Evaluation of Furnishes for Tissue Manufacturing

Johan Kullander

Karlstad University Studies | 2012:42

Chemical Engineering

Faculty of Technology and Science

Karlstad University Studies | 2012:42

Evaluation of Furnishes for Tissue Manufacturing

Johan Kullander

Distribution:Karlstad University Faculty of Technology and ScienceDepartment of Chemical EngineeringSE-651 88 Karlstad, Sweden+46 54 700 10 00

© The author

ISBN 978-91-7063-449-9

Print: Universitetstryckeriet, Karlstad 2012

ISSN 1403-8099

Karlstad University Studies | 2012:42

LICENTIaTE ThESIS

Johan Kullander

Evaluation of Furnishes for Tissue Manufacturing

www.kau.se

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Abstract

Water removal on a tissue machine becomes progressively more difficult and expensive in each successive zone. Since a large part of the cost is allocated to the drying section, more extensive water removal in the wet end may lead to huge savings in the manufacturing process. This can be accomplished by selecting suitable raw materials and optimizing the treatment of the fibres in the furnish. The aim of the work described in this thesis was to investigate the influence of three particular furnish properties on the dewatering of low grammage paper in the forming and press section: fibre species, beating and additives. The focus was to evaluate how the solids content varies as these furnish properties are changed, and also how the quality of the end product is affected. Water removal during suction is affected by the choice of pulp due to structural differences in the networks caused by differences in the morphology of the fibres. The total area of straight pores between the fibres is much greater for softwood pulps than for hardwood pulps, and this facilitates the transport of both water and air through the sheet. Beating has a negative effect on the solids content reached in vacuum dewatering, due to internal and external fibrillation of the fibres. Water removal during pressing is affected by the choice of pulp controlled by the pore structure of the fibres and their ability to sorb water. More available water before pressing leads to more water that can be removed. Beating mainly delaminates macropores with little effect on the micropores. Both water between the fibres and water in macropores is removed during wet pressing. The dryness after wet pressing is increased by the addition of a wet strength agent (PAE) to the stock, probably due to crosslinking in the fibre wall. PAE-resins decrease the volumes of both micro- and macropores, leaving less water in the fibre wall. Tensile strength is increased with a wet strength agent and further increased by the addition of a flocculant and a micropolymer to the stock. The absorption capacity is reduced by the addition of PAE-resins due to the formation of covalent bonds in the fibre wall.

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Sammanfattning

Vid framställning av mjukpapper blir avvattningen allt mer komplex och kostsam i varje successiv zon. Eftersom en stor del av kostnaden är allokerad till torkpartiet på tissuemaskinen kan en ökning av torrhalten i våtändan leda till stora besparingar i tillverkningsprocessen. Detta kan uppnås genom att välja lämpliga råmaterial eller genom att optimera behandlingen av fibrerna i mälden. Syftet med arbetet som beskrivs i denna avhandling var att undersöka inverkan av tre specifika mäldegenskaper på avvattning i vira- och presspartiet: fiberslag, malning och additiv. Fokus var att undersöka hur torrhalten påverkas när dessa egenskaper ändras, men också vilken effekt de har på produktegenskaperna. Vakuumavvattning påverkas av valet av fiberslag, vilket kan förklaras med strukturella skillnader i fibernätverken orsakade av skillnader i fibrernas morfologi. Den totala arean raka porer mellan fibrerna är mycket högre i ark producerade av långfibermassor jämfört med kortfibermassor vilket underlättar transport av både vatten och luft genom papperet. Malning inverkar negativt på torrhalten i vakuumavvattning vilket kan kopplas till intern och extern fibrillering av fibrerna. Torrhalten efter pressning påverkas av valet av massa och kontrolleras av fibrernas porstruktur och således deras förmåga att absorbera vatten. Mer tillgängligt vatten innan pressning leder till att mer vatten kan avlägsnas. Malning delaminerar huvudsakligen makroporer med liten effekt på mikroporer. Både vatten mellan fibrerna och vatten i makroporer avlägsnas under pressning. Torrhalten efter pressning kan ökas genom tillsats av våtstyrkemedel (PAE) till mälden, vilket troligtvis kan förklaras av tvärbindning i fiberväggen. PAE minskar volymen av både mikro- och makroporer vilket lämnar mindre mängd vatten i fiberväggen. Dragstyrkan kan ökas genom tillsats av våtstyrkemedel och ytterligare genom tillsats av en flockulant och en mikropolymer till mälden. En lägre absorptionsförmåga erhålls vid tillsats av PAE på grund av formering av kovalenta bindningar i fiberväggen.

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List of papers

The following publications are included in this licentiate thesis. Paper I Kullander, J., Nilsson, L., Barbier, C. (2012): Evaluation of furnishes for tissue manufacturing: Suction box dewatering and paper testing, Nord. Pulp Paper Res. J. 27(1), 143-150 Paper II Kullander, J., Nilsson, L., Barbier, C. (2012): Evaluation of furnishes for tissue manufacturing: Wet pressing, Accepted for publication in Nord. Pulp Paper Res. J. Paper III Kullander, J., Nilsson, L., Barbier, C.: Evaluation of furnishes for tissue manufacturing: Additives, In manuscript Results related to this thesis have been presented at the following conferences:

1. Kullander, J., Nilsson, L., Barbier, C: The impact of furnish composition on vacuum dewatering in tissue manufacturing, Poster presentation, 2011 TAPPI PaperCon Conference, Cincinnati, USA, 1-4/5-2011

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The authors’s contribution to the papers

Paper I The experimental work was performed by Johan Kullander with the exception of the scanning electron microscope studies that were conducted in collaboration with Dr Christer Burman at Karlstad University.

Paper II The press trials were performed by Johan Kullander and Christian

Oraassari at Aalto University. The remainder of the experimental work was done by Johan Kullander.

Paper III Johan Kullander carried out all the experimental work.

Johan Kullander is the main author of these three papers.

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Table of contents

Abstract ...........................................................................................................................i

Sammanfattning ........................................................................................................... ii

List of papers ............................................................................................................... iii

The authors’s contribution to the papers ............................................................ iv

Table of contents ......................................................................................................... v

List of figures .......................................................................................................... vi

List of Tables ......................................................................................................... vii

Abbrevations ............................................................................................................. viii

1. Introduction ....................................................................................................... 1

1.1 Overview of tissue production ................................................................... 1

1.1.1 Raw materials and stock preparation .......................................................... 2

1.1.2 Headbox and forming systems .................................................................... 2

1.1.3 Pressing ..................................................................................................... 3

1.1.4 Drying ....................................................................................................... 4

1.1.5 Creping ..................................................................................................... 5

1.1.6 Converting ................................................................................................. 6

1.2 Wood fibres ................................................................................................... 6

1.2.1 Fibre structure and composition ...................................................................... 6

1.2.2 Water in fibres .............................................................................................. 8

1.3 Beating ............................................................................................................ 9

1.4 Suction box dewatering .............................................................................. 10

1.5 Wet pressing ................................................................................................ 12

1.6 Additives for tissue manufacturing .......................................................... 16

1.7 Tissue paper properties .............................................................................. 19

1.7.1 Bulk ....................................................................................................... 19

1.7.2 Tensile strength ........................................................................................ 19

1.7.3 Wet strength ............................................................................................ 20

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1.7.4 Absorption .............................................................................................. 20

1.7.5 Softness ................................................................................................... 21

2. Aim ..................................................................................................................... 22

3. Materials and methods ................................................................................. 23

3.1 Raw materials .............................................................................................. 23

3.2 The vacuum dewatering apparatus ........................................................... 24

3.3 The press simulator (Material Test System) ............................................ 25

3.4 Water retention value (WRV) ................................................................... 25

3.5 Thermoporosimetry with differential scanning calorimetry ................. 25

3.6 Paper testing ................................................................................................ 26

3.6.1 Bulk ....................................................................................................... 26

3.6.2 Tensile properties ..................................................................................... 26

3.6.3 Wet strength ............................................................................................ 26

3.6.4 Absorption capacity ................................................................................. 27

4. Summary of papers ........................................................................................ 28

4.1 Paper I - Evaluation of furnishes for tissue manufacturing; suction box dewatering and paper testing ............................................................................... 28

4.2 Paper II - Evaluation of furnishes for tissue manufacturing; wet pressing ................................................................................................................... 30

4.3 Paper III – Evaluation of furnishes for tissue manufacturing; additives .................................................................................................................. 31

5. Conclusions ..................................................................................................... 33

6. Future studies.................................................................................................. 34

7. Acknowledgements ....................................................................................... 35

8. References ........................................................................................................ 36

List of figures

Figure 1. A simplified drawing of a tissue machine............................................................ 1 Figure 2. Pressure curves of a roll press and a shoe press ................................................ 4 Figure 3. The mechanism of creping ................................................................................... 5 Figure 4. The structure of a wood fibre cell wall................................................................ 7

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Figure 5. Location of water in a saturated web of fibres .................................................. 8 Figure 6. The effects of beating on the fibre. ..................................................................... 9 Figure 7. The mechanisms of suction box dewatering .................................................... 10 Figure 8. The four phases of pressing and the three pressure distribution curves...... 13 Figure 9. Deformation of the Kelvin element .................................................................. 14 Figure 10. Cross-section image of a low grammage sheet............................................... 16 Figure 11. The chemical reaction between the azetidinium group and cellulose. ........ 17 Figure 12. SEM-picture of low grammage sheets ............................................................. 20 Figure 13. The vacuum dewatering apparatus ................................................................... 24 Figure 14. Demonstration of differences in the pore structured networks .................. 28 Figure 15. SEM-picture of a spruce pulp beaten at 2000 revolutions ........................... 29 Figure 16. Water removed by pressing as a function of total available water .............. 30 Figure 17. Amount of water (W) as a function of pore radius. ...................................... 32

List of Tables

Table 1. Fibre characterization of the four pulps ............................................................ 23 Table 2. Properties of non-creped low grammage papers ............................................. 29 Table 3. Location of water before pressing ...................................................................... 31 Table 4. Properties of non-creped low grammage papers ............................................. 32

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Abbrevations

CD Cross Direction CTMP ChemiThermoMechanical Pulp DADMAC DiAllylDiMethyl Ammonium Chloride FSP Fibre Saturation Point L Lumen MD Machine Direction ML Middle Lamella MW Molecular Weight P Primary wall PAE PolyAmide-Epichlorohydrin S1/S2/S3 Secondary wall SC Solids Content SEM Scanning Electron Microscope SR Schopper Riegler TAD Through Air Drying TSA Tissue Softness Analyzer WRV Water Retention Value

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1. Introduction

Tissue is a category comprising products made from low grammage, dry creped and some non-creped papers such as toilet paper, kitchen towels, napkins, facials, handkerchiefs, hand towels and wipes. Even though tissue is recognized by its low grammage, the properties of the paper may vary a lot depending on the type of end product. High wet strength is for instance very important for kitchen towels while it is an undesirable property in toilet papers. Softness is very important for facials and toilet papers but is less important for towels. Tissue has been used for hygienic purposes since the 1940s and its manufacture is today a fast growing industry in the world.

1.1 Overview of tissue production

On a tissue machine, the web made of wood fibres is formed and dewatered from an ingoing solids content (SC) of 0.1-0.5% to a dry tissue paper, Figure 1. The difference between a tissue machine and a conventional paper machine is mainly the length, but features associated with tissue manufacturing are also the Yankee cylinder drying and the creping process. There are a number of different machine concepts in tissue machines running today. This chapter gives a brief overview of the concepts of tissue production and the most general machine configurations used.

Figure 1. A simplified drawing of a tissue machine. (Redrawn from United States patent

6921460).

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1.1.1 Raw materials and stock preparation

Both virgin and recycled pulps are used in tissue production, Kimari (2000). Softwood pulps normally consist of spruce or pine while hardwood pulps consist of birch, eucalyptus or aspen. Virgin pulps for tissue are produced chemically by either the sulfate or sulfite process. Chemithermomechanical pulp (CTMP) can also be added to the stock to make the paper more absorbent and give it greater bulk. Recycled pulps for tissue production are mainly made from office waste or mixed waste (newspapers and magazines). Selected recycled fibres from chemical pulp fibres (office waste) give an end product with better quality and especially a higher brightness. Depending on the requirements of the end product, the pulp can be bleached, but this is most common for virgin pulps. Bleaching increases the brightness but may also enhance the absorption properties of the end product. The level of impurities is also decreased by bleaching and this is very important for sanitary products like tissue. Virgin pulps are often subjected to moderate refining to increase the tensile properties of the final product. The energy input in the refiners is normally lower than in conventional papermaking since it is desirable to preserve the bulk and softness. Agents that can improve the runnability or the paper characteristics are normally added to the stock. The most common additives are wet strength resins, pigments, tinting dyes and chemical brighteners. Antifoaming agents, chemicals for pH control and retention aids further improve the runnability of the tissue machine. All the chemicals used in tissue production must be approved for use in conjunction with food.

1.1.2 Headbox and forming systems

Headboxes on tissue machines are designed for relatively low concentrations but otherwise they are similar to the headboxes used on conventional paper machines, Gavelin et al. (1999). The purpose of the headbox is to distribute the stock evenly across the wire to achieve a good formation. Turbulence is introduced in the pressurized box to prevent flocculation of the fibres. The cross direction (CD) profile is controlled by sectionalized slice screws and/or by sectionalized dilution across the headbox, Karlsson (2006). The orientation of the fibres can be controlled by the ratio of the jet speed to the wire speed. In a multilayer headbox, the stock can be divided into two or three layers to control the characteristics of the paper, Kimari (2000). For instance, one layer can be made up of a softwood pulp to add strength to the paper while the other

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layer can consist of a hardwood pulp to promote formation, optical properties and printability. The forming section on a tissue machine can be constructed in many ways but four different types of system are typically recognized, Kimari (2000). The crescent former is today the most commonly used configuration. The stock is sprayed between a wire and felt which results in superior runnability since no transfer between wire and felt is needed. The machine speed can be up to 2200 m/min. Twin-wire machines are also commonly used where the stock is sprayed from the headbox in between two wires where it is dewatered effectively. Suction breast roll machines were dominant until the 1970s in which water removal is controlled by suction where water is forced out of the web in the space between the top of the headbox and the wire. The oldest tissue machines are Fourdrinier machines where dewatering is one-sided by gravity and suction. The machine speed is limited since friction between mix and air tends to create an unstable liquid surface. Vacuum systems are often used in the production of tissue via suction boxes or suction rolls to increase the dryness to the press section for improved runnability.

1.1.3 Pressing

On a conventional tissue machine, the sheet is carried on a felt at a dryness of 15-25% and transferred to the Yankee cylinder. The nip pressure is typically 2 to 4 MPa in the production of tissue. Pressing is typically done with one or two press rolls against the hot Yankee cylinder. When two press nips are used, the first roll is normally a suction roll. The use of a single press roll has however recently become the most common press configuration since it gives an end product with higher bulk and softness. The use of a shoe press in the nip is also becoming increasingly common. The nip of the shoe press is longer than a conventional press roll and the nip pressure is at the same time lower, providing bulk to the web, Gavelin (1999) Figure 2. The pressure drop after the maximum nip pressure is also much steeper, reducing rewetting of the web.

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Figure 2. Pressure curves of a roll press and a shoe press.

1.1.4 Drying

Drying in tissue manufacturing is done on a steam-heated Yankee cylinder that increases the solids content of the web from 40-45% to 94-98%, Gavelin et al. (1999). The cylinder is designed to meet the need, but since the drying capacity has increased with faster machines in recent years, the cylinders have become bigger and bigger. A typical Yankee cylinder is 5-6 m in diameter with surface temperatures close to 100°C. The Yankee supplies energy for efficient drying of the web but has three other functions:

- To transport the sheet during the drying process - To function as a roll in the hot pressing operation - To provide the base for the creping process

Heat is provided by steam that enters the cylinder through the front journal. The steam condenses on the walls of the inner surfaces of the cylinder and is picked up by small pipes. The condensate is collected in transversal headers and led through long bent pipes to the internal shaft and exits the cylinder at the back journal. A Yankee hood is normally found in connection with the Yankee cylinder and this blows hot air onto the web at temperatures up to 550°C, Kimari (2000). The Yankee cylinder is also equipped with a chemical spray system in which adhesives and release agents are sprayed onto the surface to give a higher contact area between the sheet and a good separation of the sheet from the cylinder during the creping process.

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Tissue can also be manufactured with the Through Air Drying (TAD) process. In this process, the tissue machine has no press section. Water removal before the drying section is instead achieved with vacuum dewatering on suction boxes to a dryness of about 25%. The web is then transferred to a perforated through air drying cylinder where air is blown through the web as it passes. The partially dried web is finally transferred to a Yankee cylinder where it is dried and creped. This process greatly increases the bulk, softness and absorption of the tissue paper. The energy use in a TAD-process is however extremely high compared with a conventional process through its high demands for gas and electricity. Hybrid systems have therefore arrived recently to preserve the bulk at a lower energy cost.

1.1.5 Creping

Creping is probably the process element that is most associated with the manufacturing of tissue. A doctor blade is used to scrape the web off from the Yankee cylinder. The energy from the blade wrinkles the paper and breaks the physical structure of the sheet, Boudreau (2009). Microfolds are created and piled up on top of each other, and when the pile is high enough it falls down and creates a macrofold, Figure 3. This mechanism is repeated continuously on the web resulting in a structured end product. Creping gives the paper higher bulk, softness and stretch, Gavelin et al. (1999).

Figure 3. The mechanism of creping (exaggerated).

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1.1.6 Converting

Tissue converting includes embossing, printing, perforation, winding and tail sealing, log sawing, folding and packaging, Kimari (2000). Most tissue products are embossed where the plies are pressed together either completely or just at the edges. The purpose of embossing is to increase the softness and absorbency of the end product. Traditionally, the plies are embossed simultaneously by two rolls of steel or rubber. The plies can also be embossed separately and then bonded together, termed nested embossing. This technique gives an end product with higher bulk. A more advanced form of nested embossing is foot-to-foot embossing which increases the absorption properties of the end product. Suction pockets are created between the plies since the highest points of the surface patterns on the rolls are situated opposite each other. Printing is done on the converting line or on a separate printing press for decorative or informative purposes. Most converting lines are capable of printing up to four colors, although some lines exist with up to eight colors. Perforation is done on the converting line for tissue products on rolls to make the sheets easier to separate. Products such as toilet paper and kitchen towels get their final shape by winding where the desired amount of paper is wound onto a paper core and sealed at the tail. The rolls are then cut into the desired width, typically 10 to 60 cm. Sheet products such as facial tissue and paper towels are instead folded to give the desired shape before packaging. Packaging is finally done by wrapping the products in plastic, paper or boxes.

1.2 Wood fibres

1.2.1 Fibre structure and composition

Wood fibres contain layered walls with a complex inner structure and chemical composition. The lumen (L) is the central cavity which is surrounded by the inner layer (S3), the main layer (S2) and the outer layer (S1) of the secondary wall followed by the primary wall (P). The region between the fibres which holds them together is the middle lamella (ML), Figure 4.

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Figure 4. The structure of a wood fibre cell wall. (Redrawn from Karlsson 2006).

The fibre wall is composed mainly of cellulose, hemicellulose and lignin. Depending on the species, the amounts of these different components vary between 40 and 50%, 15 and 30% and 20 and 30% respectively, Karlsson (2006). The secondary wall consists mainly of cellulose and hemicellulose while the primary wall and middle lamella are composed more or less of lignin. Cellulose is a straight-chain polymer consisting of glucose units and is the main component of the fibre. Cellulose is insoluble in most solvents and is the principal source of fibre-to-fibre bonding in paper. Hemicellulose is built up of branched molecular chains of glucose and other monosaccharides. This polymer can be removed by mild chemical action but is very important to promote bonding between the fibres and increase the adhesion to the Yankee cylinder. The hemicellulose chains are much shorter than cellulose chains. Lignin is found between the fibres and within the fibre wall. Lignin is a complex molecule that can be made soluble by chemical action. Lignin prevents the formation of bonds between the fibres. Besides the three main components extractives are also present in the wood. In chemical pulping, the fibres are liberated by the degradation and dissolution of the lignin in the middle lamella. The primary wall and the S1-layer are also affected but may remain after the pulping process depending on the yield. The fibres can also be liberated by a mechanical process where most of the lignin is preserved. CTMP is produced by a combination of these processes.

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1.2.2 Water in fibres

In a saturated web of fibres, water is located in the pores between the fibres, in the fibre lumen and in the porous structure of the fibre wall, Laivins and Scallan (1994), Figure 5.

Figure 5. Location of water in a saturated web of fibres.

In the fibre wall, the water is classified as non-freezing and freezing water in micropores and bulk water in macropores, Maloney and Paulapuro (1999). The micropores are voids within the fibre wall where the water has different thermodynamic properties, i.e. a depressed melting temperature. Micropore water includes water that is absorbed into the amorphous regions of the fibre wall. About 0.15-0.20 g water / g fibre is attached to the cellulose by relatively strong bonds, termed hydration water, Goring (1977). This water is dependent on the fraction of accessible cellulose, is unaffected by beating and is expected to be more difficult to remove than the rest of the water in the fibre wall. The amount of non-freezing water is thought to be related to the number and type of accessible hydration sites, Maloney and Paulapuro (1999). Macropores contain water that has thermodynamic properties similar to those of bulk water. This water is found in pores outside the amorphous regions of the fibre wall which are too large to cause a melting temperature depression. Only micropores are originally present in wood fibres while macropores are formed by the dissolution of lignin and hemicelluloses during chemical pulping. The amount of water held by the fibre depends on the kind of pulp, on the degree of beating and on the chemical environment, Lindström and Carlsson (1982).

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1.3 Beating

Beating is commercially used to increase the strength properties of the paper by making the fibres more flexible and increasing the available bonding sites of the fibres, Annergren and Hagen (2007). The changes in fibre properties induced by beating depend on a number of different parameters such as the type of refiner, furnish properties and energy input. In Figure 6, the different effects of beating on the fibre can be seen. Generally speaking, both internal (A) and external fibrillation (B) are considered to be positive effects since they lead to an increase in paper strength. Internal fibrillation makes the fibres swell by delaminating the cell wall, thereby making the fibres more flexible. At the same time, their capacity to hold water is increased and this may impair dewatering. External fibrillation makes parts of the outer layers of the fibres unravel, resulting in an increased relative surface area of the fibres. This may enhance paper strength but slow down dewatering since the network becomes more closed. Beating may also cut the fibres (C) which is unwanted since strength is then lost. Longer fibres have more contact points available for fiber-fiber bondings.

Figure 6. The effects of beating on the fibre. (Redrawn from Annergren and Hagen 2007).

Fines creation (D) is generally seen as an unwanted effect since it has a negative influence on dewatering. Fines have a much larger specific surface area than fibres and have been shown to carry almost twice the amount of water per unit dry mass as fibres, Laivins and Scallan (1996). Due to their size, fines also block channels between fibres where water can be drained. Fines can be classified as primary fines (present before beating) and secondary fines (created by beating). Primary fines have poor bonding properties while secondary fines improve the

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strength of the paper. However, primary fines have been shown not to affect the dewatering to the same extent as secondary fines, Cole et al. (2008). Beating also deforms the fibres (E) by either straightening or curling. Straighter fibres have a better ability to withstand load since deformed fibres remain in an unloaded condition when subjected to a tensile or compressive load. A larger amount of straight fibres consequently contributes to a higher strength of the paper.

1.4 Suction box dewatering

Vacuum systems are essential parts of tissue and paper machines used in the wire, press or drying sections depending on the configuration of the machine, Räisänen (1998). In the wire section, suction boxes and couch rolls are used to form the web by dewatering. In tissue manufacturing, suction boxes are normally associated with TAD-machines since there is no press section and vacuum is a crucial element to raise the solids content before drying. There are however other configurations of tissue machines where the web is dewatered through some kind of vacuum system. It is therefore important to understand the mechanisms behind the process of vacuum dewatering. Vacuum dewatering on a suction box takes place by three mechanisms: compression dewatering, displacement dewatering and rewetting, Åslund and Vomhoff (2008), as shown in Figure 7.

Figure 7. The mechanisms of suction box dewatering. (Redrawn from Åslund and Vomhoff

2008).

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When the web first passes over the suction box, the pressure drop leads to compression of the fibre network forcing water out from the network, Campbell (1947). The dewatering is dependent on the compressibility of the pulp which in turn is dependent on the properties of the fibres in the network. As long as the pores between the fibres are filled with water, the air pressure will act to compress the web. As the water is removed, the web becomes less and less compressible. At some stage, when the solid fraction has increased to a certain limit by decreasing the size of the water-filled pores, air starts to penetrate the web. Räisänen et al. (1995) suggested that water removal by vacuum is mostly due to compression of the web, while airflow only marginally removes water. The time between the application of the suction pulse and the air penetration was experimentally determined by Granevald (2005). The “time to air penetration” was in the range of 0.2 to 1.9 ms for sheets of grammages between 17 and 51 g/m2. A higher grammage led to a longer time for the air to penetrate, while an increase in vacuum level gave a shorter time to air penetration. In displacement dewatering, the applied pressure drop must exceed the capillary pressure to allow air to break through the initially saturated web, Räisänen (1996). Water removal through displacement can be seen as a viscous drag of air flowing rapidly through the network. After the web has passed over the suction box, expansion of the web takes place and rewetting from the fabric into the web can occur. Mcdonald (1999) examined rewetting from a couch roll at grammages varying from 25 to 88 g/m2 and found that a considerable amount of water is transferred from the wire to the web at the time of separation. To compensate for the rewetting he suggested augmenting the drainage forces in the forming unit or using a finer surface of the forming fabric. Åslund et al. (2008b) studied external rewetting of sheets made of both mechanical and chemical pulps with grammages ranging from 50 to 200 g/m2. A considerable rewetting was observed to occur primarily directly after the end of the suction pulse and this accounted for a dry content decrease of between 3 and 6%. Evaluation of vacuum dewatering with different set-ups has been undertaken by several researchers, e.g. Britt and Unbehend (1980), Neun (1994), Räisänen et al. (1996), Granevald (2005) and Åslund et al. (2008a). Based on pilot machine and laboratory data, studies support the fact that the sheet solids content increases with increasing dwell time at a given vacuum level. The dewatering occurs at a faster rate initially and levels off to a final sheet solids

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content as the dwell time is increased. With a higher applied vacuum level, a higher solids content can be achieved. Räisänen et al. (1996), Mitchell and Johnston (2000) and Åslund et al. (2008a) all carried out dewatering studies on mechanical and chemical pulps. They found that a higher solids content was reached with chemical pulps, explained by the higher compressibility. Cheng and Paulapuro (2006) performed both low and high vacuum trials on a wheat straw pulp and compared the results with those from a pine and birch pulp. The wheat straw pulp showed by far the lowest dryness at both vacuum levels. This was explained by the denser web structure created by its slender and short fibres and also its high fines content. The solids content was negatively affected by beating due to fines creation. Wang (2006) claimed that chemical pulp refining should aim at increasing internal fibrillation, straightening fibres, and minimizing the amount of fines and external fibrils in order to achieve an optimum combination of dewatering and tensile strength.

1.5 Wet pressing

Wet pressing is an operation in which water is expelled from the wet fibre web by mechanical compression. In addition to water removal, pressing is important for a permanent densification of the fibre network, Paulapuro (2000). Research in wet pressing has gained a lot of attention in the past. One of the reasons is probably the great potential for improving the energy efficiency. Around 80% of the cost on a paper machine is allocated to the drying section. The energy for drying is roughly proportional to the amount of water evaporated, so that an increase in dryness of 1% after the last press will reduce the drying energy demand by 3-4%. Important process variables for wet pressing are the nip pressure, nip residence time, temperature, ingoing moisture content and sheet properties. Important equipment parameters are the nip type, roll parameters, felt parameters and press section configuration. One of the most widespread theories of wet pressing was developed by Nilsson and Larsson (1968). They presented a method of characterizing pulp with respect to pressability and a quantitative understanding of flow conditions in a press nip. They divided the transversal flow press nip into four phases and gave three separate pressure distribution curves for the different phases: total nip pressure, hydrodynamic pressure and fibre structure pressure, Figure 8.

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Figure 8. The four phases of pressing and the three pressure distribution curves.

(Redrawn from Nilsson and Larsson 1968).

In the first phase, the total pressure on the sheet starts to grow but no water flows since there is still air in the sheet. No hydraulic pressure is built up until the saturation point has been reached. The entire pressure on the sheet is taken up by the fibre structure itself. In the second phase, the saturation point is reached and water starts to leave the sheet. The hydraulic pressure starts to build up. The pressure in the fibre structure increases as long as the dryness of the sheet increases. The hydraulic pressure reaches its maximum before the maximum point of the nip pressure curve is achieved. In the third phase, the total pressure decreases and the pressure on the fibre structure reaches its maximum, which corresponds to the point of highest dryness in the nip. In the final phase, the web starts to expand and rewetting occurs until the web and the felt are separated. In wet pressing, two types of nip are often identified, compression-controlled and flow-controlled nips, Wahlström (1960). In a compression-controlled press nip, the outgoing dryness is constant regardless of ingoing conditions. In a flow-controlled press nip, on the other hand, the amount of water removed is constant regardless of the ingoing dryness. Compression-controlled conditions typically arise with thin webs with a low moisture content, whereas a combination of thick webs and high moisture content makes flow-controlled conditions more likely. In reality, a combination of these two limiting cases is most likely to occur. The two extreme cases are often illustrated by a spring and a dashpot as seen in Figure 9.

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Figure 9. Deformation of the Kelvin element when either the spring or the dashpot

determines the compression response. (Redrawn from Carlsson 1983).

In the so called Kelvin model, the applied pressure is divided between an elastic element (spring) and a viscous element (dashpot), Ward (1979). The load borne by the fibre constituents is assumed to correspond to the load on the spring (compression-controlled) while the load borne by the water is assumed to correspond to the load on the dashpot (flow-controlled). The maximum dryness is obtained when the web is subjected to the maximum pressure in a compression-controlled situation but it is achieved at the end of the pulse in flow-controlled situations since the springback is lacking. Carlsson (1983) found that the flow resistance in the web increases when the solids content is increased by a higher nip pressure, generating a higher hydrodynamic pressure in the structure. When the residence time in the nip is increased, the time-dependent flow phenomena become less important than the elastic modulus of the fibres. Thus, with longer dwell times, the web exhibits a more compression-controlled behavior. The hydraulic pressure increases with beating and the total pressure is taken up almost completely by the water in the structure in the case of highly beaten fibres. The greater swelling of the fibres makes the process more flow-controlled, and this leads to a lower expansion of the sheet. At higher grammages, the distance for dewatering in the thickness direction of the sheet increases and this creates a higher flow resistance in the

15

pores between the fibres. A higher hydrodynamic pressure leads to a lower sheet expansion because of the increased resistance of flow of water back into the sheet. The performance of wet pressing is often coupled to the amount of water available in the walls of the fibres which is normally referred to as swelling. Water in the fibre wall can be measured in many ways, but the two most frequently used methods are the Water Retention Value (WRV) and the Fibre Saturation Point (FSP). Busker and Cronin (1984) showed that the solids content after pressing correlates very well with the WRV of the pulp. This relation supports the suggestion by Wahlstrom (1990) that the solids content after pressing is limited by the rate at which water can be removed from the cell wall in pressure controlled situations when the resistance to flow through the structure is insignificant. Maloney et al. (1998b) examined the role of fibre swelling and pore structure in wet pressing using differential scanning calorimetry, solute exclusion and static and dynamic pressing as analytical techniques. They found that the behavior of a pulp during pressing depends not only on the pore structure but also on the way in which the swelling of the fibres is changed. They also showed that nearly all the water that is removed from the cell wall comes from the macropores under ordinary press conditions, i.e. below 50% solids content. Laivins and Scallan (1994) separated the water leaving a pad of pulp into the fraction leaving the porous structure of the cell wall (intra-wall water) and that leaving the spaces external to the wall (inter-wall water). They found that both the inter-wall water and the intra-wall water start to leave the pad at the lowest applied pressure. However, the removal of inter-wall water is almost complete at about 2 MPa, leaving the cell water as the predominant form of water remaining at higher pressures. They identified this water as the factor that both controls the process and limits the extent of dewatering in a press. Moreover, they claimed that a paper web could be rendered almost completely dry in the press section if no water existed inside the fibres. He et al. (2003) examined the behaviour of fibres in wet pressing and quantified the density increase as an effect of gap closure, fibre collapse and fibre twist. They found that the three mechanisms occur simultaneously at low pressures (less than 0.5 MPa), and that gap closure is the predominant mechanism in paper structure densification. Increasing the pressure only slightly increased the apparent density, and this was believed to be due to additional twisting and

16

collapse of the fibres. Figure 10 shows a cross-section image of a sheet (20 g/m2) pressed at a nip pressure of 2 MPa.

Figure 10. Cross-section image of a low grammage sheet (Spruce)

pressed at a nip pressure of 2 MPa.

1.6 Additives for tissue manufacturing

Various additives are available on the market for use in the manufacturing of tissue. They are used to facilitate the operation of the tissue machine and to improve tissue paper properties such as wet strength, softness and water absorbency. Among the most commonly applied chemicals in the process are wet strength resins that are used to retain 10 to 30 percent of the paper’s original dry strength. Several types of wet strength resin are commercially available. The polyamide-epichlorohydrin (PAE) resins are the most widely used wet strength agents and they have optimum performance under neutral and alkaline conditions. The wet strength development of PAE-containing cellulose sheets can be attributed to the covalent ester bond formation between the carboxyl groups of the fibres and the azetidinium groups of PAE, Obokata and Isogai (2007). The reaction can be seen in Figure 11.

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Figure 11. The chemical reaction between the azetidinium group and cellulose.

Westfeldt (1979), Häggkvist et al. (1998), and Liu (2004) all claimed that the PAE-resins protect existing fibre-fibre bonds by forming a network around the fibres or by creating covalent bonds between and within the fibres. The urea-formaldehyde and melamine-formaldehyde resins were the first synthetic wet strength agents, and these were used under acidic papermaking conditions. They act to retard loosening of the fibre-fibre bonds by forming a network around the fibres. The use of these resins has significantly declined in the recent decades due to environmental concerns of formaldehyde and the reduced need for its application in acidic medium, Peters (2000). Liu (2004) studied the effect of a wet strength agent (PAE) on various sheet properties of light weight paper. It was found that the wet strength agent significantly improved sheet strength and wet strength while the absorbency and softness decreased. Another common class of wet end chemicals used in tissue manufacturing are debonders. These additives are added to increase the softness with a loss of strength as a result, Liu and Hsieh (2000). The function of debonders is to sterically hinder the bonding between the fibres in a web. The traditional cationic debonders are usually quaternary ammonium compounds. Liu (2004) showed that debonders reduce the strength and stiffness while improving the sheet softness. The application of a debonder together with a wet strength resin was found to combine the advantages of individual additives and to produce sheets with high wet strength, low dry strength and high softness. Softness can also be achieved by softeners or lotions that are sprayed onto the web and affect the surface characteristics of the paper.

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To obtain a sufficient dewatering on the tissue machine, chemical retention and drainage programs can be created. Räisänen et al. (1995) studied the effect on high-vacuum dewatering of adding a retention aid (cationic polyacrylamide). They found that the airflow through the sheet was increased due to the poorer formation achieved. Despite the increased airflow, no differences in solids contents could be seen. Wågberg et al. (1990) investigated the effects of retention aid systems on the retention and dewatering of a wheat-straw pulp. Some of the systems used gave an increase in both retention and rate of dewatering while others increased the retention without any effect on the rate of dewatering. The suggested explanation of the improved dewatering was that crosslinking of the additives decreased the size of formed flocs. Springer et al. (1991) examined the influence of numerous commercial additives on dynamic pressing, and they showed that the primary deterrent to water removal by pressing is the amount of water held in the fibre wall. The chemical additives did not seem to influence the dryness after pressing since they lacked the ability to penetrate the fibre wall. Wågberg et al. (1990) conducted a number of press experiments with different retention aids and found that none of the additives had any effect on the maximum dryness reached in the press. They were however able to deswell a wheat straw pulp by adding a highly charged low molecular mass polymer. Swerin et al. (1990) studied the effect of cationic polymers on the degree of swelling of hardwood kraft fibres. Two highly charged cationic polymers were used, polybrene (9000 MW) and poly-diallyldimethyl ammonium chloride (DADMAC) (200,000 MW). The deswelling of the fibres by both polymers was substantial but it was larger in the case of the polybrene which had access to all the charged groups. The deswelling was explained by an ion-exchange process between the cationic polyelectrolyte and the cationic counterions to the ionic groups in the fibre wall. The increase in maximum solids content reached during wet pressing showed a good correlation with the decrease in WRV on addition of polymer. An increase in press solids of 5% or more was observed in the highly swollen fibers. Stratton (1982) carried out press dewatering experiments with a number ot different additives. An increase in solids contents of 1-2% was achieved. High charge density polymers (PEI) produced better results than low charge density polymers (5 mole % cationic acrylamide copolymers). The mechanism for the effect was not determined.

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1.7 Tissue paper properties

1.7.1 Bulk

Bulk is an important property of some tissue grades since the absorption and bulk softness often correlate well with the thickness of the paper, Karlsson (2006). The aim is usually to produce paper with the highest possible specific volume, leaving compaction of the fibre network as an unwanted effect. A higher bulk can be obtained by for instance using a lower pressure in the press nips, by eliminating wet pressing (TAD-process) or by using a milder beating of the fibres.

1.7.2 Tensile strength

Tensile strength is determined by measuring the force required to break a narrow strip of paper stretched at a constant and specified speed, Karlsson (2006). The tensile strength of paper depends to a high degree on the fibre properties. Beating of the pulp increases the tensile strength since the fibres become more flexible and conform to each other, improving their bonding abilities. Fibre length is an important property to enhance tensile strength since the number of bonding sites on each individual fibre is greater on a long fibre. The fibre length is not however the only morphological parameter influencing the tensile strength, Vincent et al. (2010). Another important factor is the number of fibres. The higher number in hardwood pulps can compensate for the shorter fibre length. The advantage of softwood pulps for tensile strength has also been reported to diminish as the grammage is decreased as for tissue grades, I’Anson et al (2007). This is probably a consequence of structural differences at low grammages. The probability of encountering a weak spot must for example be higher in a softwood sheet due to the more open network structure. The relative bonded area may also be higher in hardwood sheets due to better fibre coverage, Figure 12.

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Figure 12. SEM-picture of low grammage sheets. Left: Sheet made of softwood pulp (Spruce), Right: Sheet made of hardwood pulp (Eucalyptus).

1.7.3 Wet strength

The purpose of most tissue products is to be wetted. A sufficient wet strength is therefore required to prevent the paper from falling apart when it is moist or soaked. The hydrogen bonds between the fibres are water sensitive and can easily be disrupted by water molecules so that more than 90% of the paper’s original strength is lost, Liu (2004). To achieve a sufficiently high wet strength of tissue products, wet strength resins like polyamide-epichlorohydrin (PAE) must be used. They protect existing fibre-fibre bonds by forming a network around the fibres and they create covalent bonds between and within the fibres, Häggqvist et al. (1998).

1.7.4 Absorption

Absorption is a very important property for a lot of tissue products where the purpose is to wipe up liquid of some sort. Absorption is generally divided into absorption capacity and absorption rate. The absorption capacity reflects how much water the paper can absorb (g water / g fibre) while the absorption rate measures how quickly the product can absorb water, Gavelin et al. (1999). Single-ply tissue products made from recycled fibres normally absorb around 4 g/g while premium tissue grades can reach absorption capacities as high as 18 g/g, Paulapuro (2006). The absorption properties are highly influenced by the chemical properties of the fibre surface and can be controlled by the choice of fibres, beating and additives. A more porous structure of the web is often related to a higher absorption since there are more voids in the sheet in which water can be held.

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1.7.5 Softness

Softness is a subjective property for which there are few generally accepted methods of measurement. Softness is often divided into how velvet-like the paper is (surface softness) and how easily it yields when crumbled (bulk softness). Bulk softness can be predicted by some of the physical properties of the paper such as bending stiffness or tensile stiffness. Another method is the use of softness panels where both the bulk softness and surface softness are assessed on a scale by comparing different grades. In 2004, a new measuring method was developed by Emtec, Germany, that was found to correlate well with results obtained from human test panels, Gruner (2011). The Tissue Softness Analyzer (TSA) is the first measurement device able to provide accurate information about the surface softness. It collects data regarding the three parameters that have the greatest influence on the human sensation of hand feel: fibre softness, texture and sheet stiffness. It then uses special algorithms to calculate standard hand feel values based on the three parameters together with the grammage and the thickness of the paper.

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2. Aim

The aim of the work described in this thesis has been to test three hypotheses relating to how furnish properties influence dewatering of low grammage paper. The three hypotheses are:

- During water removal by suction or wet pressing, different fibre species influence dewatering due to differences in morphology.

- Beating negatively influences vacuum and press dewatering due to the delaminating effect on the fibre wall and the creation of fines.

- Additives can be used to raise the dryness in the wet end without any detrimental effect on the tissue paper properties.

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3. Materials and methods

3.1 Raw materials

Four different pulps have been used in this work. The fibre characteristics of the pulps can be seen in Table 1. Table 1. Fibre characterization of the four pulps.

Södra black 85Z A northern bleached softwood kraft pulp from roundwood (70% spruce, 30% pine). The fibres are thin-walled and collapse easily which gives good bonding properties. The fibres are easily refined, retaining softness and reducing energy. Södra black 85Z is used in product areas such as speciality and tissue paper. Södra gold eucalyptus A hardwood kraft pulp consisting of eucalyptus globulus species. The fibres are short and rather thin-walled, providing a combination of high tensile strength with little bulk trade-off. The short fibres provide high tensile strength, Z-strength, formation and light scattering properties. Södra gold eucalyptus is used in several paper applications such as free-sheets, tissue grades, board and speciality papers. Botnia RMA pine 90 A softwood kraft pulp consisting 90% pine (Pinus sylvestris) and 10 % spruce (Picea abies). The role of the pulp is to provide sufficient strength to the base sheet throughout the process. The softwood fibre requires a minimum amount of refining energy to reach the desired strength level in order to minimize generation of fines and maximize dewatering properties and bulk. Botnia RMA pine 90 is used as raw material in tissue and fine papers. Botnia AKI birch A hardwood kraft pulp made of 100% birch (Betula). 1-2% aspen is sometimes mixed in the pulp. The role of the pulp is to provide good strength and surface properties. Botnia AKI birch is a raw material for tissue, fine papers and board.

PulpPopulation

(106/g)Mean fibre length

(mm)Mean fibre width

(μm)Coarseness

(μg/m) Shape factor

(%)Fines content

(%) Kinks/fibre

Södra black 85Z 6.2 2.0 27.8 162.0 85.9 5.1 0.9Södra gold eucalyptus 26.4 0.7 16.9 66.2 91.2 4.9 0.4

Botnia RMA pine 90 4.9 2.1 29.5 169.4 85.7 3.7 0.9Botnia AKI birch 13.0 0.9 21.0 105.0 91.5 2.6 0.4

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3.2 The vacuum dewatering apparatus

This bench-scale laboratory equipment is used to perform vacuum dewatering trials and simulate the suction boxes on a tissue or paper machine, Figure 13.

Figure 13. The vacuum dewatering apparatus.

The apparatus consists mainly of a plate (1) with a slot (2), a vacuum pump (3) and a vacuum tank (4). The sheet is formed separately on a wire in a modified handsheet mould. The vacuum tank is evacuated to the desired vacuum level, after which the sheet is formed and placed in the sample holder (5). The plate with the slot is accelerated up to a constant velocity, and a suction pulse is created when the slot passes between the sheet and vacuum tank. The vacuum level can be set up to 60 kPa and the dwell time can be as short as 0.5 ms. Plates with different configurations can also be used to get desirable simulations like multiple slot configurations or wider slots. A pressure transducer (6) is placed in the pipe between the plate and the vacuum tank to make it possible to monitor the pressure during the pulse. The increase in pressure can be used to calculate the volume of air passing through the sheet and the flow rate. A more detailed description of the equipment including a validation against data from a pilot paper machine can be found in Granevald et al. (2003). A vacuum level of 50 kPa was used during the trials in this work. The trials were performed at room temperature with a 5 mm single-slotted plate at dwell times of 0.8, 1.4

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and 2 ms. These dwell times are typically used on suction boxes in the tissue manufacturing process.

3.3 The press simulator (Material Test System)

The MTS810 is a hydraulic materials testing device capable of applying 100 kN forces. The device was modified by Saukoo (2006) for wet pressing simulations and research purposes. The configuration of the pressure plates can be changed and heat can also be applied. The system is equipped with a DASYlab control program recording data with a response time of 0.2 ms. In this work, nip pressures between 2 and 6 MPa were used. Standard plates were used without heating and the residence time in the nip was around 25 ms.

3.4 Water retention value (WRV)

WRV is an empirical measure of the capacity of a test pad of fibres to hold water. The WRV-value increases with increasing beating because of internal fibrillation which occurs concurrently with the development of external fibrils, which serve to hold additional water. This test may be useful as a guide to the influence of the pulping process on the fibre produced and the papermaking potential of the pulp. It is also useful for the papermaker as a measure of the efficiency of pulping and refining. WRV was measured with a centrifuge (Hettich Universal 320R) according to ISO 23714:2007 in this work.

3.5 Thermoporosimetry with differential scanning calorimetry

To measure the hydration and swelling of fibres, thermoporosimetry in a differential scanning calorimeter (DSC) can be used. The technique is applied to enhance the understanding of how pores are formed and modified in pulping, beating and hornification. Thermoporosimetry is based on the fact that water in narrow pores has a depressed melting temperature. The principle of the isothermal melting technique is to raise the temperature in a frozen sample to a preset value where it is held constant until the melting transition is completed. The sample is then immediately cooled down and the melting is repeated at a slightly higher temperature. Maloney et al. (1998b) were first to introduce this method. In this work, thermoporosimetry was used to determine differences in pore structure between different fibre species and to evaluate the effect of beating and additives. The following thermal sequence was used: Cooling to -45°C, heating at 5°C/min to -30°C and holding for 5 minutes and then recooling to -45°C. The cycle was repeated with the isothermal melting set

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point at -8, -5, -3, -2, -1, -0.6, -0.4, -0.2, -0.1 and 0°C with enough time for the melting transition to come to completion. Temperature calibration was done with distilled water. Further details of the method used can be found in Maloney and Paulapuro (1998a).

3.6 Paper testing

In order to obtain paper properties relevant for tissue manufacturing, the paper testing included in this thesis was conducted on 20 g/m2 non-creped papers. The creping process gives the paper high stretch, absorption and softness. Another element not included in the study is embossing which increases the absorption slightly and gives the product a softer handfeel. Despite these omissions, the testing of non-creped papers was expected to give useful information about how the properties are affected by changes in the furnish, and to indicate to some extent how the end product behaviour may be predicted.

3.6.1 Bulk

The bulk was calculated from grammage and thickness measurements with a STFI thickness tester according to SCAN-P 88:01. The test piece is fed through a nip between two spherical probes at constant speed and the thickness profile is continuously recorded. The average thickness is then calculated from the profile. The bulk is calculated as the thickness divided by the grammage of the material. The length of the samples varied between 100 and 160 mm.

3.6.2 Tensile properties

The tensile properties were measured according to ISO 1924-3 in a Zwick/Roell Z005 table top machine.

3.6.3 Wet strength

Wet strength was measured according to ISO 12625-5. A polyamide-epichlorohydrin (PAE) wet strength agent, (Kymene 217LX, Ashland Hercules), was added to the stocks investigated in this work. The paper samples were cured at 80°C for 30 minutes before testing.

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3.6.4 Absorption capacity

Absorption capacity was measured by the basket immersion method according to ISO 12625-8. The weight of the test piece was reduced to 0.4 g for the low grammage paper. The isotropic sheets were cut in half after which the pieces were attached and rolled in the basket. The basket was constructed on-site according to standard dimensions.

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4. Summary of papers

4.1 Paper I - Evaluation of furnishes for tissue manufacturing; suction box dewatering and paper testing

The purpose of paper I was to investigate how different furnishes influence water removal in suction box dewatering. Four pulps beaten to different levels were investigated both separately and blended. Paper testing was included to evaluate properties relevant for tissue production. Vacuum dewatering in tissue manufacturing was found to be affected by the choice of pulp, due presumably to structural differences in the networks caused by differences in the morphology of the fibres. The structural differences were demonstrated with a light microscope. The total area of straight pores between the fibres was found to be much higher for softwood pulps than for hardwood pulps, Figure 14. A larger area of straight pores in the network facilitates transport of both water and air. These structural differences were linked to the properties of the fibres, especially the fibre length, fibre width and fibre population. The structural differences could be confirmed with air measurements during the suction pulse. More air was transported through sheets containing softwood fibres, Figure 14.

Figure 14. Demonstration of differences in the pore structured networks. Left: Light microscope pictures of the four unbeaten pulps, Right: Volume of air passing through the sheet as a function of dwell time for the four unbeaten pulps.

Beating was found to negatively influence vacuum dewatering. No secondary fines were created as an effect of beating, so internal and external fibrillation was believed to be the main cause of the decrease. External fibrillation was shown with a scanning electron microscope, Figure 15, and internal fibrillation was related to the increase in dewatering resistance with beating.

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Figure 15. SEM-picture of a spruce pulp beaten at 2000 revolutions.

Mixing of the pulps showed that birch is preferable for use rather than eucalyptus when a hardwood pulp is part of the furnish. A higher solids content could also be reached if one of the constituents in the furnish was unbeaten. The solids contents reached with the mixtures were close to the arithmetic means of their constituents. Paper testing was done on 20 g/m2 non-creped papers. Sheets made of hardwood pulps had the highest tensile strength, due presumably to the large number of fibres and greater fibre coverage. The same trend was seen for wet tensile strength. The absorption capacity of the eucalyptus pulp was lower, probably due to its less open structure. The strength and wet strength increased with beating while absorption and bulk decreased. The results of the paper testing are summarized in Table 2.

Table 2. Properties of non-creped low grammage papers (20 g/m2).

PulpDewatering resistance

(SR°)Density (kg/m3)

Tensile index (Nm/g)

Wet tensile index (Nm/g)

Absorption (g/g)

Pine 14 566 21.2 3.9 7.0

Pine 1000 rev 17 585 39.7 7.3 5.6

Pine 2000 rev 19 620 51.8 8.3 5.2

Spruce 14 560 22.2 5.6 6.6

Spruce 1000 rev 17 595 38.5 7.0 5.4

Spruce 2000 rev 20 624 50.1 12.2 4.8

Birch 15 622 20.4 3.5 7.2

Birch 500 rev 18 630 34.4 5.9 5.4

Birch 1000 rev 21 644 50.6 8.0 5.0

Eucalyptus 18 611 25.9 5.2 5.9

Eucalyptus 500 rev 27 610 32.7 9.4 5.5

Eucalyptus 1000 rev 33 612 44.9 13.0 5.0

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4.2 Paper II - Evaluation of furnishes for tissue manufacturing; wet pressing

A large part of the cost is related to the drying section on a tissue machine. The energy demand for drying is roughly proportional to the amount of water evaporated, so that an increase of 1% in dryness after the last press will reduce the drying energy demand by 3-4%. An increase in dryness after pressing can therefore be a good way to reduce the energy use in the process. In the first paper, dewatering in the wet end was investigated but the behavior of the sheet during the pressure pulse was not included. The purpose of this second paper was therefore to investigate this further, including pulp types and process conditions typically used in tissue production. The water retention value and thermoporosimetry were used to characterize the pore structure of the fibres. Wet pressing in tissue manufacturing was shown to be affected by the choice of pulp, due to differences in pore structure of the fibres and consequently to their varying ability to sorb water. Hardwood pulps showed a greater ability to retain water in the fibre wall than softwood pulps. The water in the fibre wall of hardwood pulps is however expected to be more easily removed since more water is located in macropores than in micropores. A clear correlation was found between the total available water and the amount of water removed during pressing, Figure 16. The pressure nip was believed to be more or less compression-controlled since the relation between the amount of pressed out water and the amount of total available water was linear with a gradient close to one.

Figure 16. Water removed by pressing as a function of total available water.

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Beating was found mainly to delaminate macropores with a small effect on micropores, Table 3. The increase in solids content during pressing was not significantly affected by beating under the conditions used, i.e. mild beating and low grammages. The negative effect of beating could however be seen after vacuum dewatering where a higher degree of beating led to a lower solids content. This order was maintained after pressing. Thermoporosimetry showed that both water between the fibres and water in macropores in the fibre wall was removed during pressing. Table 3. Location of water before pressing (g water / g fibre).

4.3 Paper III – Evaluation of furnishes for tissue manufacturing; additives

Additives are widely used in the tissue manufacturing process to facilitate the operation of the tissue machine and to improve tissue properties such as wet strength, softness and water absorbency. The effect of fibre properties on dewatering in suction and pressing was investigated in Papers I and II. In the third paper, the effects of four different additives on these two process elements were evaluated, together with an evaluation of paper properties. The solids content after vacuum dewatering and wet pressing was shown to be unaffected by the addition of any of the four additives used. The dryness after wet pressing was however increased by the addition of a PAE-resin to the stock, probably due to crosslinking in the fibre wall. Thermoporosimetry showed that the PAE-resin reduces the volumes of both micro- and macropores, leaving less water in the fibre wall, Figure 17. The fact that none of the four additives affected the press dryness may indicate that additives only influence pressing if they have the ability to penetrate the fibre wall.

Pulp Non-freezing water (NFW) Micropores Macropores WRV Water between fibres Total available water (TAW)Pine 0.42 0.67 0.42 1.09 3.43 4.52

Pine 2000 rev 0.45 0.74 1.04 1.78 3.51 5.29Spruce 0.43 0.71 0.36 1.07 3.25 4.32

Spruce 2000 rev 0.45 0.79 0.93 1.72 3.45 5.17Birch 0.28 0.54 0.58 1.12 2.55 3.67

Birch 1000 rev 0.29 0.59 1.06 1.65 2.72 4.37Eucalyptus 0.13 0.46 0.84 1.30 2.43 3.73

Eucalyptus 1000 rev 0.14 0.52 1.29 1.81 1.90 3.71

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Figure 17. Amount of water (W) as a function of pore radius.

The tensile index was increased by the addition of the PAE-resin and further increased by the addition of a flocculant and a micropolymer to the furnish. Wet strength was increased while absorption capacity was decreased with the PAE-resin. No further effect on these two properties could be seen when the other chemicals were added to the furnish. The results of the paper testing are summarized in Table 4. Table 4. Properties of non-creped low grammage papers (20 g/m2).

PulpDewatering resistance

(°SR)

Density (kg/m3)

Tensile index (Nm/g)

Wet tensile index

(Nm/g)

Water absorption

capacity (g/g)

Spruce 20 586 37.8 0.3 5.9Spruce (PAE) 17 580 43.1 8.7 5.3

Spruce (PAE, Nanoparticle) 18 585 40.5 8.6 5.2Spruce (PAE, Flocculant) 17 577 47.1 8.0 5.0

Spruce (PAE, Micropolymer A) 17 600 51.3 7.5 5.1Spruce (PAE, Micropolymer B) 18 583 42.6 8.5 4.7

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5. Conclusions

The solids content reached after vacuum dewatering is significantly influenced by the choice of pulp. The fibre networks have different structures due to differences in the morphology of the fibres. A larger area of straight pores between the fibres facilitates the transport of both water and air and this increases the dryness. Eucalyptus fibres are short and slender and eucalyptus pulps have a fibre population about five times larger than that of softwood pulps. This creates a fibre mat with a relatively closed structure which lowers the potential for dewatering. The solids content reached after vacuum dewatering is negatively influenced by beating even though no secondary fines are created. Internal fibrillation makes the fibres swell by delaminating the fibre wall, increasing their capacity to retain water. External fibrillation unravels the outer layers of the fibres and this increases the relative surface area and impairs water removal. Wet pressing is affected by the choice of pulp due to differences in pore structure of the fibres and consequently a varying ability to sorb water. Hardwood pulps have a greater ability to retain water in the fibre wall than softwood pulps. The water in the fibre wall of hardwood pulps is however expected to be more easily removed since more water is located in macropores than in micropores. The removal of water during pressing is also dependent on the total amount of available water. Beating mainly delaminates macropores with a small effect on the micropores. The increase in solids content during pressing is not significantly affected by beating under the conditions used, i.e. mild beating and low grammages. The negative effect of beating is however seen after vacuum dewatering where a higher degree of beating leads to a lower solids content. This order is maintained after pressing. Both water between the fibres and water in macropores in the fibre wall is removed during pressing. The solids content after wet pressing is increased by the addition of a PAE-resin to the stock, probably due to crosslinking in the fibre wall. Thermoporosimetry shows that the PAE-resin reduces the volumes of both micro- and macropores.

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6. Future studies

The potential for improving the energy efficiency on a tissue machine is great. By choosing fibres with a superior morphology and by limiting the energy inputs to the refiners, a significant increase in dryness in the wet end can be achieved. These studies have been carried out in a laboratory environment so that future studies could focus on a scaling up of the experiments to pilot machine trials to validate the results. The increase in dryness could be even higher with a sufficient treatment of the fibres with additives. Future studies in the field of dewatering could therefore focus on achieving a higher solids content with efficient fibre treatment. Enzymes are interesting since the surface of the fibres can be modified, and this will probably affect dewatering. Research in this field directly relating enzyme addition to dewatering is scarce. It would also be very interesting to investigate the effects of fibre morphology, beating and additives on creped tissue paper. It is however difficult to simulate high speed creping on a laboratory scale in a satisfactory manner. With useful equipment it would be possible to compare non creped and creped papers and evaluate how the process affects the properties of the paper.

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7. Acknowledgements

First, I would like to express my sincere gratitude to my supervisors Prof. Lars Nilsson and Dr Christophe Barbier. You have supported me through thick and thin with encouraging and fruitful discussions. I can’t imagine any more suitable persons as supervisors. Thank you for your faith in me and our work! I would also like to thank the members of the Tissue Education and Research Programme (TERP) at Karlstad University for the financial support and feedback to the project. The cooperation with all of you has been very helpful for the progress of the work. I would like to acknowledge Timo Ylönen and Christian Oraassari at Aalto University for all the support during the press trials. Even though we had some difficult times with the trials you never stopped believing. Thank you! Development engineer Pia Eriksson is acknowledged for all the help and support during the experimental work. Dr Christer Burman is greatly acknowledged for the help with the scanning electron microscope. Dr Anthony Bristow is thanked for the linguistic revision of the first part of the thesis and the papers. To all my new friends and colleagues at Karlstad University: I am very thankful for all the support and guidance during my work. Most important, it has been a joy going to work every day during these years with moments memorable for life. Finally, I would like to express my gratitude to my family and friends. Thanks for always being by my side!

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