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Potential of Papermaking Fibers


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Potential of Papermaking Fibers

  1. 1. Pele Oy Potential of Papermaking Fibers 22 October 2015
  2. 2. Pele Oy Preface  This presentation aims to review the effects of stock treatment, drying and recycling on fiber properties.  It is common knowledge that the pulp made in the mill process is inferior to the laboratory-made reference pulp.  Another big difference is in never-dried pulp compared to dried bale pulp.  Third interesting thing related to the previous changes is the difference between virgin pulp compared to recycled pulp.  The purpose of this review is to present some information related to all these three topics.  My LinkedIn profile can be found by following this link:  2
  4. 4. Pele Oy Wood fibers  Wood is a complex natural composite built up of fibers that are glued together by lignin.  Fibers consist of fibrils that are held together by lignin and hemicellulose.  Fibrils are built up of bundles of microfibrils. 4
  5. 5. Pele Oy Volumetric composition of hardwood and softwood Softwoods:  Fibers 90 - 95 %  Ray cells 5 - 10 % Hardwoods:  Fibers 27 - 76 %  Vessels 7 - 55 %  Ray cells 5 - 25 % 5
  6. 6. Pele Oy Wood cell types  Fibers/tracheids are best for papermaking. Other cell types cause mainly problems. This is one reason that softwood fibers are best and nonwood fibers worst fibers. 6 Softwood Hardwood
  7. 7. Pele Oy Breaking lengths of some materials  Breaking length is a very good measure of material strength because it takes the density of the material into account. The following list of breaking lengths is interesting:  Graphite 37 km  Eastern white pine wood 23  Paper from bleached softwood 8-10  Steel 4.5  Aluminum 3.4  Newsprint 2-5  We can notice that wood strength is more than double compared to softwood paper strength which is about double to steel strength.  There is good potential to improve paper strength closer to wood strength. 7 Picture: Hubbe
  8. 8. Pele Oy Laboratory pulp compared to mill pulp  Tear strength with same tensile strength in lab pulp is always better than in mill pulp. What are the basic reasons to this difference?  How could we improve mill pulps? 8
  9. 9. Pele Oy Laboratory vs. industrial digester  It is well known that a laboratory digester (left picture) produces stronger fibers than an industrial digester (right picture).  Chemically and mechanically the treatment of fibers is different in laboratory vs. mill. 9 Picture: YokogawaPicture: Thwing-Albert
  10. 10. Pele Oy Fiber length of laboratory pulp compared to mill pulp  Measured fiber length of mill pulp is shorter than with laboratory pulp.  Induced damage points, fiber deformations and porosity changes occurred during processing are taught to be responsible for most of the observed strength loss when industrial and laboratory pulps from softwood are compared. 10
  11. 11. Pele Oy Fiber kappa of mill vs. laboratory pulps  Laboratory cooking can produce much more homogenous delignification than industrial cooking. 11 Lab-cookedvs. commercial pulp 0 5 10 15 20 25 30 0 10 20 30 40 50 60 70 80 90 kappa %fibers labcooked29.5 commercial 31.4 COV = 0.54 COV = 0.34 Picture: TAPPI Journal Dec 2002
  12. 12. Pele Oy Pulping and fiber wall strength  Fiber wall is like a brick wall. In the wood the strong cellulose groups are surrounded with lignin and hemicellulose.  During cooking the wall looses most of lignin and hemicelluloses.  There will be lot of porosity in the structure and it is vulnerable to mechanical forces in the following processes.  At the end of cooking more than 50 % of the wall volume is dissolved.  Fiber length can be original, but coarseness is only half and strength of individual fiber is much less than in the beginning. 12 Lower kappa – more lignin has been eaten from the fiber wall Capillary structure of a delignified fiber (McIntosh 1950)
  13. 13. Pele Oy Defects and failure zones in fibers  Examples of non-homogeneous zones in fibers include curl, kink, dislocation, microcompression and twist.  In mill operation, fiber damages can be induced accidentally or intentionally, by shearing at high consistency. Some pulps are highly susceptible to damages, others are more resistant.  The most important damage might be curl which is not necessarily stable. It is readily removed from some pulps but not from others. Curl can be stabilized by certain treatments, notably by heat treatment at high consistency.  Curl and other damages are often disregarded because they cannot be easily measured. Yet in practice their effects often dominate the properties of pulp suspensions, wet webs and dry sheets. Ignoring these effects has led to costly surprises, both in research and mill operation. 13
  14. 14. Pele Oy Fiber curl and paper properties  Curled fibers decrease tensile strength but increase strain, tear strength, bulk and porosity. Very small amount of curled fibers can have big influence.  Some papers can benefit of this e.g. sack kraft, absorbent papers and tissue papers.  Cartonboard middle layer can also benefit of higher bulk and better board stiffness. 14 Straight fibers of commercial softwood bisulphite pulp of 62% yield Highly curled fibers of commercial flash-dried bleached softwood kraft pulp Moderately curly fibers of commercial dried bleached softwood kraft pulp Pictures: D. H . Page, R. S. Seth et al.
  15. 15. Pele Oy Fiber curl and kinks made with kitchen mixer  Spruce kraft pulp fibers were beaten with PFI beater 2000 revs and deformed in Hobart kitchen mixer for 0, 15 and 45 minutes. The aim was to introduce fiber deformations without changing fiber swelling.  Homogenization was carried out at room temperature 25 °C and consistency of 9%. 15 Olli Joutsimo, KCL Finland
  16. 16. Pele Oy Tensile strength deterioration after cooking  Immediately after cooking fibers have good tensile strength, but later in the process tensile strength is deteriorated already before refining.  The reason could be mechanical forces due to blowing, pumping, mixing etc.  The picture shows how mixing in the Hobart mixer increases fiber curl and the number of kinks thus decreasing tensile strength. However, paper strain is increased at the same time.  This seems to be quite similar behavior as latency of mechanical pulp. 16 Olli Joutsimo, KCL Finland
  17. 17. Pele Oy Improvement of tear strength after cooking  When tensile strength is decreased due to forces in mechanical mixing, tear strength is increased at the same time. 17 Olli Joutsimo, KCL Finland
  18. 18. Pele Oy Reduction of TMP fiber curl  In the figure below is tensile index versus fiber curl before and after LC-refining and tested after no, cold and hot disintegration. The handsheets were made without whitewater recirculation.  This shows how latency removal and low consistency refining can improve tensile strength and reduce fiber curl. 18 864 Nordic Pulp and Paper Research Journal Vol 27 no.5/2012
  19. 19. Pele Oy Bleached pulp process  There are too many high turbulence shearing forces to fibers in the pulp mill.  Should we use completely different pumping and mixing technology.  At least pumping against valves should be avoided. May be volumetric pumps could be better. 19
  20. 20. Pele Oy Latency removal of mechanical pulp  Latency of mechanical pulp has been known for a long time. This experience should also be used in chemical pulp treatment.  Latency removal involves removal of fiber curling, which occurs at high-consistency treatment, by means of mixing the pulp a certain period of time at a lower consistency (2 - 4 %) and a temperature of 70 – 80 °C. 20
  21. 21. Pele Oy Typical process in a softwood stock preparation  To save pulp strength there should be different low turbulence mixing and pumping principles than today. Minimum amount of turbulence and low consistencies.  No valves, use of speed controlled pumps, minimal stock mixing.  Two refiners in series or newest refiner designs would give more even result (fiber straightening) and lower SEL. 21 Why not only one mixer instead of two chests? Are these really needed? Low specific edge load Low consistency pulper
  22. 22. Pele Oy Online measurement of single fiber properties  Measurements of single fiber properties are new tools to develop and control papermaking process and paper quality. 22 Cell wall thickness Kappa Length Curl Surface charge Single fiber properties Width Kink Fiber performance Pulp behavior
  23. 23. Pele Oy FIBER BONDING 23
  24. 24. Pele Oy Fibrillation effects on fibers  Target of a good refining is to get external fibrillation (hairy surface and fibril fines) and internal fibrillation (delamination of fiber layers). 24 Unrefined softwood Refined hairy softwood Dimas Dwi Prasetyo Nugroho (2012) Adam A. Brancato Delaminated hairy fiber
  25. 25. Pele Oy Bonding of refined fibres  External fibrillation can make very long bonds compared to fiber thickness.  There are more bonds in fiber crossings.  Good bonding requires fiber flexibility and lumen collapse to get more intimate contact.  Fiber fines and higher surface tension enhance bonding especially at fiber crossings.  Curled fibers decrease relative bonded area by geometrically preventing bonding. Low consistency refining straightens fibers.  Electrostatic environment in refining can have a large effect on internal fibrillation and refining result. 25
  26. 26. Pele Oy Electrostatic environment in refining  Water retention value (WRV) is a good measure how much fibers are delaminated after refining.  The picture below shows how much better refining and papermaking can be in sodium environment compared to acidic environment. 26 Bäckströn and Hammar, Counterions & refining, 2010
  27. 27. Pele Oy Tensile index and fiber fines content  It is quite common opinion that fiber length is most important to get high tensile strength but also fines content is very important.  Small amounts of fiber fines have big effect on tensile strength of chemical pulp.  Fines in the picture means material through mesh № 200 (75 μm).  Picture below is simplified and redrawn from the Thesis of Yana Zaytseva (2010).  So called crill is much finer than this fines. 27 50 55 60 65 70 75 80 85 90 95 100 0 2 4 6 8 10 Tensileindex,Nm/g Fines content, %
  28. 28. Pele Oy Crill and tensile strength  Crill consists of thin fibrils that are partially or completely loosened from the fibers. Crill fibril thickness (250 nm) is about one hundred of fiber thickness.  Despite the fact that crill represents only approx. 1% by weight of the particles in a suspension, it may contribute to as much as 50% of the free surface.  Research studies at Innventia have shown that crill is the single variable having the strongest connection to paper strength. Lab results in the figure below show a strong correlation to paper tensile strength. 28
  29. 29. Pele Oy High definition image analysis vs. tensile strength  Valmet has introduced a new pulp analyzer based on microscopic online testing.  The picture below shows how sensitive the measurement is. Small changes in external fibrillation percentage have large influence on tensile strength. Pictures: Valmet 29
  30. 30. Pele Oy Wet paper vs. wet soil structure  When the solids content of paper is 20 - 50% the behavior of the suspension is very similar as soil suspensions, which are very well studied. The picture on the right and the following text is from soil behavior:  “Due to its surface tension, water molecules in the interparticle voids bond the soil grains at their interface with the air that is present in the voids and where menisci develop.  The smaller the grain size, the greater the bonding or apparent cohesion. For example, suction effects on uniformly graded gravel would be negligible while the effects on well-graded gravel could be significant.  Even a small amount of fines in sand can result in measurable cohesion. In the context of reinforced walls and slopes”.  This explains very well why small amount of fiber fines have great effect on paper strength. This also explains that fines will be transferred to the contact points with water where it will be most effective for bonding. 30 0610_f2_slopes.html
  31. 31. Pele Oy Drying stresses and tensile strength  Machine made papers always have higher tensile strength in the machine direction. How much higher depends on fiber orientation (normal range of tensile ratio 2-4).  In addition, sheet stress under drying has great effect on tensile strength. 31 Kärenlampi, P., Niskanen, K., Fapet
  32. 32. Pele Oy Structural changes in fiber and paper properties  The effects on the table are my subjetive opinions and valid only for chemical pulp.  Mechanical pulp fines is less bonding compared to chemical pulp fines. 32 SUMMARY OF EFFECTS Tensile Internal Light Smooth- strength bond scattering ness Internal fibrillation↑ External fibrillation↑ Fiber length↑ Fiber fines↑ BulkProperty +++ + – ++ – + = positive change – = negative change ++ +++ –– + –– ++ +++ –– + –– ++ + – – +
  33. 33. Pele Oy Nonwood fibers and linting in offset printing  Nonwoods and some special hardwoods have very much vessels and fine fiber material including parenchyma, epidermal and ray cells.  Several studies have shown that most of the linting material in uncoated paper is parenchyma and ray cells.  These cells are so small and light that they follow accept in screening and cleaning. Refining has practically no effect on small cells.  Old paper machines have Fourdrinier wire at least in the beginning of the wire section. This means that fines and dusting material is more on the top side of the paper.  In addition, press section often has last felt on the bottom position. This means that there will be more bonds between fibers on the bottom side.  Old wire and press section concepts lead to high linting tendency on the top side. If now filler content is high, it also means that the filler concentrates to the top side and prevents fiber bonding even more. 3 Lint from offset blanket
  34. 34. Pele Oy Nonwood fiber problems  Nonwood fibers normally have about 50 % of the area other cells than fibers, such parenchyma, epidermal and vessel cells. These cells are not good for papermaking.  Nonwood pulp has high fines content and fibers can have thin cell walls. These together lead to dewatering and runnability problems as well as to low paper stiffness. Low paper porosity Dewatering problems Thin fiber walls Flexible fibers Wide vessel cells Lumen collapse High fines content High press load Low PM speed Low paper bulk & stiffness Low fiber content Runnability problems Dust and linting 34
  35. 35. Pele Oy Softwood/hardwood refining  For woodfree papers it is common to refine softwood and hardwood separately (left figure).  Good practice is to refine first softwood, then blend hardwood to softwood and refine them together (right figure).  Sequential refining seems to give better tensile strength with same energy consumption. 35 VAIL MANFREDI: 2006 Pan Pacific Conference Advance in Pulp and Paper Sciences & Technologies
  36. 36. Pele Oy Optimal SW/HW refining  This sequential refining is more flexible than common refining, gives better result than completely separate refining and requires less equipment and energy than completely separate refining. 36 To PM
  37. 37. Pele Oy Original refiners vs. modern refiners  Old refining method was so called stamper, where fibers were beaten with big hammers. Fiber flocks got during long time treatment of pressure forces but very little shear forces. This saved fiber length.  Modern refiners such as Valmet Optifiner Pro have high capacity and lower energy consumption.  Should the refiners create even more pressure forces and less shear forces? 37
  39. 39. Pele Oy Hornification in pulp drying  Internal fibrillation can be described as the breakage of the crosslinks between microfibrils during beating. It reduces the effective moment of inertia of the cell wall, thus increasing wet fiber flexibility, conformability, and collapsibility. As a result, internal fibrillation mainly enhances inter-fiber bonding and improves paper tensile strength.  A schematic illustrating possible change in pore structure resulted from beating previously dried pulps. It shows that even though the pore volume of previously dried pulps can be recovered by beating, the permanent changes to pore structure have occurred. That is, some pores, which are closed in drying, are not reopened by normal levels of beating. Microfibrils In dried pulp Beatingin Papermaking Pulp Drying Microfibrils in never-dried pulp Microfibrils after papermaking Internal Fibrillation in Never-dried and Once-dried Chemical Pulps XINSHU WANG, THAD C. MALONEY AND HANNU PAULAPURO Helsinki University of Technology, Espoo, Finland 39
  40. 40. Pele Oy Inter-fiber hydrogen bonding and debonding  After pulping and bleaching never- dried pulp fibrils in fiber wall are better separated than in a dried and refined pulp.  After pulp drying, slushing and refining there are still some irreversible hydrogen bonds inside the fiber walls.  This means that the fiber is stiffer and not as prone to collapse as never dried pulp. Inter-fiber bonding of dried pulp is not as good as with never-dried pulp. Fibril surface Fibril surface Fibril surface Fibril surface Never-dried Pulp Once-dried Pulp After Refining Fibril surface Fibril surface 40 Pictures: Hubbe
  41. 41. Pele Oy Fibrillation and hornification in drying  Simplified presentation of external and internal fibrillation and hornification in drying.  These explain the differences between never-dried and dried pulps.  Integrated paper and board mills can benefit compared to mills using bale pulp. 41 Pictures: Hubbe et al. (2007) Good bonding Less bonding
  42. 42. Pele Oy Never-dried fiber versus recycled  Never-dried fiber has more free fibrils and OH-groups for bonding to other fibers than dried fiber.  Especially recycled fiber shows hornification i.e. less free OH-groups and fibrils. Recycled pulp also has more frayed fibrils and fiber curl. 42 Picture: Valmet Juan Cecchini 2015
  43. 43. Pele Oy Tensile strength and paper density  Never-dried pulp is stronger and requires less refining energy for a certain tensile strength and freeness.  It is well known that paper density and tensile strength correlate when kraft pulp is refined.  For almost all papers bulk and tensile strength are both desired properties.  It is interesting to know, if this combination is better with never-dried pulp.  The curve on the right shows that even if the never-dried pulp is stronger, the combination of strength and bulk is same with dried and never-dried pulps i.e. the points follow a linear relation. 43 Picture: Xinshu Wang (2006)
  44. 44. Pele Oy Pore volume of never-dried and once-dried hardwood pulps  Even though the pore volume of previously dried pulps can be recovered by beating (i.e., the fibers can be reswollen), some pores are not reopened by normal levels of beating. In other words, beating does not completely reverse hornification.  The likely explanation is that strong irreversible hydrogen bonding is formed between microfibrils in drying which is not broken when the fiber is beaten.  Thus, beating mainly disrupts and loosens macrofibrils (aggregated microfibrils), creating large-sized pores in the cell wall. Internal Fibrillation in Never-dried and Once-dried Chemical Pulps XINSHU WANG, THAD C. MALONEY AND HANNU PAULAPURO Helsinki University of Technology,Espoo, Finland hw-nd = hardwood never-dried hw-od = hardwood once-dried 44
  45. 45. Pele Oy Tensile strength of bamboo pulp  Tensile strength of never-dried bamboo pulp is better than after first drying. 45 Heijnesson-Hulten et al. (2013)
  46. 46. Pele Oy Xinshu Wang’s dissertation  Drying of pulps greatly reduces pulp swelling, enhancing dewatering but impairing tensile strength. Dried pulps offer a far better combination of dewatering and tensile strength than never-dried pulps. One possible reason is that some small hard-to-dewater pores in the fiber wall are irreversibly closed by drying, which enables better dewatering.  However, pulp drying is energy-consuming. Pressing pulps to high dryness may provide an economical way to improve dewatering, while maintaining paper strength properties. Pressing hornifies pulps, which promotes dewatering but impairs tensile strength to a certain extent.  On the other hand, pressing causes fibers to flatten, with the flattened fibers providing more surface contact for bonding, thus increasing density and tensile strength. Never-dried pulps which were pressed before refining were found to give both improved dewatering and better tensile strength. 46
  47. 47. Pele Oy Xinshu Wang’s Dissertation (cont.)  The refining results support the earlier view that internal fibrillation is largely produced by a cyclic compressive action. It is suggested that fibers need to be turned over in refining and compressed from different directions in order to disrupt their internal structure and cause internal fibrillation.  Compression also facilitates fiber straightening, but does not promote external fibrillation and fines generation.  At the same swelling level, more straightened pulps give higher tensile strength, and pulps with less fines and external fibrillation enable better dewatering. Hence, to achieve an optimum combination of dewatering and tensile strength, chemical pulp refining should aim at increasing internal fibrillation, straightening fibers, and keeping the amount of fines and external fibrils at a low level.  If pulps are refined so that moisture content after press section is same, once-dried pulp requires 100, pressed never-dried pulp about 40 and never dried pulp about 25 kWh/t. 47
  48. 48. Pele Oy Beaten fibers of kraft pulp and hornification  The fibers in the picture are freeze- dried to keep the fibrils visible and out of the fiber surface.  When fibers are dried there will be irreversible bonds between fibrils and fiber surface. This is one part of hornification and cause reduced bonding. 48 Picture: Fleming
  49. 49. Pele Oy Effect of pulp drying on papermaking process and paper quality Pulp Pressing and Drying Stiffer fibers, less collapse Better drainage Less bonding Lower wet and dry strength Less filler can be used Lower drying shrinkage Less curl and cockling Slushing needed Wider trim width Vessel cells destroyed Lower steam demand Higher bulk and stiffness Better opacity and brightness Less vessel picking Better copy paper Dry pulp bales Low transport cost Free of micro- organism growth More refining needed Higher papermaking cost Higher pulp production costs Cutting damage 49
  50. 50. Pele Oy How many times fibers are recycled?  It is a very common misunderstanding that recycled fibers are recycled 5 to 10 times. The picture below shows that only one out of every eight fibers is recycled more than two times, even at a 70% closed-loop recovery rate. 50
  51. 51. Pele Oy Effect of recycling on CTMP  In this trial Eastern spruce CTMP was used.  It can be seen that there is more bonding with recycling.  Increased bonding causes higher strength and density, but lower tear and scattering coefficient. 51 R.C. HOWARD and W. BICHARD
  52. 52. Pele Oy Effect of recycling on chemical pulp  In recycling bonding potential of bleached chemical pulp fibers decrease due to hornification. This means that tensile and burst strengths can decrease up to 15%.  Tear strength and scattering coefficient increase as usual when bonding decreases. 52 R.C. HOWARD and W. BICHARD
  53. 53. Pele Oy Summary  Fiber lines in pulp mills and stock preparation lines in paper mills should be more designed towards gentle fiber treatment and saving of fiber strength. New types of pumps and tank agitating systems are needed.  Main refiners should be developed to create more normal forces and less shear forces. Probably all fibers do not need refining at all. Refining only softwood chemical pulp could be enough in many cases.  Fines content is important for bonding. It might be easier to get enough fines by refining only a small part of pulp to a very low freeness.  Another possibility could be to use a small part such a pulp which is easy to refine and make fines. Several nonwood pulps and some hardwoods would have this kind of potential.  However, it should be remembered that best fines is so called secondary fines, which is fibril material from S2 layer of the fiber.  Recovered paper and board is still today very good material for paper and board industry. Correct treatment of recycled fibers is the key to get good products from 100% recycled material. 53