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Fiber and Pulp Characteristics for Papermaking

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1. Pele Oy Fiber and Pulp Properties for Papermaking Pekka Komulainen Pekka.Komulainen@clarinet.fi 20 August, 2015 2. Pele Oy Hardwood vs. softwood cells  Hardwood…
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  • 1. Pele Oy Fiber and Pulp Properties for Papermaking Pekka Komulainen Pekka.Komulainen@clarinet.fi 20 August, 2015
  • 2. Pele Oy Hardwood vs. softwood cells  Hardwood fibers are about third of the softwood fiber length (1 vs. 3 mm) and 2/3 of softwood fiber thickness (20/30 μm). In addition, hardwood includes lot of vessel and ray cells, which can cause so called vessel picking and linting in offset printing. Picture: Prof. Wimmer 2
  • 3. Pele Oy Wood and fiber properties  The big difference between softwoods and hardwoods is amount of real fibers (tracheids). Only tracheids can form fiber network and help papermaking.  Biggest problem with nonwood fibers is low share of real fibers (commonly less than 50%). 3 Wood properties Unit Picea abies Pinus sylvestris Pinus radiata Populus tremula Betula pendula Eucalyptus globulus Amount of wood volume Fibers (tracheids) % 95 93 89 61 65 49 Vessels % 0 0 0 26 25 21 Ray cells etc. % 5 7 11 13 10 30 Average fiber dimensions Length mm 3,3 3,1 3,3 1.0 1.1 1.0 Diameter µm 31 30 40 20 21 16 Cell wall thickness µm 3,1 3,0 7,0 3,2 3,8 3,8 2 x cell wall/width % 20 20 35 32 36 48 Wood Density (dried) kg/m3 405 550 515 450 640 820
  • 4. Pele Oy Example of softwood fiber basket 4
  • 5. Pele Oy Roles of different papermaking pulps  Softwood chemical pulps  Wet and dry runnability for papermaking, finishing and converting  Ensuring strength and stiffness for packaging materials  Hardwood chemical pulps  Good end use properties of woodfree printing papers and tissues  Good formation, brightness, opacity and printability  Decrease the costs of the fiber furnish  Mechanical pulps  Good runnability and end use properties of mechanical grades  Formation, printability  Better yield and lower costs of the fiber furnish  Bulk and stiffness, especially for filler ply of multilayer products  Recycled and nonwood pulps  Decrease fiber furnish costs, can be more environmentally friendly  Enlarge the raw material base for papermaking 5
  • 6. Pele Oy Roles of different raw materials  Fillers and coating pigments  Improve the end use printability properties of paper  Decrease costs, carbonate widely available  Save forests  Additives  Improve the papermaking process (performance chemicals)  Improve the end use properties of the paper (functional chemicals)  The desired paper properties can be obtained by  Selecting the proper furnish components  Adjusting the fiber furnish composition  Adjusting the properties of the different fiber furnish components used  Properly controlling the total papermaking and finishing processes 6
  • 7. Pele Oy Wood, fiber and paper properties 7 FIBER PROPERTIES IN PULPS Fiber length Cell wall thickness of fibers Fiber coarseness Hemicellulose content Nr of Fibers per unit weight Bulk, Stiffness, Wet strength Light scattering coefficient (opacity) Formation, Porosity, Orientation Bonding, Density, Dimension stability Fiber Stiffness Dry strength Wet Strength RAW MATERIAL PROPERTIES PAPER PROPERTIES
  • 8. Pele Oy Pulps and paper grades  Actual fiber furnishes may vary largely and can be quite different especially in small unintegrated paper mills  Very often the price of fiber seems to be more important than the performance of fiber in the product; within each end-product the quality and the price of end-products may vary largely  It is important to understand how each furnish component contributes the quality of the product and the performance in the paper machine, finishing, and converting 8 Paper Grades Short fibers for printability Long fibers for runnability Mechanical grades GW, PGW, TMP, BCTMP, DIP Long fiber softwood (BSKP)Woodfree grades BHKP, DIP Non-wood grades Several non-woods (bagasse, wheat straw etc.) Bamboo, kenaf etc.
  • 9. Pele Oy Tensile strength of different pulps  DIP has normally better tensile strength than TMP even if it contains filler.  Standard newsprint contains 50-100 % DIP.  TMP fibers of Pinus radiata are coarse and tend to form bulky, low tensile and porous sheet.  Tensile index of all pulps improve when pulp is made to lower freeness. 9
  • 10. Pele Oy Effects of refining on fibers  Internal and external fibrillations as well as creation of fines are the main positive effects of refining. 10 Removal of primary fiber wall and S1 layer More fiber hairiness (external fibrillation) Delamination and swelling of fibers (internal fibrillation) Creation of fines Fiber cutting and shortening Dissolving of material Straightening of fibers
  • 11. Pele Oy Final bonding in paper  Picture on the right describes bonded fibers after refining and drying.  External fibrils and fines from refining are an important part of bonding.  Secondary fines (fibrillar) has the most positive effects.  Collapsed lumen in the ribbon-like fibers increase bonded area.  Crimped section at fiber crossings have effect on the rheological behavior of paper. 11 Picture: Hubbe
  • 12. Pele Oy Paper density and strength  How to get bonding without density increase? Dry strength chemicals, surface sizing and micro-fibrilled cellulose are some possibilities.  Gentle refining or low Specific Edge Load (SEL) gives good bulk and bonding at the same time. Low SEL for never dried hardwood is < 0.5 J/m. 12 Fiber Lumen More internal fibrillation, collapsed – good bonding but low bulk More external fibrillation, not collapsed – good bulk and bonding
  • 13. Pele Oy 13 Effect of chemical pulp refining on paper Positive effects  Wet web strength   Fiber bonding and strength   Better formation  Coating coverage   Porosity and ink demand   Smoothness and gloss  Negative effects  Water removal and solids content   Bulk and stiffness   Paper compressibility   Opacity and brightness   Drying shrinkage  dimension stability   Tear strength   Energy consumption   CAPEX and maintenance costs  Pics: E.Gruber Internal fibrillation External fibrillation Fiber bonding + =
  • 14. Pele Oy Wood, fiber and pulp properties for papermaking  It is important to understand the effect of wood properties on paper quality.  Wood density is a simple measure and well suited to predict paper strength.  Fiber length is not the only characteristics correlating with paper strength.  Latest studies show that even more important are external fibrillation and crill content after stock preparation. Fibers should be easily refined to save energy. 14 Main Wood Properties Average dry density Tracheid, ray cell and vessel shares Fibers after Pulping Cell wall thickness Tracheid length Coarseness Fibril angle Fibers to Paper Machine Ratio fines to fibers (crill) External fibrillation Cell wall collapse, density Pitch and stickies Wood for Pulping Cellulose, hemicellulose and lignin content Extractives content
  • 15. Pele Oy Pulping process External pressurePulp drying Pulp refining Pulp yield Lateral force Surface tension Sheet density Surface properties Optical properties Strength properties Lateral conform- ability Wood vessels Wood density Degree of collapse Bonded area Wood extractives Fiber diameter Wall/dia ratio Wall thickness Forces during papermaking:Fiber processing: Wood, fiber and paper properties Wood properties: Paper properties: Wood chemistry 15
  • 16. Pele Oy Fiber collapse and paper properties  Differences in relative cell wall thickness have effect on paper properties.  An example here is Pinus radiata compared to Norway spruce. TMP fibers of Pinus radiata are coarse and tend to form bulky, low tensile and porous sheet with reduced coating and ink holdout.  Spruce fibers have lot of thin-walled early wood fibers, which form more dense, smooth and strong paper.  Dense paper, however, can have lower light scattering and stiffness.  From hardwoods eucalyptus is more thick-walled while birch and acacia are more thin-walled. 16 Norway spruce Pinus radiata
  • 17. Pele Oy Fiber collapse and flexibility  It is important to increase flexibility and collapsibility of thick-walled fibers.  The main means to improve collapsibility is refining.  When using stratified headbox it is possible to fractionate fibers and put coarse fraction to the middle layer. 17 Good fiber for printing paper Suitable fiber for tissue, copy paper and cartonboard middle ply
  • 18. Pele Oy Hardwood chemical pulp 18  For woodfree paper grades mainly hardwood kraft is used. Thick-walled eucalyptus can be better than thin-walled or birch.  On the left side paper is bulky and thicker giving better stiffness, which is important e.g. for copy paper.  Thick-walled fibers are better in cartonboard middle ply because smoothness is not as important there as bulk and stiffness. Picture: Celso Foelkel
  • 19. Pele Oy Primary and secondary fines of hardwood  Primary fines of hardwood pulp (ray and vessel cells) cause picking, linting and reduce bonding.  Secondary fines formed in refining is mostly thin fibrils and enhance bonding. 19 Picture: Martin A. Hubbe Primary fines Secondary fines
  • 20. Pele Oy 20 Effect of softwood kraft on paper properties Positive  Strength properties Increase (also surface, tear and wet strength)  Folding endurance Increases Negative  Formation Less uniform  Smoothness Decreases  Porosity Increases  Ink holdout Lower  Bulk and stiffness Decrease  Dimensional stability Decreases  Energy consumption Increases  Costs Increase Picture: Canfor
  • 21. Pele Oy Fiber type and wet end runnability  Printing papers with mechanical pulp have better runnability on paper machine than woodfree grades.  The main reason is that mechanical pulp fibers are stiffer in wet condition.  The good tensile stiffness of the wet fiber network is mainly due to elastic pressure and friction forces between fibers.  It is easy to make a model from four sticks and note the rigid structure without bonding.  Good runnability is always more important than we could imagine. 21 No bonding but very rigid structure!
  • 22. Pele Oy Dryer section runnability  There should be high residual tension after first wet strain to avoid web slackness and breaks.  Low tension after first dryers due to relaxation may lead to slackening of the wet web.  This causes wrinkling, bagging, fluttering and weaving of the web which can lead to web breaks.  In modern single felted dryer sections, the problematic areas of paper with low tension level are mainly found in converging and diverging gaps between the dryer cylinders and the fabric. 22
  • 23. Pele Oy Fiber fines and residual tension  The studies of Dr. Retulainen suggest that the residual tension is more closely related to the tensile stiffness than to the tensile strength. Therefore also the factors, such as the fiber stiffness, straightness and the activity of the network to bear load, affect the residual tension.  It is interesting to note that stiff TMP fibers blended with only 10 % kraft fines form stronger wet sheet than kraft fibers and kraft fines. 23 On the left the effect of fiber and fines type on the residual tension (tension at 1% strain after 0.475 s relaxation) of wet sheet (compared at 55% dryness).
  • 24. Pele Oy 24 Advantages of low break frequency Improved runnability Lower raw material costs Longer wire life Increased machine speed Less steam & energy/ton Lower furnish cost Higher press solids Less starch etc. needed Better CD- profiles Less shade & caliper variation Stronger paper Better bulk & stiffness Better printability Constant filler content Productivity Cost Efficiency Easy wet end chemistryProduct Quality Low Break Frequency Less effluent and fresh water/ton Better and less variable raw materials Less Dry Broke Stable and better paper quality More Net Tons Lower Chemical Consumption Decreased use of chemicalsCleaner system Steam & el used only once
  • 25. Pele Oy TMP fibers and bonding  TMP fibres are normally not very well fibrillated. Fibrillation is needed to get bonding. Sometimes there is still latency left to the paper machine. Latency (curling of fibers) reduces strength and bonding and increases porosity.  Fibrillation can be increased by using fresh/moist wood, alkaline pH, high amount of reject refining and also with post-refining (however may cut fibers too much).  Freeness as such is a measure of fines content, but not a good measure of fibrillation. Fibrillation should be checked from microscope fiber pictures. Picture: Knowpap Long TMP fibers with thick cell wall are normally not very well collapsed or fibrillated. Internal bond of paper tends to be low. 25
  • 26. Pele Oy TMP fibrils and flakes  A good quality TMP for printing paper includes fibrils to give bonding and strength as well as flakes (mainly unbonded material) to improve light scattering and paper opacity.  For cartonboard middle ply flakes are not needed. Picture: KARI LUUKKO AND HANNU PAULAPURO TAPPI JOURNAL FEBRUARY 1999 26
  • 27. Pele Oy Effect of TMP fines to paper properties  It is important to note that fibrils increase tensile strength without reducing light scattering and flakes increase light scattering without decreasing tensile strength.  A good TMP includes both flakes and fibrils and thus increases both light scattering and tensile strength. Picture: KARI LUUKKO AND HANNU PAULAPURO TAPPI JOURNAL FEBRUARY 1999 27
  • 28. Pele Oy Fiber wall thickness vs. roughness & opacity  Fiber wall thickness determines very much paper smoothness and opacity.  Increased fiber splitting without fiber cutting (lower freeness) can improve the situation but not totally. Picture: PFI 28
  • 29. Pele Oy Optimal mechanical pulp  Optimal mechanical pulp for improved publication papers should have:  Thin fiber walls (raw material selection)  Fiber fines and large degree of fiber splitting (thin slot screening and reject refining)  Fibrillated fiber surfaces (reject refining)  Low amount of shives, and especially latewood shives (reject refining and thin slot screening)  Reasonable fiber length (thin slot screening and reject refining)  Such fiber properties will give publication papers with improved print quality, formation and runnability. 29
  • 30. Pele Oy Fiber Wall Thickness of Norway Spruce  Average fiber wall thickness of Norway spruce TMP is almost 2 µm but there are some fibers with wall thickness of 3-5 µm.  Mechanical pulps made from mature Pinus radiata can have fiber wall thickness of about 6 µm, which is three times the Norway spruce value. Reme, P. A., Kure, K.-A., Gregersen, O. W., Helle, T., 1999 International Mechanical Pulping Conference Picea Abies 30
  • 31. Pele Oy Importance of fiber wall thickness  It is important to have several fiber layers in a thin paper to get good formation, smoothness, opacity and gloss. This correlates with thin fiber wall.  Fiber wall of Norway spruce is about 2 µm while Radiata pine has 4-6 µm. In a 40 gsm paper there can be only 3-4 fibers of Pinus radiata in the total paper thickness. Area = Perimeter x Wall Thickness, A=P*T Fiber volume = Area x Length, V=A*L=P*T*L Coarseness = Fiber weight/Length, C=W/L C = Volume*Density/Length, C=V*ρ/L=P*T*L* ρ/L= P*T*ρ Fiber Grammage (g/m2) = Coarseness/fiber Width = P*T*ρ/P*2 = 2*T*ρ = 3*T (µm) ~ P/2 Fiber Fiber Wall Grammage µm g/m 2 1 3 2 6 3 9 4 12 5 15 6 18 T Wall density ~ 1500 kg/m3 31
  • 32. Pele Oy Example: pulp characteristics for containerboards 32  There is no common criteria for pulp quality. It depends on end products to be manufactured.  Linerboard  Compression strength, burst, stiffness and porosity are most important  For white-top grades printing properties are important  Corrugating medium (fluting)  Compression strength is critical
  • 33. Pele Oy What makes strong kraftliner  It is quite common understanding that fiber length and fiber collapse are the most important fiber properties having effect on important linerboard properties. This means that kraft pulp must be strongly refined to get lumen collapse and higher bonded area. But there is also a third variable what Innventia in Sweden has studied.  PulpEye has recently introduced its CrillEye online crill measurement. Crill is finely divided cellulosic material - finer than external fibrillation - that is liberated during refining process. The crill particles are about a hundred times thinner than the fibers.  In spite of the fact that only about one per cent of the weight of fibers and other particles in the furnish is crill, it can correspond to as much as fifty per cent of the total free surface area. This shows the importance of crill for the strength properties of paper.  Valmet has also introduced its own online strength measurement, which is based on external fibrillation or hairiness of fibers. 33
  • 34. Pele Oy Crill and tensile strength  Research studies at Innventia have shown that crill is the single variable having the strongest connection to paper strength. Laboratory results in the figure below show a strong correlation to paper tensile strength index. 34
  • 35. Pele Oy 35 Fibers are never identical
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