus of elasticity. This approach to maximize the stiffness can be connected to the I-beam principle, see figure 7 [23].

Figure 7. Optimization of stiffness [24].

2.7 Fracture mechanics of paperboard

Fracture of laminated composites may be separated into translaminar (transverse, across the plies) and interlaminar (delamination between the plies) failures. Despite that both can occur in a single failure, one mode generally precedes the other. Interlaminar failures appear as a separation of the plies from one another and the crack path is therefore in the matrix. Translaminar fractures inherently involve fiber fracture and hence depend more on fiber properties than on matrix properties [25].

It is useful to consider three prototypical modes of loading that can drive crack growth in different ways, see figure 8. These three modes are differentiated by the manner in which the breaking forces are applied relative to the crack area.

Mode 1: Opening or tensile mode. The crack surfaces move directly apart.

Mode 2: Sliding or in-plane shear mode. The crack surfaces slide over one another in a direction perpendicular to the edge of the crack.

Mode 3: Tearing or antiplane shear mode. The crack surfaces move relative to one another and parallel to the leading edge of the crack [25].

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Figure 8. Basic modes of loading involving different crack surface displacement [26].

2.8 Paperboard properties

Paperboard can be described with many different physical properties based on basic properties, strength properties and stiffness properties.

The properties can sometimes be calculated as an index value. This is a way to compensate for different grammage, meaning that the absolute value of the property relates to the amount of material.

The index of any property is calculated according to eq. 4.

𝑃𝑟𝑜𝑝𝑒𝑟𝑡𝑦 𝑖𝑛𝑑𝑒𝑥 =L$M0%$N2 O#&MPQN% ?OPQ%

;$RSSOT% (4)

When testing anisotropic materials showing different properties in MD and CD, a geometric mean value, GM, is sometimes calculated. This is a combined value based on properties in both MD and CD. The geometric mean value is calculated according to eq. 5 [15].

𝑃_;U= 𝑃_UV∗ 𝑃_XV (5)

where

PMD = Property in MD P_CD = Property in CD

2.8.1 Basic properties

The basic properties of paperboard include grammage, thickness and density. Grammage, w, is calculated as the mass per unit area and thickness, t, is measured as the distance between the two surfaces of the paperboard.

𝑤 =^S_Z [_S^T_\] (6)

where

m = mass [g]

A = area [m²]

Having the grammage and thickness, the density, ρ, can be calculated as the grammage per unit thickness according to [15]

𝜌 =^_

N[^`T

S^a] (7)

where

w = grammage [kg/m²] t = thickness [m]

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2.8.2 Strength properties

2.8.2.1 Tear strength

Tearing resistance, which is a way of evaluating the crack sensitivity, is defined as the mean force required to tear a single thickness of a sample of paper or paperboard with the presence of an initial cut. Tearing consumes energy, and the force needed to continue the tearing is assumed to correlate with important end-use properties. Tearing corresponds to mode 3 of loading and dependent on if the cut is made in the machine direction or the cross-machine direction, the result is given as the MD tearing resistance or the CD tearing resistance, respectively. The principle of tearing is shown in figure 9 [16].

Figure 9. Principle of tearing [27].

For pulps with a low degree of bonding between the fibers, the tearing is believed to take place by drawing the fibers out of the structure, without breaking them. If, in opposite, the degree of bonding is high, the tearing is believed to take place by breakage of the fibers. The difference can be seen in figure. 10. At a suitable distribution between pulled-out and broken fibers, the optimum tearing energy is believed to be obtained [16].

Figure 10. Tearing at low and high degree of bonding, respectively [16].

The tearing of multi-ply structures usually follows complicated paths and delamination between the layers are a common observation, see figure 11 [16].

Figure 11. Delamination between plies caused by tearing.

The general perception is that tear strength of a paper is believed to be strongly dependent on the refining step which in turn can be related to the following fiber properties:

- Fiber length - longer fibers provide higher tear strength

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- Fiber strength - stronger fibers provide higher tear strength

- Degree of bonding between fibers - the tear strength is believed to increase with increased level of bonding at low degrees of bonding, while at higher levels of bonding it is believed to be more dependent on fiber strength.

This can be explained with the theory that the tear strength passes through a clear maximum as a function of the degree of beating/tensile strength, see figure 12. The corresponding degree of bonding where this maximum occurs is higher for short-fiber hardwood pulps than for long-fiber softwood pulps.

- Degree of orientation of fibers in the paper – this is connected to different values of tear strength in CD and MD. There are some disagreements in the studied literature, some believing it to be higher in the CD, while others in the MD [15].

It should also be stated that the tearing resistance increases with the amount of layers applied to build the laminate, which is due to an increase of the grammage, according to literature [16].

Figure 12. Tear index as a function of tensile index for different pulps [16].

Tear strength, F_tear, is measured in the unit of Newton [N].

2.8.2.2 Tensile strength

To describe the general strength of any material, tensile strength is a useful property. The tensile strength is the maximum force per unit width that a paperboard strip can resist before breaking. The load is applied in a direction parallel to the length of the strip.

A tensile test usually provides absolute values of tensile stiffness, stretch at break and tensile energy absorption in addition to tensile strength [15].

Tensile strength is measured in the unit of load per unit length according to eq. 8.

𝑇𝑒𝑛𝑠𝑖𝑙𝑒 𝑠𝑡𝑟𝑒𝑛𝑔𝑡ℎ = ^!

# (8)

where

F = force [N]

b = width [m]

2.8.2.3 Z-strength

Z-strength can be described as the ability of paperboard to resist tensile loading in the z-direction and is expressed as the load at failure per unit area, see eq. 9. If the z-directional strength is exceeded, a break in the sheet occurs

13

but not at its surface. This kind of measurements are of interest since paperboard usually will be loaded in the z- direction during operations, such as converting, corrugating, folding and coating [15].

𝜎_gV=^!_Z (9)

where

σ_ZD = the maximal tensile strength [kPa]

F = the maximal tensile force [N]

A = the area of the metal pieces [mm²]

2.8.2.4 Scott Bond strength

Another way to evaluate the strength in the z-direction is to measure the delamination energy with the Scott Bond tester. The procedure can be seen in figure 13 where a pendulum hits an L-shaped block, causing the paperboard to delaminate.

The Scott Bond strength is measured in the unit of J/m² [11].

Figure 13. Principle of Scott bond tester [28].

2.8.2.5 Bending stiffness

Bending stiffness is a basic quantity in engineering mechanics. It can be described as the ability of the paperboard to resist the effect of forces that are out of its axial or planar direction. High bending stiffness contributes to rigidity and strength. Bending stiffness is mostly directly controlled by the thickness, where a high thickness gives a high bending stiffness.

The bending stiffness of a multi-ply structure is obtained by eq. 10 [11].

𝑆_# = 𝐷 −^j\

Z (10)

where

𝐴 = 𝐸_"

m

"n+

𝑧_"− 𝑧_"6+

𝐵 =1 2 𝐸_"

m

"n+

𝑧_"^r− 𝑧_"6+^r

14

3 𝐸_"

m

"n+

𝑧_"^t− 𝑧_"6+^t

where

N = Total number of layers

Ei = Elastic modulus in the k direction, which may be either MD or CD, of the ith layer z = Coordinate from bottom, meaning z₀ = -d/2, where d is the total thickness.

15

3 METHOD

3.1 Introduction

The investigation started with an evaluation of the tearing resistance values of CKB 350 g/m² for the past two years. This was done in order to find possible correlations between process variables and tearing resistance for the specific period.

In addition, a multivariate data analysis was done in a separate project for CKB 315 g/m². Multivariate data analysis is a statistical technique used to analyze data that arises from more than one process variable, in order to obtain the key variables affecting the specific property. This kind of analysis makes it possible to get a clear indication of what is going on in the process and is therefore a valuable method for process control and process optimization.

This was done in SIMCA, which is a Soft Independent Modelling of Class Analogies. The SIMCA approach is based on staggered process data to match the time of production of a certain quality [29].

The literature study together with this analysis made up the basis to decide which variables to be tested and evaluated in the laboratory.

3.2 Experimental method

All preparation and testing were performed at the Stora Enso Research Center Karlstad, RCK, and limited to the available equipment at the company. Sheets were made with two different methods: anisotropic four-ply sheets were made with a dynamic sheet former and isotropic one ply sheets were made by the standard SCAN method.

The four-ply sheets were made to evaluate paperboard properties and the one ply sheets were made to evaluate the different pulps, one by one. The anisotropic sheets are further on referred to as Formette sheets and the isotropic sheets are referred to as SCAN sheets.

Since center plies 2 and 3 in the CKB paperboard are made of the same pulp mixtures and grammage, those plies were formed as one ply in the forming process (named CS2+3). This gave a final sheet of four plies instead of the CKB’s five plies.

No calendering was able to be done in the dynamic sheet former, which excluded this step in the manufacturing.

After manufacturing of the sheets, they were dried and cut for testing in the laboratory.

3.2.1 Test plan 3.2.1.1 Formette sheets

One sheet for each test point was made. The purpose with the tests was to evaluate the recipe variables in the paperboard. Test no. 1 was made as a reference according to the original recipe of CKB 350 g/m², see table 1. The purpose with test no. 2-4 was to evaluate the effect of changing the pulp composition in CS2+3. Test no. 5-7 evaluated the effect of changing the grammage in CS1. This was done at the expense of the grammage in CS2+3, to keep the total grammage constant. Test no. 8-19 evaluated the effect of using pulps refined at different refining energies. A list of what was changed is presented below, see table 3. Only one variable was change at a time, the other ones were kept as the reference recipe. A more detailed test plan is presented in Appendix A. Yellow boxes does not add any information except making it easy to find which parameter has been changed.

16

Table 3. Test plan for Formette sheets.

Test Variable 1 Reference CKB

2 Pulp composition CS2+3 = 60% CTMP, 40% broke 3 Pulp composition CS2+3 = 50% CTMP, 50% broke

4 Pulp composition CS2+3 = 50% CTMP, 30% broke, 20% unbl. chem. pulp 5 Grammage CS1 = 46 g/m2

6 Grammage CS1 = 56 g/m2 7 Grammage CS1 = 61 g/m2 8 SEC CTMP = 0 kWh/t 9 SEC CTMP = 40 kWh/t 10 SEC CTMP = 80 kWh/t

11 SEC bl. chem. pulp = 100 kWh/t 12 SEC bl. chem. pulp = 150 kWh/t 13 SEC bl. chem. pulp = 200 kWh/t 14 SEC unbl. chem pulp = 50 kWh/t 15 SEC unbl. chem pulp = 100 kWh/t 16 SEC unbl. chem pulp = 150 kWh/t 17 SEC broke = 100 kWh/t

18 SEC broke = 200 kWh/t 19 SEC broke = 300 kWh/t

3.2.1.2 SCAN sheets

The test plan for the SCAN sheets is presented below, see table 4. For the chemical pulps, seven sheets were made for each test point. For CTMP and broke, ten sheets were made for each test point. Seven respectively ten sheets are the standard amount of sheets made to get all required tests for those pulps. The purpose of the tests was to evaluate the effect of different refining energies for each pulp. This included sheets of 100% pure pulp of bleached chemical pulp, unbleached chemical pulp, CTMP and broke. Each pulp was refined at three different levels.

Table 4. Test plan for SCAN sheets.

Test Pulp SEC [kWh/t]

30 bl. chem. pulp 100 31 bl. chem. pulp 150 32 bl. chem. pulp 200 33 unbl. chem. pulp 50 34 unbl. chem. pulp 100 35 unbl. chem. pulp 150

36 broke 100

37 broke 200

38 broke 300

39 CTMP 0

40 CTMP 40

41 CTMP 80

17

3.2.2 Stock preparation

A total of 18 different pulps were taken from the head boxes and refiners at the Skoghall mill. Those were then brought up to RCK to be prepared for sheet forming. All pulps were diluted in water to get the right concentration of fibers, 3g/lit.

3.2.2.1 Fiber properties measurements

The fiber properties were measured in an L&W Fiber Tester, Code 912, according to the associated operating instructions. Each pulp was diluted and placed in the tester, see figure 14.

Figure 14. L&W Fiber Tester, Code 912.

3.2.3 Dynamic sheet forming

One way to simulate paperboard made in the board machine is to use a dynamic sheet former. This method is based on a centrifugal force, making it possible to achieve anisotropy in the sheet, which corresponds more to the real situation than conventional sheet forming methods.

The former consists of two main components: a vertical rotating cylindrical drum and a device transferring pulp to the drum, see figure 15.

The sheet was formed by an emanation of the pulp to the wire, which is placed around the inner circumferential surface of the cylindrical drum. By varying the speed of the wire and the flow of the sprayer, the speed ratio can be adjusted to obtain the desired anisotropy. To make a multi-ply sheet, the different layers are added, one by one.

A vertical arm moves up and down in the drum to distribute the pulp evenly over the total wire.

When all layers have been added, the sheet is dewatered. After this, the sheet is cut vertically to get it out from the drum.

The dimension of the final sheet is approx. 220mm x 880mm, see figure 16.

18

Figure 15. Dynamic sheet former. Cylindrical drum to the left and device for pulp to the right.

Figure 16. Final sheet dimensions.

It is possible to run the dynamic sheet former with several different settings. Table 5 shows the settings used throughout the sheet forming. Settings for pressing and drying included.

Table 5. Sheet forming settings.

Dynamic sheet forming Pressing Drying

Wire speed [rpm]

Flow [bar]

Nozzle [type]

Dewatering time

[min] Type Pressure

[bar]

Time [min]

Temp [⁰C]

1050 2.5 2510 3 cylinder 1+1, 5+5, 5 10 100

3.2.3.1 Pressing

To prepare the sheet for pressing, it was first couch rolled between blotting papers. This was done by hand using a roller pin weighing approx. 17 kg. No pressure was added beyond the pins own weight, it just rolled across the sheet.

The sheet was pressed in a cylindrical press, see figure 17, between blotting papers and felts. To start with, it was pressed two ways under 1 bar. After this, it was pressed two ways under 5 bar. At last, the sheet was pressed one way under 5 bar, without the felts.

19

Figure 17. Cylindrical press used for the Formette sheets.

3.2.3.2 Drying

The sheet was placed in a dryer of 100⁰ for ten minutes, see figure 18. It was dried under restraint conditions to avoid shrinkage of the sheet.

Figure 18. Drier used for the Formette sheets.

3.2.4 SCAN sheet forming

The SCAN sheets were made in a cylindrical sheet former, according to a standard developed from SCAN-C 26:64 and SCAN-M 5:67. Those sheets get a circular shape with a diameter of 159mm, corresponding to a surface area of approx. 200cm². For the chemical pulps, 65 g/m² sheets were made and for CTMP and broke, 150 g/m² sheets were made, which is according to standard for different pulps.

The pulp was poured into a cylindrical container. The water was then filtered through the wire in the bottom of the cylinder, where the final sheet was formed. This is shown in figure 19.

Figure 19. Cylindrical SCAN sheet former and final sheet.

20

3.2.4.1 Pressing

The sheets were pressed two times under a pressure of 4 bar, see figure 20. The first time for five minutes and the second time for two minutes.

Figure 20. Press used for the SCAN sheets.

3.2.4.2 Drying

Sufficient amount of water was removed in the pressing step to let the sheets self-dry in the conditioned climate room for physical paper testing, see figure 21.

Figure 21. Drying of SCAN sheets.

3.2.5 Testing of sheets

All preparation and testing of the samples were done in a conditioned climate at 23⁰C and 50% relative humidity, according to ISO 187:1990.

For testing of the anisotropic Formette sheets, six samples in MD and six samples in CD were cut out by a guillotine. Usually ten samples are taken in each direction, but to be able to get all required tests from one sheet, it had to be limited to six samples in each direction for each property. The SCAN sheets were cut in the standard way where the amount of samples are dependent on which property to be tested.

3.2.5.1 Thickness and grammage measurements

The thickness was measured according to SCAN P:88-01. The sample was placed between two spherical probes and fed into the nip with a constant speed, see figure 22. The grammage was simply measured by weighing each sample.

21

Figure 22. Thickness tester.

3.2.5.2 Tear test

3.2.5.1.1 Tear test of SCAN sheets

Four samples were made in dimensions of 100 x 62 mm and tested according to ISO 1974:2012 in an Elmendorf tear tester, see left in figure 23. All samples were mounted and clamped together between two metal plates. An initial cut was made with a knife, see right in figure 23. The pendulum was released and the samples were teared together.

Figure 13. Elmendorf tear tester and initial cut.

3.2.5.1.2 Tear test of Formette sheets

The Elmendorf method is not suitable when testing multi-ply paperboard. This is due to the fact that the stiffness of the board may result in bending or that the failure mode changes from tearing to delamination between the plies during tearing. Instead, the Autoline tearing method was used, which is a method used in the Skoghall mill for paperboard testing.

Samples were made in dimensions of 90 x 50 mm, according to internal discussions about what was sufficient to perform the tearing. The tearing was done in a manual variant of the original L&W Autoline tearing, see left in figure 24. The values obtained in this variant corresponds well to those obtained from the standard Autoline at the mill.

22

Figure 24. Schematic figure of tearing procedure.

The tearing was done by a V-shaped knife, moving downwards with a velocity of 25mm/s. When measuring the tearing resistance in CD, an initial cut is made in MD, which then propagates in CD. The opposite is done when measuring in MD. Tearing of the sample occurs along four lines simultaneously, see right in figure 24.

The force required to tear the paperboard, F_tear, is obtained by eq. 11. The recorded force is divided by four, due to four tracks. The force is multiplied by the root of two to obtain the force perpendicular to the tear tool.

𝐹_N%O$=^!vwxyvzwz

{ ∗ 2 (11)

To obtain a value comparable to the Elmendorf method, eq. 11 is divided by 2. This means that the L&W Autoline tearing force can be calculated according to eq. 12.

𝐹|&~ N%O$=^!vwxyvzwz

• ∗ 2 (12)

3.2.5.3 Bending test

Samples were made in dimensions of 80 x 38 mm and tested according to ISO 2493-1:2010 in a bending tester from L&W, see figure 25. The sample was mounted, clamped and bent to 15⁰. Bending test is not usually done for SCAN sheets and was not done in this study.

Figure 25. Bending tester.

23

3.2.5.4 Tensile test

Samples were made in dimensions of 15 x 150 mm and tested according to ISO 1924-3:2005 in a tensile tester, code 064, from L&W, see figure 26. The sample was mounted, clamped and loaded until failure occurred.

Figure 26. Tensile tester, code 064, L&W.

3.2.5.5 Z-strength test

Samples were made in dimensions of 50 x 50 mm and tested according to ISO 15754:2009 in a tensile tester from Zwick, see figure 27. The sample was mounted with double sided tape between two circular metal pieces, having an area of approx. 1000mm². The metal pieces were pressed together under a pressure of 3 MPa for six seconds to ensure that the plates got well attached to the sample. This assembly was then mounted and loaded until failure.

Figure 27. Z-strength tester, Zwick.

3.2.5.6 Scott Bond test

Samples were made in dimensions of 25.4 x 25.4 mm and tested according to TAPPI T 569 pm-09 in a Scott Bond tester, see figure 28. The sample was mounted with double sided tape between a metal plate and an angle of metal.

A pendulum then hit the angle of metal and loaded the sample until failure occurred.

The Scott Bond strength was measured as the loss of energy at failure divided by the area of the sheet [15].

24

Figure 28. Scott bond tester.

25

4 RESULTS

The results are divided into three different parts: fiber properties, basic properties and strength properties. The following fiber properties are presented: length, width, shape, fines and fibril area. The basic properties include grammage and thickness. Finally, the following strength properties are presented: tensile strength, bending stiffness, tear strength, Z-strength and Scott Bond strength.

4.1 Fiber properties

The results from the fiber tester are shown in figure 29-33 below. The box of explanation shows which pulp each color represents. All fiber properties are presented as a function of the refining energy. Observe that the refining energy does not increase according to the x-axis. A table with all data are presented in Appendix B.

Figure 29 shows that, for all kind of pulps, the fiber length will decrease as the refining energy increases.

Figure 29. Fiber length as a function of refining energy for all tested pulps.

2,2 2,16 2,11 2,23 2,23 2,21

1,9 1,76 1,56 1,77 1,7 1,56

0 0,25 0,5 0,75 1 1,25 1,5 1,75 2 2,25 2,5

100 150 200 50 100 150 100 200 300 0 40 80

Mean length [mm]

SEC [kWh/t]

Fiber length

CTMP Broke

Unbl. chem. pulp Bl. chem. pulp

26

Figure 30 shows that, for the chemical pulps, the fiber width increases as the refining energy increases. The opposite is seen for broke and no trend is seen for CTMP.

Figure 30. Fiber width as a function of refining energy for all tested pulps.

Figure 31 shows that, for the chemical pulps and broke, the fiber shape decreases as the refining energy increases.

CTMP shows almost the same values at all different energies.

Figure 31. Fiber shape as a function of refining energy for all tested pulps.

30,15 30,6 30,8 30,95 31,4 31,7 31,9 31,55 31,5

37,25 36,95 37,6

2526 2728 2930 3132 3334 3536 3738 39

100 150 200 50 100 150 100 200 300 0 40 80

Mean width [µm]

SEC [kWh/t]

Fiber width

CTMP Broke

Unbl. chem. pulp Bl. chem. pulp

86 85,6 85,1

89,7 88,8 88,1

85,6 84,6 83,3

87,1 87,2 87,4

80 81 82 83 84 85 86 87 88 89 90 91

100 150 200 50 100 150 100 200 300 0 40 80

Mean shape [%]

SEC [kWh/t]

Fiber shape

CTMP Broke

Unbl. chem. pulp Bl. chem. pulp

27

Figure 32 shows that increasing the refining energy for chemical pulps and broke increases the fibril area. CTMP does not show any trend.

Figure 32. Fibril area as a function of refining energy for all tested pulps.

Figure 33 shows that the amount of fines increases with the refining energy for the chemical pulps and broke.

The highest values obtained are seen for CTMP.

Figure 33. Amount of fines as a function of refining energy for all tested pulps.

2,3 4 5,5

1,5 2,7 3,8

8,1 10,7 13,7

6,2 6,1 6,4

0 2 4 6 8 10 12 14 16

100 150 200 50 100 150 100 200 300 0 40 80

Mean fibril area [%]

SEC [kWh/t]

Fibril area

CTMP Broke

Unbl. chem. pulp Bl. chem. pulp

16,2 18,2 20,5 18,4 20,2 22,3

51,6 52,9 54,1

64,4 65,4 63,8

0 10 20 30 40 50 60 70

100 150 200 50 100 150 100 200 300 0 40 80

Mean fines [%]

SEC [kWh/t]

Fines

CTMP Broke

Unbl. chem. pulp Bl. chem. pulp

28

4.2 Basic properties

The results from the grammage and thickness measurements are shown in figure 34-37 below. The basic properties are presented as a function of each test point. Observe that the grammage of the SCAN sheets are supposed to vary due to the standard that chemical pulps are manufactured at a grammage of 65g/m² and CTMP and broke are manufactured at a grammage of 150g/m². A table with all data and standard deviations are presented in Appendix C.

Figure 32. Grammage of Formette sheets, test 1-19.

Figure 33. Thickness of Formette sheets, test 1-19.

Figure 36. Grammage of SCAN sheets, test 30-41.

300,0 320,0 340,0 360,0 380,0 400,0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Grammage [g/m2]

Test Grammage, Formette sheets

500 550 600 650 700 750 800

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Thickness [µm]

Test Thickness, Formette sheets

5060 7080 10090 110120 130140 150160

30 31 32 33 34 35 36 37 38 39 40 41

Grammage [g/m2]

Test Grammage, SCAN sheets

29

Figure 37. Thickness of SCAN sheets, test 30-41.

0 100 200 300 400 500 600 700

30 31 32 33 34 35 36 37 38 39 40 41

Thickness [µm]

Test Thickness, SCAN sheets

30

4.3 Strength properties

The results from the strength and stiffness tests are shown in figure 38-56 below, divided into results from anisotropic Formette sheets and isotropic SCAN sheets. Index or absolute values of all properties are presented as a function of each test point and as a function of density. If any trend is seen regarding the property vs. the density, this is drawn in the figure. Strength and stiffness properties include results from tear test, tensile test, bending test, Z-strength test and Scott Bond test. The box of explanation shows which variable each color represents. Tables with all data and standard deviations are presented in Appendix D.

4.3.1 Tear test

Figure 38. Tear index GM as a function of test 1-19.

20,00 20,20 20,40 20,60 20,80 21,00 21,20 21,40 21,60 21,80 22,00

Reference CKB 60% CTMP, 40% broke 50% CTMP, 50% broke 50% CTMP, 30% broke, 20% … 46 g/m2 56 g/m2 61 g/m2 0 kWh/t 40 kWh/t 80 kWh/t 100 kWh/t 150 kWh/t 200 kWh/t 50 kWh/t 100 kWh/t 150 kWh/t 100 kWh/t 200 kWh/t 300 kWh/t

Tear index GM [mN*m2/g]

Test

Tear index GM, Formette sheets

SEC broke

SEC unbl. chem. pulp (BS)

SEC bl. chem. pulp (YS)

SEC CTMP (CS2+3) Grammage CS1

Pulp composition CS2+3

Reference, CKB 350

31

Figure 39. Tear index MD and CD as a function of test 1-19.

Figure 40. Tear index GM as a function of density, Formette.

15 16 17 18 19 20 21 22 23 24 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Tear index [mN*m2/g]

Test

Tear index MD and CD, Formette sheets

MD CD

20,40 20,60 20,80 21,00 21,20 21,40 21,60 21,80 22,00 22,20

4 8 0 5 0 0 5 2 0 5 4 0 5 6 0 5 8 0

Tearing index GM [mN*m2/g]

Density [kg/m3]

Tear index GM vs. Density, Formette sheets

Reference, CKB 350 Pulp composition CS2+3 Grammage, CS1 SEC CTMP, CS2+3 SEC bl. chem. pulp, YS SEC unbl. chem. pulp, BS SEC broke

32

Figure 41. Tear index as a function of refining energy.

Figure 42. Tear index as a function of density, SCAN.

13,6

11,1 10,9 12,8

10,5 10,4

13,8 13,5 12,4

9,5 9,8 8,5 6

7 8 9 10 11 12 13 14 15 16

100 150 200 50 100 150 100 200 300 0 40 80

Tear index [mN*m2/g]

SEC [kWh/t]

Tear index, SCAN sheets

SEC CTMP SEC broke

SEC unbl. chem. pulp SEC bl. chem. pulp

6 7 8 9 10 11 12 13 14 15 16

2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0

Tear index [mN*m2/g]

Density [kg/m3]

Tear index vs. Density, SCAN sheets

Bl. chem. Pulp Unbl. Chem. Pulp Broke

CTMP

33

4.3.2 Tensile test

Figure 43. Tensile index GM as a function of test 1-19.

Figure 44. Tensile index GM as a function of density, Formette.

60 62 64 66 68 70 72 74 76 78 80

Reference CKB 60% CTMP, 40% broke 50% CTMP, 50% broke 50% CTMP, 30% broke, 20% … 46 g/m2 56 g/m2 61 g/m2 0 kWh/t 40 kWh/t 80 kWh/t 100 kWh/t 150 kWh/t 200 kWh/t 50 kWh/t 100 kWh/t 150 kWh/t 100 kWh/t 200 kWh/t 300 kWh/t

Tensile index GM [Nm/g]

Test

Tensile index GM, Formette sheets

SEC broke

SEC unbl. chem. pulp (BS)

SEC bl. chem. pulp (YS)

SEC CTMP (CS2+3)

Grammage CS1

Pulp composition CS2+3 Reference, CKB 350

66 67 68 69 70 71 72 73 74 75 76 77

4 9 0 5 0 0 5 1 0 5 2 0 5 3 0 5 4 0 5 5 0 5 6 0 5 7 0

Tensile index GM [Nm/g]

Density [kg/m3]

Tensile index GM vs. Density, Formette sheets

Reference, CKB 350 Pulp composition CS2+3 Grammage, CS1 SEC CTMP, CS2+3 SEC bl. chem. pulp, YS SEC unbl. chem. pulp, BS SEC broke

34

Figure 45. Tensile index as a function of refining energy.

Figure 46. Tensile index as a function of density, SCAN.

73,6783,82 90,18 82,8395,82100,47

56,07 61,6 64,39

21,1 23,34 25,59 0

10 20 30 40 50 60 70 80 90 100 110

100 150 200 50 100 150 100 200 300 0 40 80

Tensile index [Nm/g]

SEC [kWh/t]

Tensile index, SCAN sheets

SEC CTMP SEC broke

SEC unbl. chem. pulp SEC bl. chem. pulp

0 20 40 60 80 100 120

0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0

Tensile index [Nm/g]

Density [kg/m3]

Tensile index vs. Density, SCAN sheets

Bl. chem. Pulp Unbl. Chem. Pulp Broke

CTMP

35

4.3.3 Bending test

Observe that no bending test was done on the SCAN sheets.

Figure 47. Bending resistance index GM as a function of test 1-19.

Figure 48. Bending resistance index GM as a function of density, Formette.

2,00 2,20 2,40 2,60 2,80 3,00 3,20 3,40 3,60 3,80 4,00

Reference CKB 60% CTMP, 40% broke 50% CTMP, 50% broke 50% CTMP, 30% broke, 20% … 46 g/m2 56 g/m2 61 g/m2 0 kWh/t 40 kWh/t 80 kWh/t 100 kWh/t 150 kWh/t 200 kWh/t 50 kWh/t 100 kWh/t 150 kWh/t 100 kWh/t 200 kWh/t 300 kWh/t

Bending resistance index GM [mN*m2/g]

Test

Bending resistance index GM, Formette sheets

SEC broke

SEC unbl. chem. pulp (BS)

SEC bl. chem. pulp (YS)

SEC CTMP (CS2+3)

Grammage CS1 Pulp composition CS2+3

Reference, CKB 350

0,00 0,50 1,00 1,50 2,00 2,50 3,00 3,50 4,00

4 9 0 5 0 0 5 1 0 5 2 0 5 3 0 5 4 0 5 5 0 5 6 0 5 7 0

Bending resistance index GM [mN*m2/g]

Density [kg/m3]

Bending resistance index GM vs. Density, Formette sheets

Reference, CKB 350 Pulp composition CS2+3 Grammage, CS1 SEC CTMP, CS2+3 SEC bl. chem. pulp, YS SEC unbl. chem. pulp, BS SEC broke

36

4.3.4 Z-strength test

Figure 49. Z-strength as a function of test 1-19.

Figure 50. Z-strength as a function of density, Formette.

150 170 190 210 230 250 270 290 310

Reference CKB 60% CTMP, 40% broke 50% CTMP, 50% broke 50% CTMP, 30% broke, 20% … 46 g/m2 56 g/m2 61 g/m2 0 kWh/t 40 kWh/t 80 kWh/t 100 kWh/t 150 kWh/t 200 kWh/t 50 kWh/t 100 kWh/t 150 kWh/t 100 kWh/t 200 kWh/t 300 kWh/t

Z-strength [kPa]

Test

Z-strength, Formette sheets

SEC broke

SEC unbl. chem. pulp (BS)

SEC bl. chem. pulp (YS)

SEC CTMP (CS2+3)

Grammage CS1

Pulp composition CS2+3

Reference, CKB 350

200 210 220 230 240 250 260 270 280 290 300

4 9 0 5 0 0 5 1 0 5 2 0 5 3 0 5 4 0 5 5 0 5 6 0 5 7 0

Z-strength [kPa]

Density [kg/m3]

Z-strength vs. Density, Formette sheets

Reference, CKB 350 Pulp composition CS2+3 Grammage CS1 SEC CTMP (CS2+3) SEC bl. chem. pulp (YS) SEC unbl. chem. pulp (BS) SEC broke

37

Figure 51. Z-strength as a function of refining energy.

Figure 52. Z-strength as a function of density, SCAN.

501 551 536

76 89 110

0 50 100 150 200 250 300 350 400 450 500 550 600

100 200 300 0 40 80

Z-strength [kPa]

SEC [kWh/t]

Z-strength, SCAN sheets

SEC CTMP SEC broke

0 100 200 300 400 500 600

0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0

Z-strength [kPa]

Density [kg/m3]

Z-strength vs. Density, SCAN sheets

Broke CTMP

38

4.3.5 Scott Bond test

Figure 53. Scott Bond MD as a function of test 1-19.

Figure 54. Scott Bond as a function of density, Formette.

50 70 90 110 130 150 170 190 210 230 250

Reference CKB 60% CTMP, 40% broke 50% CTMP, 50% broke 50% CTMP, 30% broke, 20% … 46 g/m2 56 g/m2 61 g/m2 0 kWh/t 40 kWh/t 80 kWh/t 100 kWh/t 150 kWh/t 200 kWh/t 50 kWh/t 100 kWh/t 150 kWh/t 100 kWh/t 200 kWh/t 300 kWh/t

Scott Bond MD [J/m2]

Test

Scott Bond MD, Formette sheets

SEC unbl. chem. pulp (BS)

SEC bl. chem. pulp (YS)

SEC CTMP (CS2+3)

Grammage CS1

Pulp composition CS2+3

Reference, CKB 350

0 50 100 150 200 250

4 9 0 5 0 0 5 1 0 5 2 0 5 3 0 5 4 0 5 5 0 5 6 0 5 7 0

Scott Bond MD [J/m2]

Density [kg/m3]

Scott Bond MD vs. Density, Formette sheets

Reference, CKB 350 Pulp composition CS2+3 Grammage, CS1 SEC CTMP, CS2+3 SEC bl. chem. pulp, YS SEC unbl. chem. pulp, BS SEC broke

39

Figure 55. Scott Bond as a function of refining energy.

Figure 56. Scott Bond as a function of density, SCAN.

401

508 564

65 65 70

0 100 200 300 400 500 600

100 200 300 0 40 80

Scott Bond [J/m2]

SEC [kWh/t]

Scott Bond, SCAN sheets

SEC CTMP SEC broke

0 100 200 300 400 500 600

0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0

Scott Bond [J/m2]

Density [kg/m3]

Scott Bond vs. Density, SCAN sheets

Broke CTMP