Karlstads universitet (KAU) 651 88 Karlstad Tfn 054-700 10 00 Fax 054-700 14 60
Information@kau.se www.kau.se Faculty of Science and Technology Department of Chemical Engineering
Syed Ali Hassnain Shah
The effect of different recovered fibres on mechanical properties
of board
Master Thesis of 30 Credits
Master of Science in Chemical Engineering
Date: 2012-08-17
Supervisors: Dr. Christophe Barbier (KAU) Johan Kullander (KAU)
Dr. Federica De Magistris (Fiskeby Board AB)
Examiner: Prof. Dr. Lars Järnström (KAU)
© Syed Ali Hassnain Shah, 2012 Fiskeby Boards AB
Title: The effect of different recovered fibres on mechanical properties of board Publisher: Diva – Academic Achieve On-line 2012
www.diva-portal.org
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ACKNOWLEDGEMENTS
This thesis work has been performed at Karlstad University in collaboration with Fiskeby Boards AB between Jan 2012 and June 2012. This work has been done for the partial fulfillment of the degree in M.Sc. Chemical Engineering.
First and foremost I would like to thank my family for being with me through thick and thin, their continuous encouragement has been a source of
motivation for me throughout my degree.
I would like to extend my gratitude to Karlstad University for providing me an opportunity to study in this prestigious institution. The faculty and all the students have been very cordial and encouraging. I would also like to thank Fiskeby Board AB for their support in this work. I would like to thank Karlstads Teknik-center for providing the facilities for fibre testing. I want to thank StoraEnso, Karlstad Research Centre for providing the plates which were used for z-strength testing.
I would also like to thank my internal supervisors Christophe Barbier and Johan Kullander. They have always been very helpful, encouraging and committed towards this work. Without their presence this thesis work would not have been possible. I also want to thank Federica De Magistris, my
supervisor from Fiskeby Boards AB for providing me an opportunity to work on this topic.
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Abstract
The objective of this work was to determine the influence of recovered fibres on the mechanical properties of board. Industrial board was provided by Fiskeby Board AB and laboratory board was made by using an isotropic sheet former. Board properties such as tensile strength, tensile stiffness, z-strength and bending stiffness were evaluated. Variation in the grammages of the middle and bottom layers of laboratory board was done and influence on the mechanical properties was studied. Each layer of multiply board was also tested separately at industrial board grammage and standard grammage.
Results showed an increase in the tensile properties with the increase in the grammage of bottom layer, an increase in the z-strength of the board was also observed. Bending stiffness calculated by laminate theory also indicated an increase with the increase in the grammage of bottom layer. While testing of the separate layers showed an increase in the tensile properties of the layers with increasing the grammage while a decrease in z-strength was observed.
Cracks occurred in the bottom layer of the multiply boards but an increase in the mechanical properties of board was observed.
Keywords: Z-strength, Bending stiffness, couching method, recovered fibres, 4-plyboard
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Sammanfattning
Returfibrer är miljövänliga och ger ett försprång framför nyfiber på grund av låg kostnad. Det är dock allmänt känt att returfibrer inte uppvisar samma egenskaper som nyfibrer, i själva verket försämras fiberegenskaperna efter varje återvinningsprocess. Mekaniska egenskaper är mycket viktiga för kartongproducenter och därmed läggs en hel del energi på förbättringar utan kostsamma investeringar.
I detta examensarbete utvärderades mekaniska egenskaper för 4-skikts kartong såsom dragstyrka, dragstyvhet, böjstyvhet och z-styrka. Den industriella kartongen som användes kom från Fiskeby Board AB och laboratoriekartong tillverkades med en isotrop arkformer. Separata skikt formades och guskades därefter samman. Ytvikten av det mellersta och undre skiktet ändrades och effekten på kartongens egenskaper utvärderades. Varje lager testades också separat vid ytvikter representativt för den industriella kartongen samt vid standardiserade ytvikter. Styrkan i tjockleksriktningen utvärderades med z- styrka och Scott Bond. Böjstyvheten beräknades med hjälp av laminatteorin.
Skillnaderna i dragstyrka mellan kartong framställd i industrin och laboratorie var små, men dragstyvheten var lägre för laboratoriearken på grund av torktekniken. Skillnaderna i z-styrka var inte signifikanta, men baserat på trenden uppvisade laboratoriearken en lägre styrka på grund av ett lägre presstryck. Delamineringen i de testade proverna skedde i det undre skiktet och inga sprickor uppstod mellan skikten.
Genom ökning av ytvikten av de separata skikten ökade styrkeegenskaper såsom dragstyrka och Scott Bond medan z-styrkan minskade. Genom att minska ytvikten av det mellersta lagret och öka ytvikten av det nedre kunde en ökning av styrkeegenskaperna observeras. Ökningen av z-styrkan var inte signifikant men baserat på trenden kan en ökning förväntas. En betydande ökning av Scott Bond kunde dock ses. Böjstyvheten beräknad genom laminatteorin visade också på en ökning. Även om sprickor fortfarande förekommer i det undre skiktet efter ökning av ytvikten, kan förbättrade kartongegenskaper uppnås.
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Executive Summary
Recycled fibres are environmentally friendly and provide an edge over virgin fibres because of low cost. But it is widely known fact that recycled fibres don’t possess the same characteristics of the virgin fibres, infect after every recycling process the fibre properties are deteriorated. Mechanical properties of board are very important for board producers and therefore a lot of efforts have been put towards the improvement of mechanical properties without much investment. Improved board properties can be obtained by fractionation, refining and using virgin fibres along with recycled fibres but more refining can lead towards the dewatering problems and virgin fibres can increase the cost.
In this work the effect of variation in the grammages of middle and bottom layers of 4-ply board on the mechanical properties such as tensile strength, tensile stiffness, bending stiffness and z-strength was studied.
Industrial board was provided by Fiskeby Board AB and laboratory board was made using an isotropic sheet former. Separate layers were formed and then were couched together. Grammages of middle and bottom layers were changed and their effects on the board properties were evaluated. Each layer was also tested separately at industrial board grammage as well as standard grammage. Tensile strength, tensile stiffness and z-strength of the board were tested. Bending stiffness of the board was calculated using laminate theory.
Comparison of industrial board and laboratory board showed not much difference between tensile index of both boards but tensile stiffness index was lower in case of laboratory board because of free drying was adopted in case of laboratory board. Z-strength of boards was evaluated by z-directional tensile strength and Scott Bond method. Difference in z-strength was not significant but based on trend laboratory board showed lower strength because of lower pressing pressure. Location of the cracks in the samples which were subjected to z-strength and Scott Bond testing was in the bottom layer and no crack occurred between the boundary of layers.
By increasing the grammage of each layer when tested separately, tensile properties of the board increased while strength in thickness direction decreased with an increase in the grammage.
By decreasing the grammage of middle layer and increasing the grammage of bottom layered increased the tensile properties of the board. In case of z- strength the increase was not secured but based on trend an increase was expected with increasing the grammage of bottom layer. An increase was significant in case of Scott Bond value. Irregularities in the thickness of the samples after Scott Bond testing was more visible and evident than z-strength because of the partial breakages which occur within the sheet in case of Scott Bond testing. Bending stiffness calculated by laminate theory also indicated the same trend that by increasing the grammage of bottom layer bending stiffness of the board will increase. It was concluded that though cracks are still occurring in the bottom layer after increasing the grammage of bottom layer but overall an improved board can be produced by increasing the bottom layer grammage.
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List of Figures
Figure 2-1 Sketch of LB Multilayer handsheet former. (Redrawn from Karlsson
2007) ... 15
Figure 3-1 Flat circular tester plate... 22
Figure 3-2 Schematic sketch of mounted piece between the plates in a tensile tester ... 23
Figure 3-3 Schematic sketch of Scott Bond apparatus ... 24
Figure 4-1 Tensile index of two boards ... 27
Figure 4-2 Tensile stiffness index of two boards ... 27
Figure 4-3 Z-strength of the two boards ... 28
Figure 4-4 Scott Bond of two Boards ... 29
Figure 4-5 Tensile index of the board with different configurations... 34
Figure 4-6 Tensile stiffness index of the board with different configurations ... 35
Figure 4-7 Z-strength of the board with different configurations ... 35
Figure 4-8 Scott Bond of the board with different configurations ... 36
Figure 4-10 Thickness profile of the sample after Scott Bond testing ... 38
Figure 4-12 Thickness profile of the sample after z-strength testing ... 38
Figure 4-9 Fractured sample after Scott Bond testing ... 38
Figure 4-11 Fractured samples after z-strength testing with Zwick tester ... 38
List of Tables
Table 4-1 Configuration of the layers ... 26Table 4-2 Board type and strain at break ... 27
Table 4-3 Thickness of the board types ... 28
Table 4-4 Properties of separate layers at industrial board grammage ... 31
Table 4-5 Properties of separate layers at standard grammage... 33
Table 4-6 Grammage and thickness of boards ... 34
Table 4-7 Different configurations of layers and Bending stiffness index ... 36
Table 4-8 Thickness measurements of fractured samples ... 37
Table 4-9 Fibre characteristics of each pulp ... 39
8
List of Abbreviations
CD Cross direction
ISO International organization for standardization MD Machine direction
MS Middle layer
ÖS Top layer
SS Second layer
TAPPI Technical association of the pulp and paper industry US Bottom layer
ZDTS Z-direction tensile strength
9
Contents
1. Introduction ... 11
1.1. Background ... 11
1.2. Aim ... 11
1.3. Fiskeby Board AB ... 11
2. Theoretical Background ... 12
2.1. Paperboard ... 12
2.2. Recycled Fibres ... 13
2.3. Multilayering Sheet Formers ... 15
2.4. Board Properties ... 16
3. Materials and Methods ... 20
3.1. Materials used ... 20
3.2. Stock Preparation ... 20
3.3. Sheet Forming ... 20
3.4. Paper Testing ... 21
3.4.1. Grammage ... 21
3.4.2. Thickness ... 21
3.4.3. Tensile Properties ... 22
3.4.4. Z-directional Tensile Strength... 22
3.4.5. Scott Bond ... 23
3.4.6. Fracture Measurement ... 24
3.4.7. Theoretical Bending Stiffness ... 24
3.5. Fibre Characterization ... 25
4. Results and Discussions ... 26
4.1. Comparison between Industrial board and Laboratory board ... 26
4.1.1. Tensile index and tensile stiffness index ... 26
4.1.2. Z-strength ... 28
4.1.3. Scott Bond ... 29
4.2. Separate layers at Industrial board grammage ... 29
4.3. Separate layers at standard grammage ... 31
4.4. Comparison with changed configuration ... 33
4.4.1. Tensile index and tensile stiffness index ... 33
4.4.2. Z-strength ... 35
4.4.3. Scott Bond ... 35
4.5. Bending stiffness ... 36
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4.6. Fracture measurements of 4 ply board ... 37
4.7. Fibre Characteristics ... 38
5. Conclusion ... 40
Appendix ... 43
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1. Introduction
1.1. Background
Usage of recycled fibres in the production of multiply board is environmentally friendly and provides an extra economic edge. But recycled fibres don’t possess the same quality as virgin fibres. Mechanical properties are very important for the board manufacturers and therefore a lot of effort has been put in the past to devise a method which could give better properties without costly investments. Using virgin fibres along with recycled fibres is a good method to enhance the strength properties but at the same time it can prove costly.
1.2. Aim
4-ply board used in this study was made from 100 % recycled fibres and the effects of changing the grammages in the layers were investigated. Grammages of the individual plies were changed in such a manner that overall grammage of the board remained constant. The effect of changing the grammage was discussed in the study which gave a better understanding of the effect of fibres on the properties of the board. Board properties such as tensile index, tensile stiffness index, z-strength and bending stiffness were evaluated during the study.
1.3. Fiskeby Board AB
The Fiskeby mill was established in 1637 and is one of the largest board producers in Sweden. Their production volume is 155600 tons. Use of recycled fibres and renewable energy make them one of the biggest contributors towards sustainable environment. 100 % recycled fibres are used in the manufacturing of the board at Fiskeby Board AB. It includes the recovered papers from non printed office papers for top layer, printed fibres layered with plastic for secondary layer. Middle layer consists of fibres obtained from mixed office grades except newsprint grades for bulk of the board while bottom layer consists of the waste from production. Multiboard is the product name of Fiskeby’s comprehensive range of cartonboard. Multiply construction is involved in the production of multiboard and is available in different grades with different properties. There are five different basic grades of multiboard i.e. Multiboard Offset, Multiboard Kraft, Multiboard EcoFrost, Multiboard EcoKraft and Multiboard Barrier. Multiboard Offset is a versatile board for general packaging solutions and is suitable for gift packaging, clothes
12 and foods. Multiboard EcoFrost is suitable for frozen and chilled foods and it is fully sized grade featuring moisture resistance. Multiboard Kraft is used for strong packaging and it is an extra strong board with high tearing resistance and is suitable for heavy items such as tools, beverage packs, spare parts and household articles. Multiboard EcoKraft is used for chilled and frozen food in strong packaging. It is a strong board with high tearing resistance and fully sized with moisture resistance. Multiboard Barrier is used for products which require barrier properties and is suitable for fatty, frozen and moist products (De Magistris 2012).
2. Theoretical Background
2.1. Paperboard
Today almost everything is packed in some sort of package during its entire life or some parts of it. Every product to be packaged varies in nature and requires specific solutions using appropriate material and structure. The most used packaging materials are wood, glass, plastic, metal and paper.
The traditional role of the package has been to store and transport the goods but it has been changed to a silent salesman too, as it contains essential information and data on the goods it protect. There has been different kind of printings and colors on the packages to make it look attractive and soothing to the eyes so a customer is invited to buy the goods.
Cartonboard is usually defined as a paper material ranging from about 200 g/m2 up to 600 g/m2. The material less than 200 g/m2 is usually called paper and above 600 g/m2 is described as millboard (Korin 2009).
Cartonboard is often made as a multiply board which gives an opportunity for the manufacturer to design a ply of the board according to his requirements.
Normally chemical pulp with higher strength and better printing and optical properties is placed on the top of the board while a low cost mechanical pulp is placed in the middle layer to achieve the bulk. This composition results in a carton board with high bending stiffness, high strength and smooth surface (Karlsson 2007).
The process to produce a package from cartonboard includes several converting steps such as cutting, creasing, embossing, folding, printing and sealing etc.
13 2.2. Recycled Fibres
Recycled fibres can be used as raw material in board production.
Disappearances of raw material sources, rapid decrease in the forests and the human awareness have lead to the use of recycled fibres in the pulp and paper industry. Also the economics of the raw materials is one of the major reasons for more emphasis on the recycling of the fibres.
46 % of the fibres used in the paper industry worldwide are recycled fibres (Kirwan 2005). Recycled fibres can be obtained from different sources such as old news print, old magazine grades, mixed office waste, other high grade papers, old corrugated containers and post consumer paper. In Western Europe 65% of the recovered paper is used in packaging materials and utilization rate is close to 90%. While in newsprint 16.9% of the recovered fibre is used and hygiene paper has 8.3% usage of recovered fibres. Utilization rate is a ratio of amount of recovered fibre used in the paper industry and total paper production (Ek et al. 2006).
Recycled fibres have a large spectrum of qualities; Waste contains fibres which are chemically and mechanically separated and fibres which already may have been recycled once or more times. All fibres cannot satisfactorily be used for all the paper products. The highest value waste paper is white virgin fibre paper which is unprinted and wood free because it does not contain any mechanical or recycled pulp instead it consists of chemically separated bleached pulp. Paperboard waste obtained from industrial and commercial areas is usually clean and it can be easily collected regularly in reasonable quantities. Most difficult waste to collect is post consumer use and that includes newspaper, brown corrugated cases and printed white papers. These are low valued grades of waste and are more likely to contain pigments, inks and adhesives etc (Kirwan 2005).
Pulp properties deteriorate during recycling mainly due to the irreversible structure changes in the fibre wall because of drying (Laivins & Scallan 1993).
Loss in the strength of recycled fibres has been referred to the loss of bonding which is a function of two parameters, fibre flexibility and surface condition (Nazhad 1994). Fibre morphology is also changed by other mechanical and chemical treatments such as refining, bleaching and pulping (Oksanen et al.
2000).
14 During drying, fibres lose their swelling capacity and conformability which cannot be recovered by the rewetting of the fibres. Hornification is caused by irreversible hydrogen bonding between microfibrils. It results in inferior strength properties and bulkier sheets. Compared to chemical pulp fibres, mechanical pulp fibres can enhance their strength properties on recycling because the fibres become more flexible and hence result in increased bonded area (Laivins & Scallan 1993). Hornification is beneficial for improved dewatering and increased machine speed. Fibre flexibility increases with increased swelling in the fibres. With increasing flexibility conformability of the fibres is improved. Flexibility of the fibres correlates with the pulp yield, sheet density and sheet strength and it depends on the fibre morphology.
Recycled fibres can be upgraded through fractionation, refining, chemical additives and blending with the virgin pulps but the virgin fibre properties cannot be restored fully (Bhatt et al. 1991).
Refining can improve the strength properties of the recycled fibres as more fibrils are formed which enhance the bonding characteristics. The problem with refining is that it increases the amount of fines. Fines create dewatering problems because of their high relative surface area and thus the dewatering rate lowers (Dienes et al. 2003).
One major problem with the use of recovered fibres is the continuous change of qualities of the recovered fibres themselves. Fractionation helps in separating the fibres where longer fibres can be used in the board making process as they are attributers of the strength properties (Abubakr et al. 1995).
Bhatt et al. (1991) used high shear field (HSF) instead of common beating which modified the fibre cell walls by brushing and blending action. It increases the bonding area and causes less fines formation and freeness loss.
Fibre properties are not the sole factor that influences the strength properties;
fines also play an important role. Primary fines are present in the unbeaten pulp while the secondary fines are produced from external fibrillation during beating. Due to their high surface area, secondary fines reduce the drainage time but increase bonding between fibres. Fines with higher swellability, higher water sorption, higher compression strength, and lower crystallinity index results in higher sheet density. Beating at lower consistency gives higher tensile strength.
15 2.3. Multilayering Sheet Formers
Multilayer forming techniques are most commonly used in the manufacturing of paper board where chemical pulps with higher strength and better printing properties is placed on the outer layer while the low cost, bulky mechanical pulp is used in the middle layers. Multiply paperboard has been produced continuously for almost 150 years and during these years multiply board production techniques have evolved in order to meet up with the requirements of the board.
In 1996 the LB Multilayer handsheet former was presented and evaluated, the sheet former was originally developed at Åbo Akademi. The basic purpose was to create a sheet former which is easy to use and able to increase the production of laboratory sheet. The basic design of the sheet former is shown in the figure 2-1.
Figure 2-1 Sketch of LB Multilayer handsheet former. (Redrawn from Karlsson 2007)
The pulp container can be divided into two, three or four compartments using sliding plates. Water is added from the bottom in order to avoid the sticking of the fibres on the wire. Pulp is added from the top and lower compartment is filled with the water then plate is moved and the compartment is closed.
Second layer pulp is added and same step is repeated and it is done with the each compartment depending on the layers to be produced. There are stirring propellers attached to each compartment for the mixing of the pulps. Once all the pulps are filled then all the plates are removed from the top to bottom and the drainage valve is opened for dewatering. Pulps start making their layers respectively and multiply sheet is formed (Karlsson 2007).
16 Variation of the properties within the sheets with LB Multilayer sheet former and Isotropic handsheet former show that the grammage is approximately at the same level for the two methods but the variation in the thickness is higher in isotropic handsheet former than Multilayer former because of the more uniform sheets which are produced in the LB multilayer sheet former. While in the case of other paper properties such as tensile strength, tensile stiffness and stretch at break; the multilayer sheet former showed greater variation as compared to the isotropic sheet former (Karlsson 2007). These results lead towards the usage of isotropic sheet former. The basic purpose of using LB Multilayer sheet former was the thought that it might perform at the same level of industrial board.
2.4. Board Properties
Paperboard producers have been forced to continuously develop and improve their products because of the increased competition in the market. This has lead towards the desire of improving board strength without any costly changes in the process. Softwood pulps give high strength to the paper because of the long fibres. Dry strength resins are also used to enhance the strength properties of the paper and boards.
Long and flexible fibres contribute to the strength of the sheet; the strength with which fibres adhere to each other, together with fibre strength contributes to the paper strength. Both the specific bonding strength and bonded area affect the bonding strength. More flexible fibres increase the surface available for bonding. For fibres which are fairly straight and with few kinks, a high bendability indicates that the fibres are flexible. Longer fibres show higher strength as compared to the shorter fibres because chances of fibre-fibre bonding are higher (Karlsson 2006). Talking about machine direction and cross direction in the paperboard, it will show higher strength in the machine direction because more fibres are aligned in that direction. More fibres in machine direction would result in more entanglements of the fibres which will give higher strength.
Z-strength represents the paper’s ability to resist tensile loading in the direction perpendicular to the plane of the paper. Thus z-strength can be defined as the force required producing a unit area of fracture. When z- strength of the paper is exceeded, a break in the paper structure occurs in the sheet but not at its surface. Thus Z-strength is not equivalent to the surface strength or linting tendency of the paper (Karlsson 2006).
17 Strength in z-direction is an important property for paperboards. The mechanical properties in the thickness direction are important for the operations such as creasing, bending, printing and plastic coating. High z- strength in Offset printing is probably the most important. In offset printing the splitting resistance of tacky printing ink creates a z-directional tensile stress at the exit side of printing nip. The mechanical properties in the thickness direction are mainly influenced by number and strength of the fibre to fibre bonds, the stiffness of the fibres and the fibre strength in the transverse direction (Kajanto 1998). Since the fibre-fibre bond properties are difficult to determine with precision, density is often used for ranking the effects of pulping and papermaking parameters on the mechanical performance paper along the thickness direction (Koubaa & Koran 1995).
Z-strength can be improved by increasing the fibre-fibre bonded area or by improving the fibre-fibre joint strength. The orientation of the fibres in the z- direction can also affect the z-strength. Bad formation of the sheets can increase the z-strength because of the orientation of more fibres in the z- direction (Aaltio 1960). This theory was contradicted by Döbeln (2005) as he observed that worse formation did not impact the z-strength of the sheets. He stated that the reason it did not affect the z-strength is mainly because it is difficult to orient the long fibres in the z-direction and maybe with higher amount of fines some other picture could have emerged.
Z-strength is also affected by the density. An increase in density results in a higher z-strength due to the narrower contact between the fibres (Kajanto 1998). The narrower contact of the fibres increases the contact areas. The increase in the content of mechanical pulp in multiply board results in low density and low z-strength as compared to the chemical kraft pulp. Chemical kraft pulp with more efficient fibre-fibre bonding and more flexible fibres give higher density and higher z-strength (Döbeln 2005).
Changes in basis weight have a different effect on the delamination strength and energy. At very low basis weight in the single ply sheets the z-strength will be higher, that could be due to the changes in network structure with increasing basis weight in such a way that the bonding degree is smaller in the middle layers of the sheet. At high basis weight rupture occurs in the plane of minimum strength while at low basis weight there is a little room in the thickness direction for such variations (Kajanto 1998).
18 Out of plane strength is very sensitive to any non uniformity or layering in the z-direction, since delamination will occur at the weakest plane. The location is dependent upon the z-directional distribution of fines and fillers, density through bonding degree and distribution of size is present. In multilayer boards, the inter-layer strength is also important.
The layered structure of the fibre network has strong influence on the out of plane strength. In the layered structure sheet fibres lie on top of each other in a well defined sequence so the sheet can delaminate without breaking any fibres. In case of completely interwoven sheet, it’s impossible to find simple delamination planes that would not involve fibre breaking or pull out (Kajanto 1998). Since fibres are stronger than the bonds between them even a small number of broken fibres can significantly increase the delamination energy.
Since cartonboard to a large extent has a layered structure, measurement of the z-directional tensile strength provides an indication of the internal bond strength. As paper contains fibre entanglements, the z-direction fibre distribution affects the z-strength. There are three methods which are commonly used to measure the internal bond strength of the paperboard.
These methods include z-directional tensile test, the delamination test and the Scott Bond test. These tests don’t measure the same thing as z-directional tensile includes the energy of intrafibre bond failure. Delamination energy includes the energy dissipated in the fibrous network while the Scott Bond energy is affected by the basis weight and by the dynamic nature of the test which over evaluates this parameter (Koubaa & Koran 1995).
The z-directional tensile strength method is preferable since the Scott Bond test involves non uniform stress and shear distributions during the fracture process. However, z-directional tensile strength method involves difficulties in applying a uniform tensile load on the sample. The plates and the tensile forces have to be aligned. The gauge length of the test is the thickness of the paper and so the result is sensitive to the variations of the thickness in the sample.
Macroscopic failure due to interfibre bond failure in thickness direction is often termed as delamination. Delamination may be caused by out of plane normal and shear loading and combination of these. The failure mechanism between Scott Bond and z-strength method is different. Paper is delaminated by pure tension in case of z-strength, while in Scott Bond method gradual
19 delamination can be observed from one end to the other end of paper (Fellers et al. 2012).
Singh (2007) observed that z-directional tensile strength increases by extra refining of the pulp. He also stated that resistance of paper to failure under z- directional strength can be easily expressed in terms of in-plane tensile index.
An increase in curl index may cause an increase in z-strength but with a decrease in the in-plane tensile strength. The decreases in the in-plane strength and increase in out of plane strength was explained earlier by Gärd (2002) and he described that curlier fibres increase the z-strength while a decrease in the in-plane strength properties is observed because of shrinkage. It is important to mention that information about z-strength of recycled fibres is not defined in literature.
Bending stiffness of the sheet is one of the most important properties of the cartonboard. Lower bending stiffness results in poor appearance of the printed board and it is also difficult to handle. Improper bending stiffness also gives runnability problems. Bending stiffness problems often occur when one tries to lower the basis weight because it strongly influences the bending stiffness.
Higher bending stiffness is necessary for strong and rigid package, it is also very important for good runnability on the packaging machine (Kajanto 1998).
Bending stiffness can be calculated by using the laminate theory by using grammage, tensile stiffness index and density as the starting values.
20
3. Materials and Methods
3.1. Materials used
The pulps used in this work comprised of recycled fibres, provided by Fiskeby Board AB. Four different pulps were used in this work for four different plies which are given below:
ÖS (Used for top layer)
It is used for the top layer in the multiply board, it is recycled from the non printed office papers. Fibres are new as compared to the fibres from the other pulps and these are white in color.
SS (Used for second layer)
It is used in the secondary layer; fibres are obtained from the recycling of printed papers layered with plastic. It looks grayish in color.
MS (Used for third layer)
The fibres used in the middle layer are obtained from the liquid packaging board and other products. Fibres used in the middle layer are intended to provide bulk in the board.
US (Used for bottom layer)
The fibres used in the bottom layer are obtained from the internal broke. It is the waste from the production scale which is not used as the final product.
3.2. Stock Preparation
Concentration of each pulp was determined by using the oven method ISO 638. Due to inconsistency in the pulps the concentration of the pulps varied a lot. Another method was therefore used to measure the concentration. Stock was prepared using the concentration which was determined by using the oven method and sheets were made and placed in climate room at 23 °C and 50 % RH. The concentration was obtained with the help of the desired and obtained grammage. Stock of each pulp was prepared using 2 % concentration.
3.3. Sheet Forming
Sheets were made using isotropic sheet former following ISO 5269-1. LB Multilayer sheet former was to be used in this work initially but due to problem in dewatering. The reason for the dewatering problem may be because of the use of recycled fibres or working at higher grammage of 280
21 g/m2. It was decided to use isotropic sheet former and couched the plies together.
Water was filled up to certain mark in the sheet former and the calculated amount of stock was added. After dewatering a layer was formed on the wire, two blotting papers were placed on the layer and a couch weight was used to adhere the sheet with blotting paper. Couch weight was placed on the sheet for 15 seconds, then the sheet was picked up with the help of blotting paper, same procedure was repeated for every ply. US pulp was used to make the sheet for the bottom ply and then the sheet made from MS pulp was placed on the sheet made from US pulp, sheet made from SS pulp was placed on the MS layer while ÖS pulp gave the top layer and it was placed on the SS layer resulting in the four plies. That four ply sheet was then pressed in the flat plat pressing machine following the standard ISO 5269-1. After that sheets were placed at 23 °C and 50 % RH for drying in the climate room. Free drying was used to dry the sheet. All the sheets used in this work were made using the same procedure.
3.4. Paper Testing
Following properties were evaluated in this work
Grammage
Thickness
Tensile properties
Z-strength, Scott Bond
Bending stiffness 3.4.1.Grammage
Grammage of the paperboard was calculated following the standard ISO 5270.
Paperboard was cut in the circular shape giving the area of 0.01m2. Five Samples were weighed and then the weight was divided by the area which gave the grammage of the sheets.
3.4.2.Thickness
Thickness of the paperboard was measured in a STFI thickness tester. Five samples were used for thickness measurements resulting in 10 values.
22 3.4.3.Tensile Properties
A standard tensile tester (Zwick/Roell Z005) was used to determine the tensile properties of the paperboard; ISO 1924-3 was followed. The Paperboard was cut in the dimensions of 15 × 1.5 cm. Tensile strength index, tensile stiffness index and strain at break of the paperboard were obtained.
3.4.4.Z-directional Tensile Strength
Z-directional tensile strength was measured using a standard tensile tester (Zwick/Roell Z005). Two flat circular tester plates having the test area of
Figure 3-1 Flat circular tester plate
1057 mm2 were used. The samples were cut in the dimension of 5.5 × 5.5 cm and adhesive double sided tape was applied on both sides of the samples. The tape used in the process was same throughout the experiment. The Test piece was mounted between the circular plates and placed in the cells of the tensile tester (shown in fig. 3-1) for the determination of z-directional tensile strength.
ISO 15754 was followed in the procedure. 10 samples were tested during the procedure. The schematic sketch of mounted piece between the plates in Zwick tester is shown in figure 3-2.
23
Figure 3-2 Schematic sketch of mounted piece between the plates in a tensile tester
3.4.5.Scott Bond
Scott Bond testing apparatus is used to determine the internal bond strength of the samples. The samples were cut in the dimensions 140 × 25.4 mm. A common type of double sided adhesive tape was used in the procedure and the same tape was used throughout the experiment. The two sided tape was attached to the bottom steel anvils of the apparatus, paper was placed on the tape and then strongback loaded with five aluminum plates with their vertical surfaces towards the front were placed on the tape. Pressure was applied by pulling the cam lever forward for 2 to 3 seconds. Strongback was removed and the samples were adhered to the tape. Aluminum plates were subjected to the swing of the pendulum, which initiated a crack in the z-direction of the samples. TAPPI T 569 was followed in the procedure. Figure 3-3 shows the apparatus used for Scott Bond testing. 10 samples were tested during the procedure.
24
Figure 3-3 Schematic sketch of Scott Bond apparatus
3.4.6.Fracture Measurement
The location of the fractures created in the samples used in the z-directional tensile strength measurement and Scott Bond measurements were evaluated by using the STFI thickness tester. Fractured samples were attached to the adhesive tape and it was difficult to measure the thickness because tape was sticking with the instrument. The adhesive side sample was attached to the paper and then the thickness of the sample was measured. Thickness of the paper and tape was separately measured and subtracted from the cumulative thickness of the sample in order to find out the actual thickness of the fractured sample. The thickness of the fractured samples determined the location of the crack as at which layer the crack has occurred. The thickness of the paper and tape was assumed to be the constant.
3.4.7.Theoretical Bending Stiffness
Bending stiffness of the samples was calculated by using laminate theory (Fellers 2009) and the formula is given in equation 2.1
25 Tensile stiffness index, grammage and thickness were the starting values in the determination of the bending stiffness. Basis weight, thickness and tensile stiffness index of each layer used in the 4 ply board was used. The thickness of each layer was added and the total thickness was noted down which was divided by two as given in eq. 2.2.
Then thickness of the 1st layer was added to the value of Z0 giving the value of Z1, then value of Z1 was added to the thickness of the 2nd layer giving the value of Z2. Same step was repeated up to the value of Z4. Then the squares and cubes of Z0 to Z4 were calculated and then following equation were used where k represents layer of the board from 1 to 4. Where E is the tensile stiffness index of the layer and k represents the ply of the board.
A, B and D were used to calculated the bending stiffness in mNm using equation 2.1.
3.5. Fibre Characterization
Fibre tester at Karlstads Teknik-center (KTC) was used to investigate the fibre characteristics of the pulps. Samples of each pulp were prepared, calculated amount of pulp was placed in the bottle and de-ionized water was added in the bottle making the commutative weight of 200 mg. Samples were placed in the apparatus and machine was started. Fibre length, width, coarseness and fines content were evaluated by this method.
26
4. Results and Discussions
4.1. Comparison between Industrial board and Laboratory board 4.1.1.Tensile index and tensile stiffness index
It was important to do a comparison between the both boards before continuing with the experiments. Laboratory board was made by using the isotropic sheet former, while industrial board was provided by Fiskeby Board AB. Laboratory board was made at the same configuration of industrial board.
Industrial board was tested in both directions i.e. MD and CD. In order to compare the tensile index, tensile stiffness index and strain at break of the board with isotropic board eq. 3.1 was used to calculate the geometric mean of the properties where PGeo is the geometric mean of the properties while PMD
represents the properties in machine direction and PCD represents the properties in the cross direction.
Figure 4-1 and 4-2 show the comparison of tensile index and tensile stiffness index between the industrial board and laboratory board. Laboratory board was prepared with the same raw material and at the same configuration which was used in the industrial board. Table 4-1 shows the grammages of each layer at industrial configuration.
Table 4-1 Configuration of the layers
Layer Grammage of Board
(g/m2)
ÖS (Top layer) 25
SS (Secondary layer) 30
MS (Middle layer) 200
US (Bottom layer) 25
Total 280
27
Figure 4-1 Tensile index of two boards1
Figure 4-2 Tensile stiffness index of two boards2
It can be seen that with tensile index that both boards are showing the same results which means that isotropic sheet former with couching method is a useful method to further proceed with the experiment and it can be correlated in the end with the industrial board production on the process scale.
Tensile stiffness index shows a higher value in case of the industrial board than laboratory board which can be because of the drying method. Laboratory boards are dried freely in the climate room while restrained drying is applied in case of industrial board. Tensile stiffness index is affected by the drying
method because the structure of the board is tightened by the shrinkage in the fibres, crimps are partly removed and increase in the tensile properties is observed. While no such tightening of the structure occurs in case of free drying which results in lower tensile properties (Blomstedt et al. 2007). Strain at break of both board types are given below in the table 4-2.
Table 4-2 Board type and strain at break3
Board Type Strain at break % Industrial board 2.9±0.1 Laboratory board 6.3±0.2
1 Error bars show the standard deviation of 10 samples
2 Error bars show the standard deviation of 10 samples
3 ±Shows the standard deviation of 10 samples
28 4.1.2.Z-strength
Z-strength of the paper reflects the strength of the paper in thickness direction. Z-direction fibre distribution affects the z-strength, which is important for the converting operations of the boards. Figure 4-3 shows the comparison of z-strength between industrial board and laboratory board. It can be seen from the results that isotropic laboratory board is showing lower strength in the z-direction as compared to the industrial board. Thickness of the laboratory paperboard is one reason why it is showing less strength in z- direction. Z-strength is affected by the pressing and which directly affect the thickness and the density of the board (Döbeln 2005).
Figure 4-3 Z-strength of the two boards4
The industrial board is pressed under higher nip pressure while the pressure applied on the laboratory board is lower which is 280 kPa. Thickness of both the boards is shown in table 4-3 which shows that industrial board is having lesser thickness as compared to the laboratory board.
Table 4-3 Thickness of the board types5
Board Type Thickness (μm) Industrial board 375.8±6.2 Laboratory board 443.2±9.4
4 Error bars show the standard deviation of 10 samples
5 ± shows the standard deviation of 5 samples
29 4.1.3.Scott Bond
Scott Bond testing method was used to investigate the internal bond strength of the fibres in the paperboard. Figure 4-4 shows the comparison of the results of Scott Bond values between industrial board and laboratory board.
The results show that the value of the Scott Bond is lower in the case of the laboratory board and as compared to the industrial board. The reason is the pressing of the industrial boards which are pressed under higher pressure, which results in lower thickness giving narrower contact in the fibres and increased contact surface (Döbeln 2005). Industrial board shows higher Scott Bond value than the laboratory board.
Figure 4-4 Scott Bond of two Boards6
4.2. Separate layers at Industrial board grammage
Before changing the configuration of the plies of the board it was important to investigate the properties of the layers at the grammage of the industrial board.
Another reason to investigate the properties of the layers at the grammage of the industrial board is the determination of the tensile stiffness index to calculate the bending stiffness and thickness of the each layer for the investigation of the fracture location. Table 3-1 shows the properties of the separate layers at the grammage of the industrial board. As table 4-4 indicates the layer ÖS shows the higher tensile strength and tensile stiffness index as compared to the other layers. The reason to that may be the fibres being used in the top layer. Pulp used in the top layer consists of non printed recycled fibres. Main purpose to use those fibres is to attain stiffness in the board and
6 Error bars show the standard deviation of 10 samples
30 better optical properties. The fibres used in the top layer are recycled once and thus those fibres have better characteristics as compared to other fibres which are used in the other layers as the fibre properties deteriorate after every recycling process (Nazhad 1994). SS layer shows the lesser strength as compared to the ÖS layer while it shows higher strength compared to the MS and US layer. SS layer consists of the fibres which are relatively new fibres as compared to the MS and US layers and are printed fibres which are layered with plastic. MS layer shows the weaker behavior and the reason is that MS layer is there to provide the bulk in the board and normally the recycled fibres which give higher bulk are used in the layer. Bulkier fibres provide the bulk in the board and fractionation is used to separate the bulkier fibres from the slender fibres. Fractionation is used because there are different qualities of recovered fibres available and the pulp which is used in the middle layer is attained from liquid packaging and all other grades except news papers which provide bulk. These fibres vary in their characteristics so that is why fractionation is used to separate those fibres and bulkier fibres which are used to provide the bulk are separated from the slender fibres. Return broke (the boards which do not go to production) is used as raw material for the bottom layer.
Z-strength of the ÖS layer is higher than the other layers while the z-strength of the MS layer shows the lowest value. The reason for that is the use of bulkier fibres as compared to the other layers. As bulkier fibres provide bulk in the board and posses less density than the slender fibres therefore the decrease in the z-strength can be seen in the middle layer. Influence of the adhesive tape used in the experiment may have played a part in the higher strength of the other layers. As Girlanda and Fellers (2007) observed the decrease in the z- strength with the increased grammage which at the end they found out was the result of the influence of the adhesive tape at the low grammage and with the increase in the grammage the influence of the adhesive tape is reduced. So it can be assumed that maybe the influence of the adhesive tape has lead to the increase in the z-strength of the other layers. The grammages of the ÖS, SS and US layers are 25, 30 and 25 g/m2 respectively while the grammage of the MS layer is 200 g/m2. At high basis weight the rupture occurs in the plane of minimum strength while at low basis weight the room for such variations in the thickness direction is very small (Kajanto 1998). Scott Bond values show different behaviour because MS layer is showing the highest value. The reason might be that at approximately 80 g/m2 the out of plane strength becomes constant and the Scott Bond energy starts to increase. The structure of the
31 sheets no longer changes at these basis weights and the increase in delamination energy may be because of the consumption of energy in the plastic deformation and partial rupture throughout the sheet thickness (Kajanto 1998). Andersson and Mohlin (1980) described that increasing the basis weight up to 200 g/m2 results in the increase in the Scott Bond value and decrease in the z-strength. They evaluated that as z-strength measures the force required to cause a breakage in the z-direction whereas Scott Bond value measures the energy involved in the breakage. When basis weight is increased the increase in Scott Bond value is probably because of the partial fractures in the sheet which consume energy without causing failure in the sheet. The apparent influences of the adhesive tape lead to the investigation of the properties of each layer at standard grammage (60 g/m2).
Table 4-4 Properties of separate layers at industrial board grammage7
Paper Properties
ÖS (25 g/m2) SS (30 g/m2) MS (200 g/m2) US (25 g/m2)
Tensile index (Nm/g)
34.4±2.7 27.7±1.2 25.5±2.2 20.6±2.0
Tensile stiffness index (kNm/g)
2.15±0.16 1.91±0.12 1.47±0.09 1.44±0.25
Z-strength (kPa)
465±10.8 440±8.6 351±11.0 362±27.6
Scott Bond (kPa)
0.184±0.01 0.181±0.02 0.224±0.02 0.134±0.02
4.3. Separate layers at standard grammage
Table 4-5 shows the properties of the each layer at standard grammage (60 g/m2). It can be seen that ÖS layer shows the highest value of tensile strength and tensile stiffness index as compared to the other layers. It is due to the fact
7 ± Shows the standard deviation of the 10 samples
32 that ÖS layer comprises of the pulp which is more refined and is obtained from a better source. MS layer shows the lowest value of tensile strength and tensile stiffness index as compared to the other layers which is again due to the fact that MS layer is there to provide the bulk in the board and shows the inferior strength properties.
Z-strength of each layer shows that ÖS layer is having the highest strength.
Which is due to the fact that pulp used in ÖS layer is obtained from the non printed white paper. The fibres used in the top layer are recycled once as compared to other pulps which consist of the fibres which are recycled more than once and their properties have been deteriorated due to recycling.
Fractionation is used to separate the bulkier fibres from the slender fibres.
Generally it is believed that flexible fibres contribute towards the sheet strength as more flexible fibres provide more efficient fibre-fibre joint strength which improves z-strength (Döbeln 2005; Forsström et al. 2005).
Scott Bond values of each layer show the same trend as the other properties showed except ÖS layer showing the lower value. MS layer shows the lowest value. In the table 4-4 MS layer showed the highest value of Scott Bond which lead towards the testing of the each layer at standard grammage. It can be seen from the comparison of the tables 4-4 and 4-5 that change in grammage influenced the board properties. It is evident that MS layer has shown the lowest value for z-strength and Scott Bond as compared to other layers which could be termed to the fibre characteristics of the middle layer. Bulkier fibres show the lowest strength because of their lower density. Also the influence of the adhesive tape at standard grammage is neglected and it can be said that adhesive tape influence the strength of the board in the thickness direction.
The comparison of the tables 4-4 and 4-5 show the decrease in the z-strength as well as Scott Bond at standard grammage.
While comparison of the tables 4-4 and 4-5 show that with the increase in the grammages of ÖS, SS and US layers an increase in the strength properties of the board can be observed. While it is not true with z-strength, it is decreased when the grammages of the layers are increased.
33
Table 4-5 Properties of separate layers at standard grammage8
Paper Properties
ÖS (60 g/m2) SS (60 g/m2) MS (60 g/m2) US (60 g/m2)
Tensile index (Nm/g)
41.2±2.2 28.0±2.7 23.3±2.3 27.5±2.0
Tensile stiffness index (kNm/g)
2.26±0.24 2.03±0.31 1.75±0.36 2.12±0.25
Z-strength (kPa)
412±13.2 404±9.9 315±13.2 322±9.3
Scott Bond (kPa)
0.174±0.009 0.199±0.018 0.147±0.011 0.159±0.013
4.4. Comparison with changed configuration 4.4.1.Tensile index and tensile stiffness index
There are different ways to increase the strength properties of the multiply board such as usage of virgin fibres and beating. Shifting board production to virgin fibres represents a cost issue and further beating of the fibres could lead to dewatering problems which would slow down the production.
Changing the configuration of the layers used in the board was adapted to investigate the effect on the strength properties of the board. The grammage of MS layer in multiply board was decreased and Overall grammage (280 g/m2) of the board was kept constant by increasing the grammage of the US layer.
By changing the grammages of MS and US layers the effect of each change was investigated. The overall grammage was kept constant while the thickness did not vary much. Grammage and thickness of all the boards are shown in table 4-6.
8 ± Shows the standard deviation of the 10 samples
34
Table 4-6 Grammage and thickness of boards9
Board Type Grammage (g/m2) Thickness (μm)
Industrial board 289±2.9 375±6.2
MS 200, US 25 301±3.0 443±9.4
MS 175, US 50 301±4.7 447±4.7
MS 150, US 75 301±4.1 446±4.3
MS 125, US 100 286±6.2 428±10.2
Figure 4-5 and 4-6 show the effect of the changing configurations on the tensile strength and tensile stiffness index respectively. It can be seen that US 25, MS 200 which is the same configuration of the industrial board has shown the lowest values. While with the increase in the grammage of US layer and decrease in the grammage of MS layer the tensile properties are increasing.
The highest values can be seen with the US 100 and MS 125 which might be due to the fact that US layer with higher strength than MS layer have affected the overall strength of the board. Fibres with better quality have an influence on the overall strength properties of the board.
Figure 4-5 Tensile index of the board with different configurations10
9 ± Shows the standard deviation of 5 samples
10 Error bars show the standard deviation of 10 samples
35
Figure 4-6 Tensile stiffness index of the board with different configurations11
4.4.2.Z-strength
It can be seen from the figure 4-7 that with the decrease in the grammage of MS layer and increase in the grammage of US layer the increase in z-strength is not significantly secured but based on the trend a raise is expected. Z-strength of the board depends on the fibre-fibre bonded area and the joint strength of the fibre-fibre bonding. The increase in z-strength could be due to the ply bond strength. This might be due to more coverage of fibre because of increased grammage in the bottom layer. Ply bond strength is increased with the high contact area between the layers in the wet pressing (Kajanto 1998).
Figure 4-7 Z-strength of the board with different configurations12
4.4.3.Scott Bond
Figure 4-8 shows the comparison between the different configurations of the board and the Scott Bond values. It can be seen from the figure that MS 125 and US 100 has shown the highest value while the lowest value of Scott Bond is shown by MS 200 and US 25 which again shows the same trend as other
11 Error bars show the standard deviation of 10 samples
12 Error bars show the standard deviation of 10 samples
36 board properties showed. The higher value with the increase in the grammage of the US layer can be due to the fact that MS layer having the bulkier fibres influences the strength of the board while decrease in the grammage of MS layer reduces its effect.
Figure 4-8 Scott Bond of the board with different configurations13
4.5. Bending stiffness
Table 3-3 shows the values of bending stiffness index which were calculated using laminate theory and the formulae is given in eq. 1.1. It is clear from the results obtained that MS200, US 25 shows the lowest value of the bending stiffness index while it increases with increase in the grammage of the US layer. Bending stiffness index depends upon the grammage, thickness and the tensile stiffness index of each layer. Bending stiffness is showing an increase with the increase in US grammage which could be because of the improved tensile properties of the layers which effect the bending stiffness of the board.
Table 4-7 Different configurations of layers and Bending stiffness index
13 Error bars show the standard deviation of 10 samples
Configuration of layers
Bending stiffness index (mNm/kg)
MS 200, US 25 0.038
MS 175, US 50 0.042
MS 150, US 75 0.046
MS 125, US 100 0.051
37 4.6. Fracture measurements of 4 ply board
Cracked samples from the ZDTS and Scott Bond were used to investigate the cracks in the 4 ply boards. Thickness of the samples which were used to investigate the z-strength and Scott Bond value was determined using STFI thickness tester. The thickness of each sample was noted down. As the thickness of each layer used in the board was known the crack location was found out.
In case of all the configurations the cracks occurred in the bottom layer for z- strength and Scott Bond values shown in tables 4-8. The location of the cracks in all the samples was at the surface of the bottom layer and not in the boundary of the layers. It validates the argument of using couching method because LB Multilayer sheet former was firstly preferred to get the good strength at the boundaries. It is important to discuss the difficulties while determining the thickness of the Scott Bond cracked samples. The samples were very short and the tape which was used to test the samples was sticking with the surface of the instrument resulting in the difficulties. Figures 4-9 and 4-11 show the fractured samples after testing with Scott Bond and Zwick tester respectively. In figure 4-9 the irregularity in the thickness is more visible as compared to figure 4-11. Thickness profile of Scott Bond (shown in figure 4-10) and thickness profile of z-strength (shown in figure 4-12) show the irregularities of the thickness between both samples. Thickness profile of Scott Bond sample is more uneven than z-strength because of the partial breakages within the sample which are observed in case of Scott Bond method (Andersson & Mohlin 1980).
Table 4-8 Thickness measurements of fractured samples14
14 ± shows the standard deviation of 10 samples
Board Type Thickness of US (μm) Thickness of fractured z- strength sample (μm)
Thickness of fractured Scott Bond sample (μm)
MS 200, US 25 48.9±2.2 14.9±3.57 30.3±9.22
MS 175, US 50 82.4±1.19 33.0±5.97 35.3±7.18
MS 150, US 75 115±3.77 36.8±5.44 40.8±3.35
MS 125, US 100 197±3.45 143±4.50 100±6.08
38
Figure 4-10 Thickness profile of the sample after Scott Bond testing
Figure 4-12 Thickness profile of the sample after z-strength testing
4.7. Fibre Characteristics
Table 4-8 shows the fibre characteristics of each pulp. It can be seen from the table that all the pulps show the same mean length while one important thing which can be noted from the results is the difference between the mean lengths of recycled and virgin fibres. Normally a softwood fibre length is between 2.5-4.5 mm while hardwood pulp fibres have the length between 0.7- 1.6 mm. Therefore it can be observed that as far as fibre length is concerned recycled fibres are very short and which could be due to the fact of the continuous recycling and cutting of the fibres. Same is the case with the width of the fibres which is nowhere near the width of the virgin softwood fibre. An important result which can be seen is the amount of fines present in the pulps;
ÖS pulp shows the least amount of fines while other pulps possess more fines than the ÖS pulp. Normally fines which are obtained during beating because of external fibrillation contribute to a higher tensile strength because of high relative surface area (Dienes et al. 2003). ÖS pulp has shown the least amount of fines in table 4-8 and the results show that ÖS pulp has given higher
Figure 4-9 Fractured sample after Scott Bond testing
Figure 4-11 Fractured samples after z- strength testing with Zwick tester
39 strength. The amount of fines present in the pulps might be primary fines and not the secondary fines.
Table 4-9 Fibre characteristics of each pulp
Fibre Characterization Pulp Population
(105/g)
Mean fibre length (mm)
Mean fibre width (μm)
Coarseness (μg/m)
Shape factor (%)
Fines content
(%) Kinks/fibre
ÖS 7.01 1.11 22.5 140 90.2 5.7 0.46
SS 6.14 1.24 26.9 143 86.8 9.5 0.66
MS 6.49 1.28 26 130 89.5 8.5 0.46
US 6.1 1.28 26.1 136 89.3 8.6 0.44
40
5. Conclusion
Couching method used in this work can be used to create multiply board with mechanical properties in thickness direction close to industrial board
With the changed configuration by increasing the grammage of under layer a raise in the tensile index, tensile stiffness index and bending stiffness is observed. The increase in z-strength is not secured but based on the trend a raise is expected
By increasing the grammage of individual layer all properties show an increase while z-strength decreases because at high basis weight rupture occurs in the plane of minimum strength. While at low basis weight the room for such variations in the thickness direction is small
With an increase in the grammage of the under layer cracks are still located in the under layer and overall improved board is obtained
At lower grammage adhesive tape plays an important role while determining the strength in thickness direction in case of single ply sheets
