Rheology of Coating Colors 19 April

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1. Pele Oy Rheology in Paper and Paperboard Coating Pekka Komulainen 19 April, 2016 2. Pele Oy Rheology in Paper and Paperboard Coating…
  • 1. Pele Oy Rheology in Paper and Paperboard Coating Pekka Komulainen 19 April, 2016
  • 2. Pele Oy Rheology in Paper and Paperboard Coating Contents Page  Coated Paper and Board Grades 3  Introduction to Rheology 11  Particles in Coating Color 19  Properties of Coating Colors 44  Rheology od Paper Coatings 69  Thank You for Your Attention 102 2
  • 3. Pele Oy Coated Paper and Board Grades
  • 4. Pele Oy Chemical and mechanical pulps  Chemical pulp papers are called woodfree papers and more than 10% mechanical pulp containing grades are called mechanical grades. 4
  • 5. Pele Oy Publishing paper grades in Europe  Standard newsprint, improved newsprint and SC magazine papers are uncoated grades. 5
  • 6. Pele Oy European coated publishing paper grades  Coated papers for publishing can be single, double or triple coated. They can be mechanical papers or woodfree grades. They can be made to matt, silk or glossy finish and can be delivered in rolls or sheets.  Usual printing methods are heat set web offset, rotogravure or sheet fed offset. 6
  • 7. Pele Oy Coating rheology - important for paperboards 7 Table origin:
  • 8. Pele Oy Coated grades by finish  Matt papers are not calendered or only slightly calendered.  Silk papers are mostly calendered with two soft nips.  Gloss papers are fully calendered - at least four or more soft nips. 8 *Hunter gloss
  • 9. Pele Oy 9 Surface of paper coating Fine kaolin clay Ground Calcium Carbonate Pictures: SMI Precipitated Calcium Carbonate
  • 10. Pele Oy Coating compared to base paper  The share of coating layer thickness is only half of the share of grammage because density of coating is about double in calendered paper. Base paper must be smooth and have good formation to get a good coated paper surface. 10 Picture: KCL Coating density 2000 kg/m3 Base paper density 1000 kg/m3 38 - 40 µm 4 - 5 µm Coating is a thin layer compared to base paper
  • 11. Pele Oy Introduction to Rheology Dilatancy
  • 12. Pele Oy 12 Types of deformation Coating colors are viscoelastic Picture: Fapet
  • 13. Pele Oy 13 Viscoelastic terms Ideal elastic element – Hookean solid Deforms when stress is applied. Once stress is removed fully recovers to the original shape. Modeled as spring. Ideal viscous element – Newtonian fluid Flows as long as stress is applied and remains in that shape once stress is removed. Modeled as dashpot. Picture: Fapet
  • 14. Pele Oy Viscoelastic behavior  Coating colors are viscoelastic. Viscoelastic material has the properties both of elasticity and viscosity. It is simulated by a Maxwell element. Response of Maxwell element to strain is the following: 14 Picture: Fapet
  • 15. Pele Oy Pictures: Viscosity, shear stress and shear rate  Viscosity is resistance of a liquid to flow - molecules slide over one another.  Viscosity can be defined with a moving plate over a fixed plate. Viscosity η is Shear stress / Shear rate. In papermaking shear rates are high and viscosities then lower than with conventional low shear rate measurements. 15
  • 16. Pele Oy 16 General flow curve for suspensions Picture: Fapet
  • 17. Pele Oy Types of flow behavior  Good coating colors are pseudoplastic to get easy runnability and good quality.  If volume concentration is too high, suspension is easily dilatant.  Special binders, rheology modifiers and lubricants can help to increase volume concentration without dilatancy.  Dilatant coating colors are difficult to run leading to scratches, streaking, bleeding, too high blade load and web breaks. 17 Shear rate 1/s Shear stress
  • 18. Pele Oy Types of time-dependent flow behavior  Coating color should be thixotropic – low viscosity after long shear before and under the leveling element e.g. blade and high viscosity to start immobilization after the metering when coating has leveled. 18 Thixotropic loop Picture: Fapet
  • 19. Pele Oy Particles in Coating Color
  • 20. Pele Oy Conventional coating color compositions 20 Picture: SCG-DOW
  • 21. Pele Oy Typical coating color formulations  Pigment 100 parts, e.g. Clay (Kaolin), GCC - Ground Calcium Carbonate, PCC - Precipitated Calcium Carbonate, Talc, Gypsum, plastic pigment, TiO2 etc.  Dispersant 0.1-0.5 parts, e.g. polyacrylates, polyphosphates, lignin sulfonates  NaOH to adjust pH to 8.5-9  Binder(s) 5-20 parts, e.g. synthetic binder (XSB, SA, PVAc) and natural binders (starch and starch based particles, CMC, casein, soybean protein)  Cobinder / Thickener 0.5-1.5 parts, e.g. natural products (starch derivatives, CMC, soy protein, casein) and synthetic products (PVOH, PVP, HEUR, ASE, HASE, ACT)  Optical brightener and dyes 0.5-0.8 parts  Lubricants (calcium stearate, wax, PE- or PP-glycols, soy lecithin/oleic acid blends) 0.2-0.5 parts  Other additives, e.g. defoamer/degassing agents, biocides etc. 21
  • 22. Pele Oy Effect of volume fraction on viscosity  For random packing critical volume fraction of spheres is 0.66 i.e. volume solids is 66%. Volume fraction of dense tetrahedral packing would be about 74%.  When shear rate is high, lower than cubic packing (52 %) is needed to get workable coating color viscosity.  Mass solids of mineral pigments is always higher than volume solids because main pigments have densities about 2700 kg/m3 - titanium dioxide even much more. 22 Pictures:
  • 23. Pele Oy 23 Low shear rate rheology of GCC/Clay slurries Picture: Omya
  • 24. Pele Oy 24 Gloss development of clay/GCC mixtures Picture: Omya
  • 25. Pele Oy Solids content and volume fraction  Titanium dioxide has high particle density (rutile 4230 kg/m3) and latex low density (max. 1050 kg/m3) compared to mineral pigments. This means that volume fraction is not directly correlated with mass solids content. 25 Picture: Omya
  • 26. Pele Oy Plastic pigments and void volume  The most efficient pigment is hollow sphere plastic pigment where air filled volume is 55% of the sphere. This means that a very small mass addition increases effective solids of coating pigments. Effective solids is then much more than actual solids. 26 Pictures: Rohm and Haas
  • 27. Pele Oy Particle size effects on viscosity  The picture on the left means that with smaller particles viscosity is higher.  Wider particle size distribution = lower viscosity. The picture on the right shows viscosities for mono-modal, bi-modal and tri-modal particle size distributions.  Practical coating colors can include foreign particles such as air bubbles, fibers and fillers from base paper. These increase viscosity. Picture: www.malvern.com0.1 0.25 0.50 Volume Fraction Logviscosity 100 200 300 nm Picture: Horiba 27
  • 28. Pele Oy Particle shape effects on viscosity  More spherical shape = lower viscosity.  Viscosity is not related to the static packing density, which would be highest with cubes.  Viscosity is related to the packing volume of rotating particles.  Higher aspect ratio gives higher viscosity unless particle orientation is reached. 0.1 0.25 0.50 Volume Fraction Logviscosity spheres cubes rods Picture: Horiba plates 28
  • 29. Pele Oy Viscosity and slurry structure  High aspect ratio clay and talc can have very high viscosity if the plates are not aligned but form structures like house-of-cards.  Both chemical forces (at low shear) and mechanical shear forces can form these highly porous structures. 29 Picture: Picture:
  • 30. Pele Oy Kaolin structure, charge and pH  Anisotropic particles such as rods or plates have different charge at edges depending on slurry pH.  It is very important to control constant pH for this kind of slurries.  Aspect ratio of kaolin can vary from 10:1 (blocky kaolin) to more than 60:1 (hyperplaty kaolin). 30 Aspect ratio = L / t = Shape factor
  • 31. Pele Oy Particle shape and slurry solids  When solids contents are compared in same viscosity, GCC slurry can be made to 77% solids and several high aspect ratio clays must have <67%.  Kaolin is used for paper gloss, better coating coverage and water retention. On the other hand, higher solids as such compensates very much by improving paper quality, coating coverage, water retention, print gloss as well as reducing binder demand, which again improves gloss and smoothness. 31 Picture: Imerys GCC
  • 32. Pele Oy Aspect ratio and in-plane coating strength  It is interesting to know how platy clays can increase in-plane coating strength and thus also bending resistance. For packaging grades this is important when both barrier properties and stiffness increase by using platy grade pigments. 32 Picture: Imerys
  • 33. Pele Oy Viscosity and particle shape  With elongated particles the random orientation leads to an increase in low shear viscosity. However, under shear these elongated particles can orient themselves to be streamlined with the direction of flow. They are therefore easier to flow, resulting in a lower high shear viscosity than the spherical same size equivalent. 33 Picture:
  • 34. Pele Oy Bi-modal particle blends  When two particle sizes are blended the viscosity reaches its minimum closer to the bigger particles. 34 Picture:
  • 35. Pele Oy Viscosity and particle volume fraction  Coating colors should have high solids to get good quality and low water evaporation cost.  Good water retention also reduces evaporation energy. Moistened base paper under the coating is very difficult to dry again. Water is not free but connected to fibers with hydrogen bonds.  Fast immobilization after coating is also important for product quality and evaporation costs.  This all means that real coating colors must be very close to the shear thickening range (over 50% of maximum volume fraction). 35 Picture:
  • 36. Pele Oy Particle size and high shear viscosity  A higher number of smaller particles results in more particle-particle interactions and an increased resistance to flow.  Clearly as shear rate increases, this effect becomes less marked, suggesting that any particle-particle interactions are relatively weak and broken down at ultrahigh shear rates.  It is very important to always measure viscosity at ultrahigh shear rate. 36 Smaller particles – higher viscosity at low shear rate Picture:
  • 37. Pele Oy Blocky and platy pigments  Blocky particles (carbonates and fine glossing clays) dive into the paper pores and coating coverage is poor.  Platy clays stay on the paper surface and improve coating coverage. This is important for first coating but not as much for the second coating where pores are smaller. 37
  • 38. Pele Oy Viscosity explanations  NPSD or Narrow Particle Size Distribution GCC gives more bulky coating and higher coating color viscosity.  BPSD or Broad Particle Size Distribution GCC gives denser coating, lower viscosity and higher solids coating color. 38 Picture: Omya
  • 39. Pele Oy General viscosity order of main coating pigments  Platy coarse clay is suitable for precoating and fine clay for topcoating if needed at all. 39 Picture: Omya
  • 40. Pele Oy Particle charges and viscosity  The simplest example concerns hard spherical particles that don't interact, i.e. there are no forces acting between them.  Attraction between particles increases viscosity and can result in unstable dispersion forming aggregates.  Attraction between particles can be overcome by making the particles charged, or 'hairy' by adsorbing a polymer stabilizer on their surface.  The presence of a stabilizing layer increases the effective volume of the particles and tends to give a higher viscosity than the ideal system. 40 Picture:
  • 41. Pele Oy Clay slurry dispersion optimization  The picture shows ACAV viscosity of clay slurry with narrow particle size distribution. One can see that low shear rate viscosity is not telling very much of the real viscosity. 41 0 1x10 5 2x10 5 3x10 5 4x10 5 5x10 5 0 50 100 150 200 250 300 Dispersant level 0.22 0.24 0.26 0.28 Viscosity(mPas) Shear rate (1/s) Lowest viscosity
  • 42. Pele Oy 42  Chemically stable and low solubility in water  High light scattering and low absorption at all wavelengths i.e. good brightness and whiteness  Free from impurities, no hard and big particles  Appropriate particle size and particle size distribution  High refractive index i.e. good combination of brightness and opacity  Small binder demand  Good flow properties as an aqueous suspension (low high shear viscosity)  Mixes easily with water (wettability, dispersability)  Good glossing properties (eye-pleasing coating gloss and high print gloss)  Good compatibility with other coating components  Low density, coating coverage, bulky paper  Non-abrasive, low dusting, good strength and stiffness properties  Low water absorption, easy water evaporation  Cost effective Ideal pigment properties
  • 43. Pele Oy Conclusion on main viscosity factors  In practice mass solids content is measured. Volume fraction varies if components have different densities. Titanium dioxide is high density pigment and volume fraction is not as high as solids content shows. Solid plastic pigments have about same density as latex. 43 Picture:
  • 44. Pele Oy Properties of Coating Colors
  • 45. Pele Oy Main paper coating principles 45 Picture: Katarina Dimic-Misic
  • 46. Pele Oy Main coating methods  There are three different coating methods: blade, film transfer and curtain coating.  It depends on paper grade and speed which one is most suitable. 46 Pictures: Valmet
  • 47. Pele Oy Main blade/rod coating methods  Blade coating can have three different color application methods: applicator roll, jet and short dwell application.  Most common color leveling methods are blade or sometimes rod.  It depends on paper grade and speed which one is most suitable. 47 Pictures: Valmet
  • 48. Pele Oy Blade coating principle 48 Nozzle gap about 1 mm Incoming web Backing roll Blade and blade holder Coated paper  The most common paper coating method is blade coating. Speed of commercial coaters is up to 30 m/s (1800 m/min) and in pilot up to 50 m/s (3000 m/min).  Coating color application can be with a narrow nozzle (picture), with an applicator roll or so called short dwell time unit close to the leveling blade itself.  Typical for these methods is that the applied coating color amount must be 10-20 times the amount left on the paper surface. Application amount and absorption time before blade varies depending on the application method and speed.
  • 49. Pele Oy 49 Calculation example of coating color formulation Substance Parts Dry amount Solids Amount kg % kg Kaolin, 80% < 2 µm 50 530.5 68 780 GCC, 90% < 2 µm 50 530.5 75 707 SB-latex 11 116.7 50 233 CMC 1 10.6 10 106 Insolubilizer 0.1 1.1 10 11 Lubricant 1 10.6 50 21 Water 141 Total 113.1 1200 60 2000 Input 1Inputs 2 Results 2 Results 3 Result 4Result 1
  • 50. Pele Oy 50 Rheology of coating colors  Coating colors are complex materials:  Their viscosity is strongly dependent on shear rate  Viscosity is dependent on shearing time  They are viscoelastic  They may have significant extensional viscosity and/or normal stresses  Base paper and water retention have effect on coating color rheology  Surface chemistry and hydrodynamics determine viscosity.  Significance of different factors depends on shear rate. It is not usually possible to draw conclusion about high shear rheology by measuring low shear area, or vice versa.  Coating colors are shear thinning at low shear rate area. However, there may be dilatant behavior at higher shear rates. Picture: Voith
  • 51. Pele Oy Types of coating color flow behavior  Coating color must be shear thinning = pseudoplastic. Shear thickening = dilatant is not good for blade coating. 51
  • 52. Pele Oy 52 0.0 2.0x105 4.0x105 6.0x105 8.0x105 0 500 1000 1500 2000 Viscosity(mPas) Shear rate (1/s) Chemical Factors Hydrodynamics PCC-slurry • Van der Waals • Electrostatic repulsion/attraction • Steric factors • Particle size and size distribution • Particle shape • Viscosity of water phase • Volume fraction (solids) Coating color viscosity depends on shear rate
  • 53. Pele Oy 53 HYDRODYNAMICS AT ULTRA HIGH SHEAR  Volume fraction of particles  Particle size  Shape of the particles  Particle size distribution  Viscosity of water phase SURFACE CHEMISTRY AT LOW SHEAR  Volume fraction of particles  Electric attraction/repulsion  Van der Waals attraction  Steric repulsion  Viscosity of water phase Brookfield, rotational and capillary viscometers Capillary and Slit viscometers Dominating factors at different shear rates
  • 54. Pele Oy 54 Coating color shear rates in real process Shear Rate (1/s) Capillary and slit viscometers Coating color circulation loop 0 102 103 104 106105 107 Applicator NipPumping Mixing Blade Screens Rod Curtain hitting point
  • 55. Pele Oy Shear rate ranges of viscometers  It is easy to note that coating research without using capillary or slit viscometer is not relevant. There are still whole countries, where all research and development is made with rotational viscometers. 55 Viscometer Practical Shear rate (1/s) Comparison to process Brookfield 10 Real shear rate 200.000 times higher Rotational 40 000 Real shear rate 50 times higher Capillary 2 Million Shear rate comparable to process Slit 10 Million Possibility to simulate also highest shear rates
  • 56. Pele Oy Typical ultrahigh shear viscosity curves  Especially in blade coating it is important to know and control ultrahigh shear viscosity. Typical measurement curves of good coating colors are in the picture below. 56
  • 57. Pele Oy Suspension property effects on coating  In coating colors intermolecular forces and particle charges have effect on viscosity measured with conventional viscometers.  However, practical shear rates of coating colors are so high that more important are hydrodynamic forces arising from particle shape and rotation in the suspension.  Viscosity should be measured with capillary/slit viscometers to get practical shear rates. 57 Variable Change in variable Effect on viscosity Effect on water retention Effects on process and quality Intermolecular forces Increase Increased low shear viscosity Better There is an optimum Particle volume concentration Increase Increase Better Better quality, runnability limit Particle size Increase Decrease Lower Lower quality, lower binder demand Particle size distribution Wider Decrease Better Denser coating, lower coating coverage Particle shape Less spherical Increase Better Better gloss, runnability limit
  • 58. Pele Oy 58 0 10 20 30 40 50 60 70 80 0 200000 400000 600000 800000 1000000 1200000 Shear rate 1/s ViscositymPas 30ºC 50ºC 60ºC Logical behaviour below 100.000 1/s Coating shear rates Temperature and coating color viscosity  Theoretically increase of temperature decreases viscosity. However, higher increase of coating color temperature destroys the surface active ingredient of the latex and ultrahigh viscosity increases with temperature.
  • 59. Pele Oy Low shear viscosity of coating components 59 Picture: Michel-Sanchez Enrique Albagloss is PCC
  • 60. Pele Oy Example of flow patterns in blade coating 60 Picture: C.K.Aidun
  • 61. Pele Oy BackingRoll Base Paper Color Coating shear rates  Paper speed can be up to 30 m/s. The average gap between paper web and blade is about 20 µm. Average shear rate is then 1.5
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