The Effect of Different Xylan Contents on the Strength Properties of Softwood Kraft pulp

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Institutionen för Kemiteknik

Maria Svedinger Andersson

The Effect of Different Xylan Contents

on the Strength Properties of Softwood

Kraft pulp

Xylanhaltens påverkan på styrkeegenskaper hos

barrvedssulfatmassa

MSc Thesis in Chemical engineering 30 points

Specialized in Pulp Technology

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Abstract

The aim of this Master thesis was to investigate if the xylan content had any influence on the physical properties of softwood kraft pulps. To achieve pulps with different xylan content different kraft cooking conditions were used; two different temperatures and two different effective alkali levels.

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Sammanfattning

Målet med examensarbetet var att undersöka om och hur mycket xylaneti pappersmassan påverkar fiberns och därmed papperets fysikaliska egenskaper. Egenskaperna som

undersöktes var drag- och rivstyrka samt zero-spanstyrka. Xylaninnehållet skulle varieras genom att kokförhållandena förändrades dels genom olika koktemperaturer dels olika satsningar av effektivt alkali vid given sulfiditet. Dessa var 160ºC med 30% effektivt alkali(EA) hädanefter benämnd referenskoket och 145ºC med 17% effektivt alkali(EA) som benämns det milda koket i fortsättningen.En bestämning av koktiden gjordes för att nå 30 i kappatal och två provkok, ett vid varje temperatur behövde göras.Skillnaden i xylanhalt mellan de slutliga massaproverna låg på c:a 3% enheter.

Styrkeproverna gav inga entydiga svar på om skillnaden i xylanhalt gav någon effekt på massastyrkan. Dragproverna visade att för omald massa var massan från referenskoket starkast men att massan från det mildare koket reagerade kraftigare på malningen. Redan vid 1000 varv hade den i princip samma dragindex som referensmassan vid samma malgrad. Zero-span mätningarna visade att fibrerna hade samma styrka när de var omalda.

Resultaten från fiberanalysenverifierade resultaten från styrketesterna eftersom en tjockare fiber bör ge en styvare fiber och därmed erhålls färre bindningspunkter. Färre bindningspunkter ger en lägre dragstyrka och det krävs mindre energi för att bryta

bindningarna. Efter malningen kan man se att zero-span styrkan har minskat betydligt för massan med högre xylanhalt medan referensmassan behöll styrkan. Dessutom har

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Preface

The exprimental of this thesis was made in the cooking laboratories of Karlstad University and Metso Fiber.

I want to thank my advisor at Karlstad University professor Ulf Germgård for his support and good advices. I also want to thank my colleagues for all there help and especially Frederica De Magistris and my supervisor Helena Håkansson. Pia Eriksson and

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1 Introduction

The paper industry has different demands on the pulp depending on what type of paper they are making. Therefore it is of great importance that the pulp industry knows how to produce pulp with different qualities for example regarding strength, compressibility and printability. One way to accomplish that is to change the hemicellulose content in the pulp. A lower energy cost and higher yield is also welcomed.

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2 Background

Wood is a material that consists of fibres built up by different kinds of substances: cellulose, lignin, hemicellulose

of compounds that can be extracted by means of polar and non and G Wegener, 1989). The main compon

andsoftwood consists of 41

Hemicellulose and lignin are also important parts of the fibre wall and hemicellul together with cellulose the brick or the reinforcement of the fibre wall. Lignin is a binding agent that keeps the other components together and it also gives the its brown colour. The colour is one reason that

the lignin to be present in the pulp possibility for fibres to bind to each other

paper strength. In contrary hemicellulose contributes to fib to the pulp strength. How the

contentsat fixed lignin content was therefore interesting to study.

Table 1 A Table of the composition in wood fibre (Fogelholm C, Gull

Below is a picture of a fibre cross section were one can see the different layers in the fibre wall, Fig 1. The space

around the lumen. The space between the fi

of lignin, cellulose and pectin although lignin is the main substance. The middle lamella “glues” the fibres together. The primary cell wall consists of pectin, cellulose,

hemicellulose and extensin (glycoprot

in the network. The secondary cell wall consists of three layers S1, S2 and S3. Their chemical compositions differ

S2 is richer on cellulose an Textbook, 2006, ch.3.8)

Wood is a material that consists of fibres built up by different kinds of substances: in, hemicellulose and extractives. Extractives is a term for a large number of compounds that can be extracted by means of polar and non-polar solvents ( DFengel and G Wegener, 1989). The main component in wood fibres is cellulos

of 41-46% cellulose(Fogelholm C, Gullichsen J,

Hemicellulose and lignin are also important parts of the fibre wall and hemicellul together with cellulose the brick or the reinforcement of the fibre wall. Lignin is a binding agent that keeps the other components together and it also gives the

The colour is one reason that the producers of white p

the lignin to be present in the pulp.Another reason is the fact that lignin reduces the nd to each otherand fibre to fibre bonds are essential for the paper strength. In contrary hemicellulose contributes to fibre to fibre bonds and thereby to the pulp strength. How the strength properties are influenced by the hemicellulose

content was therefore interesting to study.

of the composition in wood fibre (Fogelholm C, Gullichsen J, Chemical pulping)

is a picture of a fibre cross section were one can see the different layers in the . The space in the middle is called the lumen and the cell wall is built up round the lumen. The space between the fibres is called the middle lamella

cellulose and pectin although lignin is the main substance. The middle lamella “glues” the fibres together. The primary cell wall consists of pectin, cellulose,

hemicellulose and extensin (glycoprotein) –which is thought to hold the cellulose fibrils network. The secondary cell wall consists of three layers S1, S2 and S3. Their

s differ; S1 has a higher concentration of lignin than S2 and S3 but S2 is richer on cellulose and hemicellulose than both S1 and S3 (The Ljungberg

Wood is a material that consists of fibres built up by different kinds of substances: term for a large number

r solvents ( DFengel is cellulose, Table1,

Fogelholm C, Gullichsen J, 2000).

Hemicellulose and lignin are also important parts of the fibre wall and hemicellulose is together with cellulose the brick or the reinforcement of the fibre wall. Lignin is a

binding agent that keeps the other components together and it also gives the sulphate pulp the producers of white paper do not want

lignin reduces the essential for the re to fibre bonds and thereby properties are influenced by the hemicellulose

ichsen J, Chemical pulping)

is a picture of a fibre cross section were one can see the different layers in the in the middle is called the lumen and the cell wall is built up

s called the middle lamella and consists cellulose and pectin although lignin is the main substance. The middle lamella “glues” the fibres together. The primary cell wall consists of pectin, cellulose,

d the cellulose fibrils network. The secondary cell wall consists of three layers S1, S2 and S3. Their

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Figure 1 Picture of a wood fibre were one can see the structure of the fibre. (G. Daniels, 2006)

2.1 Cellulose

The main component in wood is cellulose and approximately 40-50% of the wood fibrer is cellulose. Cellulose consists of glucose units; they are linked by a β–(1, 4) glycosidic linkage, Fig 2. Every second unit is turned upside down and together two units form a cellobiose unit. The cellobiose unit has a length of 1.03 nm (Dietrich Fengel and Gerd Wegener, 1989) and a cellulose chain consists of 5000-12000 glucose units.

Figure 2A cellulose chain (National encyclopedia)

The micro fibrils give the cell wall and the tree its strength. They are built up by cellulose chains packed together and bonded with strong hydrogen bonds. They consist of both amorphous and crystalline regions.

2.2 Hemicellulose

The hemicellulose molecule differs from cellulose in several ways as they are heteropolysaccharides,have much shorter chains and the chain molecules are

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are linked with glycosidic bonds. The monosaccharides are arabinose, galactose, glucose, xylose, mannose and rhamnose. There are only a few major groups of hemicelluloses and there are differences between the hemicellulose present in softwood and those present in hardwood, Table2 (The Ljungberg Textbook, 2006, ch. 5).

Table 2 Content and kind of hemi cellulose in hardwood and softwood (Anita Teleman, 2006)

Occurrence Hemi cellulose Amount(% By dry weight)

Softwood Galactoglucomannan 5-8

Softwood Glucomannan 10-15

Softwood Arabinoglucuronoxylan 7-15

Hardwood Glucuronoxylan 15-35

Hardwood Glucomannan 2-5

As seen in Table 2 there are three main groups of hemicellulose; Arabinoglucuroxylan, glucomannan and galactoglucomannan. In this work Arabinoglucuroxylan is referred to as xylan.

2.2.1 Xylan

Softwood and hardwood have different kinds of xylan. Glucuronoxylan is the major xylan present in hardwood. It has a backbone of xylose units that are linked by β-(1→ 4)-glycosidic bonds. It is the main hemicellulose in hardwood, 15-30 % of the wood is glucuronoxylan.

Figure 3 A softwood xylan molecule (A. Teleman, 2006)

In softwood xylan, Fig 3,arabinofuranose is linked to the backbone by α-(1→3)-glycosidic bond. It also lack acetyl groups, which differentiate them from hardwood xylan together with a higher portion of arabino-4-O-methylglucuronoxylan.

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170°C the removal of xylan is gradually diminished and the residual amounts is relatively sTable (R. Aurell and N.Hartler,1965).

2.2.2 Glucomannan

Glucomannan is the most common hemicellulose in softwood, Figure 4. Glucomannan is usually divided in two types, galactoglucomannan and glucomannan. Both forms have galactosyl units as substituent andglucomannan have a lower degree of substitution than galactoglucomannan. The fact that galactoglucomannan have a higher degree of

substitution makes it more soluble in water than glucomannan (A. Telemann, 2006). This could be an explanation to why the glucomannan dissolve easy even at low temperature.

Figure 4 Glucomannan molecule (A. Teleman, 2006) .

The removal of glucomannan can be divided into three well definied phases: the initial phase, the bulk phase and the final phase. In the initial phase at the beginning of the cook most of the glucomannan is in the form of galactoglucomannan (Aurell,Hartler 1965a).

2.3 Lignin

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Figure 5 Lignin structure of softwood (Jongerius, Anna L, et al,2012)

The lignin is sensitive to alkali charge and a higher charge gives a faster removal. The temperature is also of importance in all phases of the cook. When the temperature is low the rate of the removal of the lignin is slow.

2.4 Physical testing of laboratory sheets

In order to see if the strength was affected by different concentration of Xylan in the pulp the tensile index, tear index and zero-span index were measured. Viscosity is a parameter often used to get a hint of the pulp strength. Another useful tool is the fibre analysis which gives information about fibre length, fibre width, shapefactor and coarseness.

2.4.1 Tensile strength

Tensile strength is measuring the fibre bonds and the fibre strength. Depending on the strength of the inter-fibre bonds and on the fibre strength the fibres or the bonds break (K. Niskanen,1998). The beating increases the strength in the network because the bonding area increases. This occurs when the fibre surface is exposed to the rough treatment and fibrils arise from it. If the bond strength is very high the fibres break instead of the bonds. 2.4.2 Tear strength

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point where less energy is required to break the fibre then to break the bonds. When this happens the tear strength decreases and thus beating that weakens the fibres (C. Fellers and B. Norman, KTH)

2.4.3 Viscosity

Viscosity is a way to measure the resistance of a fluid to being deformed by shear stress or extensional stress. The viscosity of a fluid is dependent on the size and shape of the particles. Small round particles give lower viscosity than both longer and thicker

particles. The temperature and flow velocity is also influencing the viscosity. To measure viscosity of pulp a CED solution is used and the test gives an indication on the degree of polymerisation. At a high viscosity the degree of polymerisation is high and indicates high fibre strength.

2.4.4 Zero span

Zero-span is a method that can be used to give an indication of the fibre strength. The method is a tensile strength test with a span length of zero. The ideal span occurs when the span is so narrow that the fibre bonds are insignificant. It has been found that straight and curled fibre reacts differently to zero-span tests (R.S. Seth, 2001). The curls are given by a deformation in the fibre wall. It decreases the fibre- and the bonding strength.

Beating is known to have a positive effect on zero-span strength. It is the straightening effect on the fibre that gives the higher zero-span value after beating.

2.4.5.Fibre analysis

The fibre analysis was made with L&W STFI FiberMasterand it measures fibre length, width, shape, bend ability, kink, fines and coarseness by image analysis. The suspension is passing between two glass plates and is photographed with a video camera (H.

Karlsson and P-I. Fransson, 1997). Then the images are analysed according to the physical properties listed above and the results are recorded and saved in a file.

2.4.6. Beating

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Figure 6 A picture of a PFI mill.(Institute of biopolymers and chemical fibers)

The refining/beating is primary for strength increase and in some cases to increase high tensile energy absorption. The beating straightens the fibre and fibrils arise from the surface. The fibres become more flexible which gives the paper more binding points(The Ljungberg Text Book, G. Annergren and N. Hagen, 2006).

2.5 Hemicellulose content effect on strength properties

Increased content of hemicellulose is known to decrease tear index(U. Molin et al. 2002).Increased xylan content gives an increased bondingstrength (C. Schönberg et al, 2001). According to C.Schönberg et al. ahigher amount of xylan on the fibre surface is correlated to a higher bonding ability. For tensile strength the total amount of xylan along with the charge of the fibres isimportant.

The adsorption of xylan on to the fibre surface is proportional to the concentration of the xylan in the cooking liquor. When the temperature reaches 170°C the concentration reaches its maximum.Xylan adsorption seems to be highest at a high temp and with most of the alkali consumed (S.Yllner and B.Enström, 1957). This indicates that a low

effective alkali at a high temp for a long time will give a pulp with good bonding

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3 Materials and method

3.1 Cooking

The pulp was cooked in autoclaves attached to a rotating shaft in a polyethylene glycol (PEG) bath. Wood chips from Norwegian spruce were used. The chips had been laboratory screened. The chips were pre-treated with steam at 110ºC for 10 minutes to force air out of cavities in the chips and replace it with water. White liquor was prepared from technical grade NaOH and N2S and was added into the autoclaves. The autoclaves were then pressurised with nitrogen gas to 10 bar and were heated for 30 minutes at 90ºC in the PEG bath. After 30 minutes they were depressurised and the temperature was increased by 1.33°C/minute up to the digesting temperature. When the cook was finished the autoclaves were cooled in a water bath. The black liquor was collected from each fraction and analysed for remaining hydroxide ions, according to SCAN-N 33:94. The pulp was stored in tap water in a plastic bucket over night. The next day it was

disintegratedfor 10 minutes in a laboratory disintegrator, washed and screened and then the pulp was stored in a refrigerator until analyzed.

Kappa number was analysed according to ISO 302:2004 and the carbohydrate content was analysed by a HPLC method developed by Metso fibre.

3.2 Papermaking and physical testing

Laboratory sheets had to be prepared for the physical testing. The sheets were formed according to ISO 5269-1:2005. The papers were conditioned and tested in a climate room at 23ºC and 50 % humidity.

Tensile strength was analyzed according to ISO 1924-2:1994. The test was made with an Instron 4411.

Tear strength was analyzed according to ISO 1974:1990 and was performed with Elmendorf-type tear tester, AB Lorentzen& Wettre, Stockholm, Sweden.

Viscosity was analyzed according to ISO 5351:2004.

Zero-span strength was analyzed according to ISO 15361:2000 and the samples were analyzed rewetted.

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4 Results

The project started with a pre-study and a number of different cooking conditions were tested, but most of them were rejecteddue to too small differences in xylan content between the cooking conditions. In the end two different kind of cooking conditions were chosen, 160°C with 30% effective alkali (EA), referred to as reference cook, and 145°C with 17% effective alkali (EA), referred to as the mild cook. The cooking time was decided from the desired kappanumber and amount of xylan in the pulp. Several cooks were carried out in order to find the timewere the kappanumber and the amount of xylan where sufficient. The kappanumber had to be the same for the both pulps and the

difference in xylan content had to be at least 10 %but preferable greater.

The residual alkali was measured in order to determine if there was sufficient amount of alkali in the cook. In the reference cook there was so much alkali left it was assumable that there was no shortage of alkali in the dissolving process. In the mild cookthere was considerably less alkali left but according to the kappanumber the amount was sufficient for the lignin dissolution.

4.1 Pre-study

In Figures 7 and Figure 8 below the results from the pre-study are shown.From the outcome of the pre-study the time for the cooking was decided.The xylan content and the kappanumber were the two properties that were most important. As seen in the Figures 7 and Figure 8,neither the reference cook northe mild cook showed a significant difference in hemicellulose content, due to the varied kappa number. Therefore kappanumber 30 was chosen since it is commonly used both in laboratory cooking and in the industry.

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Figure 8 The content of xylan in % on pulp vs kappanumber and labelled with cooking time

4.2 Cooking parameters

As can be seen in Figure 7 and Figure 8 the condition that dissolves the xylan preserves the glucomannan and vice versa. Glucomannan seems to be the most sensitive

hemicellulose in the pulp with respect to time in the long mild cook. In order to dissolve both glucomannan and xylan a cook with high effective alkali at temperatures over 140°C but below 160°C should be used. It is difficult to preserve both xylan and glucomannan at the same time due to the narrow temperature interval

When the pre-study was finished and the cooking time was decided, the main cooks were made. The residual alkali, yield, kappa number and the amounts of rejectswere

determined. The results are shown in Table 3 below.

Table 3 Reject, residual alkali, yield and kappa number from the cooks are shown in the Table

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4.3 Yield

The yield increased by two percentage units in the mild cook compared to the reference cook. The mild treatment seemed to increase the yield.

Figure 9 The yield of the two cooks after screening

4.4 Hemicellulose

The cooking was designed either to preserve (mild cook) or to degrade (reference cook) the xylan. In the mild cook xylan was preserved but not glucomannan and in the

reference cook glucomannan was preserved but not xylan. This led to an equal amount of hemicellulose in both pulps but with different proportions to each other.

4.4.1. Xylan

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Figure 10 Yield of xylan in the prestudy

Figure 11 Yield of xylan in the main cook

The dissolution process of xylan is similar to how lignin dissolves which can make it hard to preserve the xylan and dissolve the lignin. However it is possible to preserve the xylan and dissolve the lignin and it is the temperature of the cook that should be varied to control the dissolution. If less alkali is charged there could be a problem with the

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4.4.2 Glucomannan

According to the pre-study the content of glucomannan was in the expected range. In Figure 12 and 13 below it is seen that in the mild cook, less glucomannan has been preserved than in the reference cook.

Figure 12 Yield of glucomannan from the pre-study

Figure 13 Yield of glucomannan of the main cook

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A short cooking time at high temperature seemed to increase more than a mild, long cook at

to the alkali charge as well but based cooking time seems to be more important.

4.5 Fibre analysis

The results if the fibre analysi

Table 4 Results from the fibre analysis

145°C

Fibre length (mm) Fibre width (µm) Shape factor (%)

Coarseness (µg/m) 145.

With higher alkali charge and temperature the width and coarseness. The milder cooked fibres had reference cooked fibres.

4.6 Viscosity andSchopper

The viscosity analysis gave an indication that the pulp from the mild

longer cellulose chains and therefore should be stronger than the reference cook pulp.

Figure 14 Viscosity of the pulp from the

The fibres from the milder cook have a higher viscosity w higher strength.

time at high temperature seemed to increase the glucomannan content long cook at low temperature. The glucomannan is probably

kali charge as well but based from the results of the cooks in this study more important.

e analysis are shown inTable 4 below.

from the fibre analysis

145°C 160°C 2.3 2.2 29.8 28.8 86.2 88.4 145.2 141.5

e and temperature the fibres were more affected regarding length, s. The milder cooked fibres had a lower shape factor than the

Schopper-Riegler

gave an indication that the pulp from the mild cook consisted of longer cellulose chains and therefore should be stronger than the reference cook pulp.

the pulp from the pre-study

The fibres from the milder cook have a higher viscosity which indicates

the glucomannan content low temperature. The glucomannan is probably sensitive

from the results of the cooks in this study the

more affected regarding length, a lower shape factor than the

cook consisted of longer cellulose chains and therefore should be stronger than the reference cook pulp.

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Figure 15 Results from viscosity analysis on pulp from the main study

The dewatering resistance was investigated by using a Schopper-Riegler apparatus which showed very little difference between the dewatering resistances of the pulps. The charge of the fibres is important for both the strength and the dewatering ability. Thus the SR indicates there are no significant differences in the charge of the fibres.

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4.7 Zerospan

The zero span tests indicated a lower strength on the refined fibres from both the

reference cook and the milder cook, Fig17. In this case the fibre from the reference cook is stronger than the fibres from the mild cook. This can be seen in the Figure below and the error bars indicate a high accuracy in the results.

Figure 17 Results from the zero-span test

4.8. Tensile index

Tensile index showed no difference between the pulps except that the unbeaten pulp of the reference cook (160°C) gave a higher tensile index than the mild cook (145°C). Figure 18 shows that the pulp from the mild cook was more sensitive to initial

refiningAfterthe initial refining the two pulps showed the same trend. One explanation for that could be that the hemicellulose dissolved in the black liquor is adsorbed on the fibre surfaces. If that is the case more fibrils would probably arise from the surface and the bonded area would increase.

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Figure 18 Tensile index vs degree of refining

The unbeaten pulp from the reference cook had a higher tensile strength than the pulp from the mild cook . There can be numerous reasons for that. When cooked at high temperature and EA the fibres may be softer and have developed more fibrils on the surface compared to when cooked at milder conditions. Lignin might be re-adsorbed on the fibre surface when cooked in mild conditions and neutralize the charge on fibre surface.

4.9 Tear index

Tear index showed a significant difference between the two pulps. In Figure 19 it is seen that the increased bonding that could be seen in Figure 18 does also affect the tear strength.

When the pulp is beaten the fibre becomes weaker. Tear strength depends on both strength of the network and fibre strength. It is a measurement of the energy required to pull out the fibre from the network and/or break the fibres. If the network bonding strength is higher than the strength of the single fibres, the fibres brake instead of being pulled out from the network. The tear index for the mild cook was lower than for the reference cook, probably due to a re-adsorption of lignin on the fibre surface.

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Figure 19 Tear index vs degree of refining

4.10 Tear index vs tensile index

In Figure 20 the tear index is plotted against the tensile index.The results are as expected. With higher bonding strengththe tensile index increases and thetear index decreases.

Figure 20 Tear index vs tensile index.

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5 Discussion

A way to accomplish a strength differences between pulp samples is to vary the xylan content and keep theglucomannan content constant. However the pulps produced in this project had different xylan content but the glucomannan content varied too, which gave the project another line of approach. Instead of comparing pulps with only different xylan contents we were able to see if the new composition had any effect on the strength

properties. Also if the higher content of xylan had any effect on the tensile strength. The strength properties did not differ much between the pulp from the mild cook andthe pulp from the reference cook. It is probably due to the cooking parameters in the mild cook which did not give the adsorption of xylan on the fibre surface that was wanted. Cooking at higher temperature with low EA probably would have given a higher bonding ability.

The zero-span index indicates that the fibres from the reference cook were stronger than those from the milder cook when they were refined. But the tensile strength increased much more for the pulp from the milder cook with initial refiningwhile the tear index decreased equally for both pulps.The difference in pulp strength was the same for all grades of refining. If the fibres from the milder cook were stiffer than the fibres from the reference cook, it could be an explanation for the results.With higher stiffness the fibres get a lower bonding strength by a decreasein the number of bonding points compared to fibres with lower stiffness. When the beating fibrillates the fibre and makes it more flexible it is possible it decreases the fibre strength too. If we assume the beating decreases the fibre strength that could be an explanation for the results.

For the milder cook the tensile index increased and the zero-span index decreased with refining. The decrease in zero-span could explain the decrease in tear index if the energy required to break the fibre is less than that required to break the bonds and pull out the fibre from the network. In the reference cook tensile index increased but not so much and zero-span index only decreased slightly. The size of the error bars indicates almost no variation between the different degrees of refining. These results could be explained by the fact that the fibres from the mild cook were thicker and had a higher coarseness and thereforprobably were stiffer. If that is the case the bonding points are fewer and that could be the reason why they have a lower tensile index and tear index but the same zero-span strength when unbeaten.

One can see only a small difference between the tensile index on the two pulps, and the xylan content does not seem to have the effect expected. This result can be due to the effects discussed in the background discovered by C. Schönberg et al. that tensile

strength is depending on the amount of xylan and the total charge of the fibre. But also on the fact that adsorption of xylan on the fibre surface increases with high temperature and low concentration of alkali(S.Yllner and B.Enström, 1957).The similar behaviour

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6 Conclusions

.

There was a significant difference in tensile strength for the unbeaten pulpwhere the reference cook was stronger. But after refining there was no significant difference in tensile strength between the mild cook and the reference cook.The milder cook gave a higher yield than the reference cook.

The zero-span index showed a significant difference between the pulps after refining where the mild cook was weaker. Before refining there was no difference in zero-span index.

The fibres from the milder cook are stiffer, have lower strength, are strongly affected by initial beating; give a lower tear index and a lower zero-span index. The fibres from the reference cook are not as stiff, they are more flexible. Therefore they have a higher strength, are not so affected by beating, have a higher tear index and a higher zero-span index.

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7 References

• AnnergrenGöranand HagenNils, Industrial beating/refining,The Ljungberg

Textbook, 2005, ch. 34

• Aurell Ronnie and Nils Hartler, Kraft pulping of pine, Part 1: The changes in the composition of the wood residue during the cooking process,

Svenskpapperstidning, no. 3/1965, p.59-68

• Daniel Geoffrey, Wood and fibre morphology, The Ljungberg Textbook,2006,ch. 3

• Fogelholm Carl-Johan and Gullichsen Johan, Chemical pulping, p. A27 • GustavssonCatrin A-S and Al-DajiniWaleedWafa, The influence of cooking

conditions on the degradation of hexenuronic acid, xylan glucomannan and cellulose during kraft pulping of softwood, Nordic Pulp and Paper Research

Journal,Vol 15/no. 2/2000, p. 160-167

• Dietrich Fengel and Gerd Wegener, Cellulose, Wood Chemistry ultrastructure

reactions, 1989, ch. 4

• Henriksson Gunnar,Lignin, The Ljungberg Textbook, 2006, ch.6 • Institute of biopolymers and chemical fibers,

Polenhttp://www.ibwch.lodz.pl/en30,laboratory_of_paper_quality.html (2013-05-13)

• Jongerius, Anna L. , Jastrzebski, Robin, Bruijnincx, Pieter C.A and Bert M. Weckhuysen,CoMo sulfide-catalyzed hydrodeoxygenation of lignin model compounds: An extended reaction network for the conversion of monomeric and dimeric substrates,Journal of Catalysis, Volume 285/Issue 1/ January 2012, Pages 315–323

• Karlsson Håkan and Fransson Per-Ivar, Innventia, http://innventia.knowitis.se (2012-06-24)

• Molin, Ulrika andTeder, Ants, Berlin Importance of cellulose/hemicellulose-ratio for pulp strength, Nordic pulp and paper Research Journal Vol. 17/no. 1/2002 p. 14-28

• National Encyklopedin ,http://www.ne.se.bibproxy.kau.se:2048/lang/cellulosa (2013-08-25)

• NiskanenKaarlo, Paper physics,1998 , Finnish Pulp and Paper Research Institute, FapetOy, Ch. 5

• Norman Bo andFellers Christer, Pappersteknik, 1998, Division of paper

Technology, The Royal Institute of Technology

• R.S. Seth, Zero-span tensile strength of papermaking fibres, PaperijaPuu-Paper

and Timber Vol.83/No.8/2001/p.597-604