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

Date/Sem: August 2013 Supervisor: Ulf Germgård Examiner: Lars Järnström

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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. The cooking conditions used were 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 first step in this study was to determine the cooking time needed for reaching a certain kappa number, i.e.30. It was also determined whether the difference in xylan content between the pulp samples was sufficient when these cooking conditions were used. When the correct cooking time and cooking conditions had been found new cooks were made using these conditions. All cooks were made at a liqour to wood ratio of 4:1. The difference in xylan content between the corresponding pulp samples was found to be 3%. The physical testing showed no significant difference in the tensile strength between the two pulps after beating. There was a difference in tear index however and the pulp with the highest content of xylan had the lowest tear index. Zero-span index was the same for the two pulps when unbeaten. After beating the zero-span index decreased for the pulp with highest xylan content but stayed unchanged for the pulp with the lower amount of xylan. These results can be explained by the results from the fibre analysis which showed that the fibres with high xylan content were longer, thicker and had a higher coarseness. Thicker fibres are probably stiffer than thinner fibres which gives the paper fewer bonding points and a lower strength. The result from the zero-span test indicated that the fibres with higher xylan content are affected more by beating than fibres with the lower xylan content.

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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 dragstyrkan ökat för båda massorna men mest för massan med högre xylanhalt. Det kan förklaras med att malningen ger små fibriller på ytan av fibern och en mjukare och böjligare fiber. Därmed ökar bindningsstyrkan då bindningsarean ökar och fibern blir mjukare och böjligare. Den ökade bindningsgraden samt den minskade fiberstyrkan kan förklara varför rivindex fortfarande var lägre för massan med högre xylanhalt trots att dragindex ökade med ökad malning.En annan förklaring kan vara att vid ett långt kok med låg temperatur är det troligt att lignin adsorberas på fibrerna.Ligninet på ytan ger en sämre bindningsförmåga vilket leder till att de är lättare att dra ur nätverket med lägre energiåtgång som följd.

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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 GöranWalan are to be thanked for all there help with troubleshooting in the laboratory at Karlstad University. And a special thanks to Maria Gustavsson and Ulla Ekström at Metso Fiber for all their help.

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ContentsAbstract ............................................................................................................................... 2 Sammanfattning .................................................................................................................. 3 Preface................................................................................................................................. 4 1 Introduction ...................................................................................................................... 6 2 Background ...................................................................................................................... 7

2.1 Cellulose ................................................................................................................... 8 2.2 Hemicellulose ........................................................................................................... 8

2.2.1 Xylan .................................................................................................................. 9 2.2.2 Glucomannan ................................................................................................... 10

2.3 Lignin ...................................................................................................................... 10 2.4 Physical testing of laboratory sheets ....................................................................... 11

2.4.1 Tensile strength ................................................................................................ 11 2.4.2 Tear strength .................................................................................................... 11 2.4.3 Viscosity .......................................................................................................... 12 2.4.4 Zero span .......................................................................................................... 12 2.4.5.Fibre analysis ................................................................................................... 12 2.4.6. Beating ............................................................................................................ 12

2.5 Hemicellulose content effect on strength properties ............................................... 13 3 Materials and method ..................................................................................................... 14

3.1 Cooking ................................................................................................................... 14 3.2 Papermaking and physical testing ........................................................................... 14

4 Results ............................................................................................................................ 15 4.1 Pre-study ................................................................................................................. 15 4.2 Cooking parameters ................................................................................................ 16 4.3 Yield ........................................................................................................................ 17 4.4 Hemicellulose ......................................................................................................... 17

4.4.1. Xylan ............................................................................................................... 17 4.4.2 Glucomannan ................................................................................................... 19

4.5 Fibre analysis .......................................................................................................... 20 4.6 Viscosity andSchopper-Riegler .............................................................................. 20 4.7 Zerospan .................................................................................................................. 22 4.8. Tensile index .......................................................................................................... 22 4.9 Tear index ............................................................................................................... 23 4.10 Tear index vs tensile index ................................................................................... 24

5 Discussion ...................................................................................................................... 25 6 Conclusions .................................................................................................................... 26 7 References ...................................................................................................................... 27

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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. The main idea of the project was to investigate if there were differences in the strength properties in pulps with different content of hemi cellulose. It was also a test to see if it was possible to get a higher xylan content and higher strength by pulping similar to industrial pulping. This was tested by cooking with effective alkali (EA), sulphidity and liquor to wood ratio (L:W) similar to what is common in the industry.

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 nonand G Wegener, 1989). The main componandsoftwood consists of 41Hemicellulose and lignin are also important parts of the fibre wall and hemicellultogether 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 theits brown colour. The colour is one reason thatthe lignin to be present in the pulppossibility for fibres to bind to each otherpaper strength. In contrary hemicellulose contributes to fibto the pulp strength. How thecontentsat 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 spacearound the lumen. The space between the fiof 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 (glycoprotin the network. The secondary cell wall consists of three layers S1, S2 and S3. Their chemical compositions differS2 is richer on cellulose anTextbook, 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 hemicellultogether 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 pthe 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 lamellacellulose 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

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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 ; S1 has a higher concentration of lignin than S2 and S3 but

The Ljungberg

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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 branched.They are mainly structural carbohydrates and consist of monosaccharides that

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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. Xylanhas a degree of polymerisation (DP) of 90-120 (A. Teleman, 2006), which can be compared to cellulose with a DPof 12000 for native spruce wood. Therefore it is easier dissolved than cellulose.Xylan is sensitive to high alkali charge and high temperature. At a temperature below 140°C only a small amount of xylan is removed. Between 140°C and 165°C the removal of xylan is very intense. This is due to alkaline hydrolysis and dissolution (C. Gustavsson and W.W. Al-Dajani, 2000). When the temperature reaches

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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

Lignin is one of the most common biopolymers in nature and in wood it acts asglue and gives the cellwall it stiffness. Hardwood and softwood both contains lignin but different kinds. Hardwood lignin has a more open shape which makes it easier to remove. The lignin structure is a three-dimensional web with the monomers connected with a number of different ether and carbon-carbon bond. It has a very complex structure with a mixture of aromatic and aliphatic moieties (G. Henriksson, 2006). Lignin contains several kinds of alcohols: p-coumaryl alcohol, coniferyl alcohol and sinapyl alcohol. The softwood lignin consists of coniferyl alcohol and small amounts of p-coumaryl alcohol but no sinapyl alcohol. The structure of lignin present in softwood is shown in the figure below, Figure 5.

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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

To evaluate the paper strength in a different way one can measure the tear strength. In a paper with low inter-fibre bonding, this is often the case for pulp with a low degree of beating, the theory is that the fibres are pulled out of the network rather than break.The energy required to pull out a fibre is decided by the fibre length and the number of bonding points. There is an increase in the number of bonding pointswhen the pulp is beaten to a higher degree.That is a result of the fibrils that rises from the fibre surfaces. This makes it harder to pull out the fibres from the network and initially more energy is required to tear the paper. However if the pulp is beaten to a very high degree it reaches a

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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

Refining or beating is a way to mechanically treat the pulp in order to change the physical properties. In this project a PFI mill has been used, Figure 6. The PFI mill is strictly used in laboratory and it has no equivalent in the industry

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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 ability.At those cooking conditions however adsorbtion of lignin also occursand this can counteract with the effects from the xylan adsorption.

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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. Schopper-Riegler value wasanalyzedaccording to ISO 5267-1:1999

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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.

Figure 7 The content of glucomannan in % on pulp vs kappanumber and labelled with cooking time

22 h 13min

20 h 13 min

18 h 42 min

16 h 42 min

48 min

58 min

68 min78 min98 min

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145°C; EA17% 160°C; EA30%

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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

Reject [%] Residual alkali[g/l] Yield [%] Kappa number

145ºC;17%EA ( 18 h 45 min) 0.003 2.93 50 28.3 160ºC;30%EA ( 2h 7 min) 0.63 30.47 48 27.2

16 h 42 min

18 h 13 min

20 h 13 min

22h 13 min

48 min58 min

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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

In the figures below (10 and11) the xylan content is plotted and result from the pre-study is shown in Figure 10 and the result from the main cook in Figure 11. The xylan content was higher in the mild cook, which indicates that a mild treatment increases the xylan content. A cook at a lower temperature could maybe increase the xylan content even more. But it would probably not have an impact on strength since the adsorption on the fibre surface is too low. It is probably hard to dissolve more xylan at the kappanumber wanted but if more xylan could be adsorbed on the fibre surfaces it could increase the strength.

46%

47%

48%

49%

50%

160ºC 145ºC

Yie

ld (

%)

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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 dissolution of lignin and the lignin content of the pulp would therefore increase.

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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 increasemore than a mild, long cook atto the alkali charge as well but basedcooking 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 hadreference cooked fibres.

4.6 Viscosity andSchopper

The viscosity analysis gave an indication that the pulp from the mildlonger 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 whigher 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 studymore 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

20

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.

hich indicates longer fibresand

21

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.

Figure 16 SchopperRieglervs degree of beating.

0

500

1000

1500

2000

26 27 28 29 30

Vis

co

sit

y [

ml/

g]

Kappa number

145°C EA 17% 160°C EA 30%

15

17

19

21

23

25

27

29

0 1000 2000 3000 4000 5000 6000 7000

Sc

ho

pp

er

Rie

gle

r [S

Rº]

PFI-beating [revolutions]

145ºC EA17% 160ºC 30%EA

22

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.

80

100

120

140

160

0 1000 2000 3000 4000 5000

Ze

ro-s

pan

in

de

x[N

m/g

]

PFI-beating[revolutions]

145°C 17%EA 160°C 30%EA

23

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.

40

50

60

70

80

90

100

110

0 1000 2000 3000 4000 5000 6000 7000

Te

ns

ile

in

dex

[Nm

/g]

PFI-beating [revolutions]

145ºC 17%EA 160ºC 30%EA

24

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.

0

5

10

15

20

25

0 2000 4000 6000 8000

Te

ar

ind

ex

[m

N*m

²/g

]

PFI-beating[revolutions]

145ºC EA17% 160ºC EA30%

0

5

10

15

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25

0 20 40 60 80 100 120

Tensile index [kNm/g]

Tear

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[mN

*m²/

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145ºC 17%EA 160ºC 30%EA

25

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 between the two pulps when beaten can be an indication on small differences in charge of the fibre.

26

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. There has to be a further study to determine what the differences in physical properties depend on.

27

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

• Schönberg C., Oksanen T, Suurnäki A, Kettunen H, Buchert J, The importence of xylan for the strength properties, Holzforschung, Vol 55/No.6/2001p. 639-644

• Teleman Anita, Hemicelluloses and pectin. The Ljungberg Textbook, 2006, ch.5