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Effects of Friction in Papermaking

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Effects of Friction in Papermaking

  1. 1. Pele Oy Effects of Friction in Papermaking 1
  2. 2. Pele Oy Preface Papermaking is a long sequential process, where all process sections should work to get best possible overall efficiency and product quality. It is very important to have top knowledge of all parts of the process to get maximum possible total efficiency and good product quality. As an example in the following, I will present how paper friction depends on the total papermaking process and also affects to it and finally to roll winding and product quality. This really needs more competence than only winding expertise. Helsinki, 9 December, 2015 Pekka Komulainen 2
  3. 3. Pele Oy FRICTION IN PAPERMAKING
  4. 4. Pele Oy Total efficiency of papermaking process  Papermaking is a long sequential process where all process sections should work to get saleable production.  If we assume five separate processes (different colors) and very good 99% material efficiency in each, the total efficiency is 95%. When the efficiency of each process is as good as 95% then the total efficiency is only 77%. 4 Stock prep Paper slitting and winding Packaging Headbox et drainage Wet pressing Paper drying Coating / sizing Calen- dering Reeling Sheeting Coat drying
  5. 5. Pele Oy Paper friction  The development of the tangential force F as a function of time required to initiate and to maintain relative movement between two solid bodies, where Fs is the static friction force and Fk is the kinetic friction force.  Coefficient of friction µ is the measured force divided by the normal force. 5 The Friction between Paper Surfaces Garoff, Niklas, Doctoral thesis, 2002
  6. 6. Pele Oy Possible difficulties of COF variation High COF  Roll bouncing, vibration and throw- out in winding  High roll hardness  Dusting, low surface strength  Wear of web contacting machine parts (wires, felts, rolls, dewatering elements, knives)  Problems in web converting  High power consumption Low COF  Web spreading difficulties and crepe wrinkles in roll winding. Low roll hardness.  Slipping between driven rolls and paper.  Sliding of sheets and packages in treatment, storage and transport.  Sheet grip problems in printing houses, copier machines and automatic teller machines. 6 Coefficient of friction (COF) should be constant in every process. The level of COF is also important. In papermaking there are mechanical and chemical reasons which affect on friction. More about friction: www.fpirc.kth.se/Documents/PDF/Friction.pdf
  7. 7. Pele Oy Measurement speed and COF  Copy paper friction force as a function of time. Measuring speeds were 0.2 and 5 mm/s. A strong stick-slip motion is visible at the lower speed. 7 ANNA JOHANSSON et al. Paper friction—influence of measurement conditions VOL. 81 NO. 5 TAPPI JOURNAL
  8. 8. Pele Oy Surface roughness structure and COF  Normally surface roughness has no effect on friction. However, paper is sometimes a different material.  Fibers rising from paper surface can originally be orientated or will be orientated after first measurement. It is important to measure paper-to-paper friction in same direction (also meaning flow from the headbox). 8 Higher COF Lower COF The Friction between Paper Surfaces Garoff, Niklas, Doctoral thesis, 2002
  9. 9. Pele Oy Friction sample contamination  Contact-free handling of the test sheets is important. This is demonstrated by comparing results for contact-free test surfaces with surfaces that had been touched with the thumb and the hand.  These results show that the friction decreased considerably when the surfaces were touched before measurement. 9 Paper friction—influence of measurement conditions ANNA JOHANSSON, CHRISTER FELLERS, DENNIS GUNDERSON, AND URBAN HAUGEN VOL. 81 NO. 5 TAPPI JOURNAL
  10. 10. Pele Oy PAPER CHEMISTRY EFFECTS ON FRICTION 10
  11. 11. Pele Oy 11 Effects of low paper COF in papermaking  Fatty chemicals reducing friction even in small amounts are mixed in the pulp. These can be fatty acids from wood, deinking chemicals, defoamers, neutral sizes etc. In addition, lubricants are used in coatings. With coated broke lubricants are returned to the wet end.  These chemicals reduce paper friction and also surface tension thus lowering wet web strength and having effect on paper winding. Fatty acids soaps etc. More broke Bursts Wrinkles Winder breaks Low winder capacity Low winder capacity Winder breaks Low winder capacity More broke Winder breaks Low winder capacity Higher web tension Higher speed More splices Wet end breaks Low fiber friction Low paper friction Low surface tension Low wet web strength
  12. 12. Pele Oy 12 COF and paper chemicals Higher COF with the following chemicals (order of effectiveness):  Silica (retention system), calcined clay and titanium dioxide  Ground calcium carbonate and PCC  Clean chemical pulp and starch  More rosin acids than fatty acids from wood (more softwood)  Generally hydrophilic/oleophobic substances with high surface energy Lower COF with the following chemicals (order of effectiveness):  Silicones/defoamers (several), lubricants and fatty acids from wood  Fatty acid surfactants which are used in the deinking process  Talc and kaolin clay (platy pigments)  Latexes, gums, waxes especially in deinked pulp  Generally hydrophobic/oleophilic (lipophilic) substances with low surface energy i.e. substances having low surface tension such as surfactants
  13. 13. Pele Oy 13 Fatty acids  Fatty acids are organic acids with a long carbon chain. Some saturated fatty acids are:  Butyric: CH3(CH2)2COOH  Lauric (dodecanoic acid): CH3(CH2)10COOH  Myristic (tetradecanoic acid): CH3(CH2)12COOH  Palmitic (hexadecanoic acid): CH3(CH2)14COOH  Stearic (octadecanoic acid): CH3(CH2)16COOH  Arachidic (eicosanoic acid): CH3(CH2)18COOH
  14. 14. Pele Oy Paper COF and type of fatty acids  The coefficient of friction of cellulose surfaces impregnated with different fatty acids plotted against the number of carbon atoms in the fatty acid. 14 The Friction between Paper Surfaces, Garoff, Niklas, Doctoral thesis, 2002
  15. 15. Pele Oy 15 Disadvantages of fatty acids  Increasing amount of fatty acids reduces friction and surface tension. This might in some cases be good for winder vibration but not good for wet and dry paper strength. Fatty acids Seika Tay: TAPPI Journal, August 2001
  16. 16. Pele Oy Surface energy and hydrophobicity  Materials with lower surface energy are more hydrophobic. More hydrophobic surfaces have lower coefficient of friction. 16 Picture: Leslie Webb
  17. 17. Pele Oy 17 Extractive content of American wood species % of dry wood  Balsam Fir 1.0 - 1.8  Douglas Fir 0.3 - 2.6  Western Hemlock 0.3 - 1.3  Eastern Hemlock 0.2 - 1.2  Western Larch 0.7 - 0.9  White Spruce 0.4 - 2.1  Jack Pine 1.9 - 4.3  Longleaf Pine 2.1 - 9.2  Silver Maple 0.2 - 0.9  White Birch 1.5 - 3.5  Trembling Aspen 1.0 - 2.7 Extractives content has effect on COF. If there is variation in wood species there is variation in COF.
  18. 18. Pele Oy 18 Surface-active agents or surfactants  Surfactants are molecules having a dual character. Part of each molecule is hydrophilic and part is hydrophobic.  The dual affinity means that surfactant molecules have a tendency to accumulate at interfaces between polar and nonpolar phases.  Surfactants can help in dispersing suspension particles, spread various liquids onto solids, and stabilize desirable foams.  Surfactants can be a nuisance to papermakers when they stabilize undesirable foams, hurt dry strength of paper, or hurt fine-particle retention.  Surfactants are added in deinking and intentionally to retention aid formulations, biocides, and various sizing agents to keep such products stable during storage.
  19. 19. Pele Oy 19 Initial wet web strength  Friction forces in the wet web form initial wet web strength. However, there must be pressure force between the fibers to get friction force.  This pressure force is formed of surface tension forces together with fiber stiffness.  There are several chemicals which reduce surface tension, fiber-to-fiber friction and paper-to-paper friction. This would help in winder vibration and bouncing but at the same time these chemicals reduce paper strength and runnability. Friction force = μN N
  20. 20. Pele Oy Influence of wood extractives on COF  S1 and S2 in the picture are static COF:s and K3 is kinetic COF. It can be noted that clean reference has highest COF.  The same is valid for virgin and recycled pulps: well washed pulp has higher COF. 20 THE INFLUENCE OF WOOD EXTRACTIVES ON PAPER-TO-PAPER FRICTION, 1999 International Paper Physics Conference Proceedings
  21. 21. Pele Oy 21 Effects of wood extractives  For winder vibration and roll bouncing lower friction is good. For crepe wrinkles higher friction is better.  Lower sheet strength especially wet web strength has effect on paper machine runnability.  Best friction level is always compromise between paper machine runnability and winder runnability as well as final paper quality. Some softwood extractives especially in the mechanical pulp have the following effects: Components Effects Triglycerides, fatty acids Lower strength and friction All hydrophobic components Foaming, low retention All hydrophobic components Lower water absorbance
  22. 22. Pele Oy 22 COF and fillers  Platy fillers decrease COF (kaolin, talc).  Synthetic silicate and calcined clay increase COF. Michael C. Withiam, The effect of fillers on paper friction properties April 1991 Tappi Journal
  23. 23. Pele Oy 23 Synthetic silicate and COF  Synthetic silicate is very efficient in increasing kinetic COF. Small amounts up to 2% have the greatest effect. Michael C. Withiam, The effect of fillers on paper friction properties April 1991 Tappi Journal
  24. 24. Pele Oy 24 Retention control and silica  Polyacrylamide and silica microparticles are often used to keep tray water consistency constant.  Small additions of silica can increase paper COF considerably. There can be high variation in silica content in the paper - more silica means higher friction.  Bentonite retention system might be more slippery – bentonite is a special type of clay.
  25. 25. Pele Oy 25 Foam control and paper COF  Foam is controlled by varying defoamer content according to air content measurement.  The most universal characteristic of any defoamer is the fact that it is surface active, but highly insoluble in water. It has to be formulated so that it will be dispersed as tiny droplets, i.e. as an emulsion.  Foam control agents in surface size or coating stay on the paper surface and have very big effect on paper COF.  Defoamers normally reduce friction. Variation of defoamer content makes variation in paper COF.
  26. 26. Pele Oy 26 Example of water circulation system  Let us suppose that fresh water consumption is high (30 m3/t paper) and there is no water lock between the pulp mill and the paper machine. Pulp Mills Paper- making Fresh Water 30 m3/t paper To effluent treatment From PM 11 m3/t paper Pulp consistency 10%, water 9 m3/t pulp, lot of dissolved &colloidal materials to paper machine To effluent treatment 10 m3/t paper 20 m3/t paper 1 m3/t paper 17 m3/t paper12 m3/t paper
  27. 27. Pele Oy 27 Water lock after pulp mill  Let us suppose that there is a double wire press between pulp mill and paper machine and the solids after double wire press is 33%. Only 2 m3 water/t pulp is then transported to the paper machine. Pulp Mills Paper- making Fresh 10 m3/t paper From PM 11 m3/t paper If consistency is 33%, water content is 2 m3/t pulp, less anionic trash to paper machine Main flow to effluent from DIP Plant  anionic trash out 0 m3/t paper 10 m3/t paper 1 m3/t paper 0 m3/t paper 9 m3/t paper
  28. 28. Pele Oy 28 DCS, fresh water and water lock  Practical fresh water usage is 10-20 m3/t. This range has very little effect on dissolved and colloidal solids concentration. However, moving from integrated system to interstate washing (water lock) has a big effect on DCS carry-over to paper machine.
  29. 29. Pele Oy 29 Base paper porosity and coating coverage  Coating coverage has effect on smoothness after calendering. Coating can have lower friction than base paper.  The final paper friction depends more on the friction difference between coating and base paper than on the final paper roughness.  Variations in fiber components and in their shares in the furnish have effect on paper porosity and coating coverage. Porous baseDense base TING HUANG AND PIERRE LEPOUTRE: Effect of basestock surface structure and chemistry on coating holdout and coated paper properties, VOL. 81: NO. 8 TAPPI JOURNAL
  30. 30. Pele Oy 30 Coating lubricants and COF  The most effective lubricant is wax. Also stearates reduce friction. Toshiharu Enomae · Naoya Yamaguchi - Fumihiko Onabe Influence of coating properties on paper-to-paper friction of coated paper
  31. 31. Pele Oy FRICTION IN WINDING 31
  32. 32. Pele Oy TNTs and two-drum winder  With a two-drum winder the nip force at the end of winding can be too high. If this is compensated by tapering web tension and torque difference, the combination of heavy nip load and low tension can create crepe wrinkles. 0 2 4 6 8 10 12 100 300 500 700 900 1100 1300 1500 1700 Roll diameter, mm Nipload,kN/m Nip load Compensation with rider roll Nip due to roll’s own weight Tapering web tension or torque can be dangerous 32
  33. 33. Pele Oy Winder types for thin printing papers  Winders have been developed to reduce the load between winder drum and paper roll (too heavy load breaks a thin paper). Glossy coated paper has high density and paper is thin. This requires better winder than standard two- drum winder (e.g. Soft drum winder, WinBelt or single drum winder). Thinner paper or larger roll diameter Two-drum Winder Belt supported Winder Multi-station Winder Multi-station belt supported Belt Belt Recommended with soft drum cover(s) 33
  34. 34. Pele Oy Winder problems and COF  There is an optimum COF especially for two-drum winders  Papermaking variables have a great influence on the COF level  COF is an important papermaking variable and should be monitored 34 Papercoefficientoffriction 0.5 1.0 Vibrating and bouncing range (uncoated & matt coated grades) Wrinkling range (thin and smooth grades)
  35. 35. Pele Oy Two-drum winder and uneven caliper  The purpose of shaftless winding is to rotate the rolls with same surface speed. In practice this is difficult. Due to axial forces the rolls try to attach to each other and try to have the same rotational speed.  The edge roll can be smaller in diameter. The situation is more difficult, when there is a smaller roll in the middle of the set. Caliper profile should be all the way slightly thinner to the edges to get better winding.  When paper friction is high, touching rolls have big effect resulting in disturbing forces = core misalignment, out-of-roundness, roll bouncing and throw-out.  One remedy is to run some supporting paper layers on the cores slitters still open. 35 CD caliper profile Winder drum Axial force Problem Picture: Voith Paper
  36. 36. Pele Oy Roll bouncing  General unstable behavior of the roll, i.e., roll rocking back and forth from drum to drum caused by roll diameter differences and high friction paper.  Appears on shaftless two-drum type of winders or belt supported winders.  Can be seen as blinking in the roll gaps or shaking of the core locks after certain roll diameter . Picture: Metso Paper 36
  37. 37. Pele Oy Effect of paper COF on coated fine paper winding  When COF is ≥0.5 there is a risk for vibration. This is very common situation when mostly carbonate is used in coating.  When COF is low, roll dishing can be a problem. 37 0,35 0,55 Paper Coefficient of Friction Riskforwindingproblems No problems
  38. 38. Pele Oy Effect of paper COF on newsprint winding  When COF is ≥0.5 there is a risk for vibration and bouncing. This is very common situation when DIP quality is good (no stickies or fatty acids).  For low grammage coated paper wrinkles and burst are a common problem. 38 0,3 0,5 Paper Coefficient of Friction Riskforwindingproblems No problems
  39. 39. Pele Oy Vibration, friction and roll hardness  When newsprint is made of mechanical pulp or lower quality DIP, coefficient of friction (COF) is low and there are no vibration problems in winding.  Nowadays, with better DIP processes and higher amount of carbonate fillers, newsprint tends to have high friction and severe vibration problems (like bond paper in the picture). 39
  40. 40. Pele Oy Example of winder vibration and bouncing  Due to vibration, speed of the winder must be increased stepwise. With normal speed there are more web breaks (example in the picture) and to avoid these, the speed must be reduced. Reduced speed, stepwise acceleration and web breaks all reduce winder capacity. Time, s Speed,m/min 1000 500 1500 2000 Break 40 Roll bouncing ALEXIS OLSHANSKY TAPPI Journal, Feb 1997
  41. 41. Pele Oy Roll deformations and vibration  If paper friction is high and/or paper compressibility is high all impacts to the roll give a permanent deformation. These tend to have a certain wavelength. If this wavelength matches roll periphery, vibration will be heavier. 41 Roll bouncing ALEXIS OLSHANSKY TAPPI Journal, Feb 1997
  42. 42. Pele Oy Vibration harmonics vs. drum’s natural frequency Winder drum natural frequency Paper roll frequencies vary due to growing roll diameter = Heavy Vibration 42
  43. 43. Pele Oy J-line and crepe wrinkles  Internal slipping of paper layers under the roll surface is measured with J-line measurement.  Shooting a radial line on roll end and then winding layers on top of that will move the marked layers first to the tightening direction and then slowly towards the winding direction (= loosening the original maximum web tension under some top layers).  If the maximum tension is low and slippage high there will be a negative tension in the machine direction resulting in a buckle of the layers (= crepe wrinkle).  Very bad combination is a high nip load, a low web tension and a low COF. This is most common at the machine reel edge, where caliper can be high and web length long (=loose edges). In addition edges can be dry and thus COF is low.  It is best to have curved CD profile = slightly lower caliper at edges to avoid crepe wrinkles and other problems. 43
  44. 44. Pele Oy Sensitivity to crepe wrinkles  There will be slippage and possibility to get crepe wrinkles, when - F > µ • p • A + S - F = buckling force from nip action = f (N and nip width) - A = area of possible sliding - S = paper stiffness force  To avoid wrinkles paper static COF must be high as well as roll hardness, but nip load should be low. Low paper stiffness and caliper increase possibilities to get wrinkles.  With a soft drum cover the buckling force and J-lines are smaller than with a hard winder drum. Possibility to get crepe wrinkles is very small. Picture: Metso Paper 44
  45. 45. Pele Oy Reasons for crepe wrinkles  Low web-to-web coefficient of friction  Slipping of web layers due to nip effect inside the roll surface  When roll weight is high (close to center if center support, close to surface if surface support)  Combination of low web tension and high/increasing nip load  Combination of loose web and high caliper/roll diameter  Low basis weight or thin paper and low MD stiffness  Hard winding on top of soft winding  Variation in CD profiles, loose edges, uneven rider roll contact Winding direction 45
  46. 46. Pele Oy No-problem channel in winding  It is important to find the correct web tension for every paper grade. If tension is too high web breaks and internal bursts are problems.  If tension is too low crepe wrinkles are problems. Increasing nip load with roll diameter No problem area Too low web tension  wrinkles Too high web tension  bursts and breaks 46
  47. 47. Pele Oy Core burst of single drum winders  Due to core and roll bottom bending shear and compression forces have most effect at roll edges against the core chuck area.  The situation is worst when roll weight is highest.  COF has effect on the slippage between layers. Gearing is easier with low COF. 47
  48. 48. Pele Oy Pressure from spool to paper  There is more pressure at reel edge when the spool is thin, reel is wide, diameter is large and paper density is high.  Thin groundwood papers are more sensitive.  Reel hardness, air between layers and machine speed have influence. The problem: Wrinkles and bursts on reel bottom. Bottom broke up to 2% = 2000 m of 100000 m. Problem areas 48
  49. 49. Pele Oy Effect of COF on roll traction  Tension change depends on COF.  Air cushion can reduce the real COF depending on speed, web porosity, capstan wrap angle and web tension.  With very high speeds and dense webs there can be a floating situation. Then the paper-roll COF has no effect anymore. 49 Picture: Getting and Losing Traction by Jerry Brown
  50. 50. Pele Oy Traction between web and roller  There are two distinct ways to improve the traction potential between web and roller: topography and chemistry.  Topography is using surface roughness to increase traction. Rough rollers will have a higher traction with rough webs and with all webs where air entrainment is significant.  However, smooth rollers might have more traction with a smooth web at speeds less than 50 mpm or where grooving is present to handle the air.  The most common roller roughening techniques are grooving and spraying with tungsten carbide. 50
  51. 51. Pele Oy Chemistry and roll traction  Chemistry also affects traction. Metal surfaced rollers tend to have a somewhat similar friction against many webs; a COF of 0.3 is typical. However, those same webs against common resilient cover materials, such as polyurethane have considerably higher COF.  The effect of chemistry on COF is very efficient. Basically only molecular layer on the surface has big effect on COF.  Adding some lubricant or high friction chemicals to the surface size or coating has more influence than chemicals in base paper. Similarly, contamination of rollers can have great effect on COF between roller and paper. For example washing of winder drums is very important to keep good traction.  The only issue here is that higher COF topography or chemistry tend to be expensive to achieve and expensive to maintain. 51
  52. 52. Pele Oy Paper grades and COF  Good deinking removes all fatty acids from the pulp. Fatty acids are lubricants decreasing paper friction. Good deinked pulp = high paper-to-paper COF = vibration in winding. Newsprint earlierImproved deinking 52
  53. 53. Pele Oy Loosening of fibers  After wire section there are several kind of forces trying to loose fibers from the paper web.  Fiber detachment depends on the balance between binding forces and detachment forces. To solve the dust problems it is important to increase binding forces and reduce detachment forces. With high friction papers the detachment forces are often higher and paper is more dusty. 53 Potential dust fiber Paper web
  54. 54. Pele Oy Conclusion  Be aware that COF is important to the runnability of the paper you work with and the performance of your product.  Be aware that COF is sensitive to changes you cannot see or feel.  Include COF as one of the paper properties routinely measured and controlled.  Relate COF to the operating parameters of your equipment. Keep records so that you can compensate for changes in paper properties.  Work out a reasonable specification for COF - and hold to it. 54

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