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Pele Oy
Rheology in Paper and Paperboard Coating
Pekka Komulainen
Pekka.Komulainen@clarinet.fi
19 April, 2016
Pele Oy
Rheology in Paper and Paperboard Coating
Contents Page
 Coated Paper and Board Grades 3
 Introduction to Rheolog...
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Coated Paper and Board Grades
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Chemical and mechanical pulps
 Chemical pulp papers are called woodfree papers and more than 10% mechanical pulp
...
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Publishing paper grades in Europe
 Standard newsprint, improved newsprint and SC magazine papers are uncoated gra...
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European coated publishing paper grades
 Coated papers for publishing can be single, double or triple coated. The...
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Coating rheology - important for paperboards
7
Table origin: www.zumbiel.com
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Coated grades by finish
 Matt papers are not calendered or only slightly calendered.
 Silk papers are mostly cal...
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9
Surface of paper coating
Fine kaolin clay
Ground Calcium Carbonate
Pictures: SMI
Precipitated Calcium Carbonate
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Coating compared to base paper
 The share of coating layer thickness is only half of the share of grammage becaus...
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Introduction to Rheology
Dilatancy
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12
Types of deformation
Coating colors are viscoelastic
Picture: Fapet
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13
Viscoelastic terms
Ideal elastic element – Hookean solid
Deforms when stress is applied. Once stress is removed...
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Viscoelastic behavior
 Coating colors are viscoelastic. Viscoelastic material has the properties both of
elastici...
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Pictures: www.viscopedia.com
Viscosity, shear stress and shear rate
 Viscosity is resistance of a liquid to flow ...
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16
General flow curve for suspensions
Picture: Fapet
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Types of flow behavior
 Good coating colors are
pseudoplastic to get easy
runnability and good quality.
 If volu...
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Types of time-dependent flow behavior
 Coating color should be thixotropic – low viscosity after long shear befor...
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Particles in Coating Color
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Conventional coating color compositions
20
Picture: SCG-DOW
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Typical coating color formulations
 Pigment 100 parts, e.g. Clay (Kaolin), GCC - Ground
Calcium Carbonate, PCC - ...
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Effect of volume fraction on viscosity
 For random packing critical volume fraction of spheres is
0.66 i.e. volum...
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23
Low shear rate rheology of GCC/Clay slurries
Picture: Omya
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24
Gloss development of clay/GCC mixtures
Picture: Omya
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Solids content and volume fraction
 Titanium dioxide has high particle density (rutile 4230 kg/m3) and latex low ...
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Plastic pigments and void volume
 The most efficient pigment is hollow sphere plastic pigment where air filled vo...
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Particle size effects on viscosity
 The picture on the left means that with smaller particles viscosity is higher...
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Particle shape effects on viscosity
 More spherical shape = lower viscosity.
 Viscosity is not related to the st...
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Viscosity and slurry structure
 High aspect ratio clay and talc can have very
high viscosity if the plates are no...
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Kaolin structure, charge and pH
 Anisotropic particles such as rods or
plates have different charge at edges
depe...
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Particle shape and slurry solids
 When solids contents are compared in same viscosity, GCC slurry can be made to ...
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Aspect ratio and in-plane coating strength
 It is interesting to know how platy clays can increase in-plane coati...
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Viscosity and particle shape
 With elongated particles the random orientation leads to an increase in low shear
v...
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Bi-modal particle blends
 When two particle sizes are blended the viscosity reaches its minimum
closer to the big...
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Viscosity and particle volume fraction
 Coating colors should have high
solids to get good quality and low
water ...
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Particle size and high shear viscosity
 A higher number of smaller particles results in more particle-particle in...
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Blocky and platy pigments
 Blocky particles (carbonates and fine glossing clays) dive into the paper pores and
co...
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Viscosity explanations
 NPSD or Narrow Particle Size Distribution GCC gives more bulky coating and higher
coating...
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General viscosity order of main coating pigments
 Platy coarse clay is suitable for precoating and fine clay for ...
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Particle charges and viscosity
 The simplest example concerns hard
spherical particles that don't interact,
i.e. ...
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Clay slurry dispersion optimization
 The picture shows ACAV viscosity of clay slurry with narrow particle size di...
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42
 Chemically stable and low solubility in water
 High light scattering and low absorption at all wavelengths i...
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Conclusion on main viscosity factors
 In practice mass solids content is measured. Volume fraction varies if comp...
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Properties of Coating Colors
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Main paper coating principles
45
Picture: Katarina Dimic-Misic
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Main coating methods
 There are three different coating
methods: blade, film transfer and curtain
coating.
 It d...
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Main blade/rod coating methods
 Blade coating can have three different
color application methods: applicator
roll...
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Blade coating principle
48
Nozzle gap about 1 mm
Incoming web
Backing
roll
Blade and blade holder
Coated
paper
 T...
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49
Calculation example of coating color formulation
Substance Parts Dry amount Solids Amount
kg % kg
Kaolin, 80% <...
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50
Rheology of coating colors
 Coating colors are complex materials:
 Their viscosity is strongly dependent on s...
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Types of coating color flow behavior
 Coating color must be shear thinning = pseudoplastic. Shear thickening =
di...
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52
0.0 2.0x105
4.0x105
6.0x105
8.0x105
0
500
1000
1500
2000
Viscosity(mPas)
Shear rate (1/s)
Chemical Factors
Hydr...
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53
HYDRODYNAMICS AT ULTRA HIGH SHEAR
 Volume fraction of particles
 Particle size
 Shape of the particles
 Par...
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54
Coating color shear rates in real process
Shear Rate (1/s)
Capillary and slit viscometers
Coating color circula...
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Shear rate ranges of viscometers
 It is easy to note that coating research without using capillary or slit viscom...
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Typical ultrahigh shear viscosity curves
 Especially in blade coating it is important to know and control ultrahi...
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Suspension property effects on coating
 In coating colors intermolecular forces and particle charges have effect ...
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58
0
10
20
30
40
50
60
70
80
0 200000 400000 600000 800000 1000000 1200000
Shear rate 1/s
ViscositymPas
30ºC
50ºC
...
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Low shear viscosity of coating components
59
Picture: Michel-Sanchez Enrique
Albagloss is PCC
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Example of flow patterns in blade coating
60
Picture: C.K.Aidun
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BackingRoll
Base Paper
Color
Coating shear rates
 Paper speed can be up to 30 m/s. The average gap between paper
...
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Ultra high shear viscosity curves
62
Area Rheology Problems
A High viscosity at low shear rates Pumping and startu...
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Curtain coating and extensional viscosity
 Instead of old air knife coating curtain coating is used when a contou...
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Flow pattern in blade coating exit
 Flow is contracted after the blade about 50% which means that the average flo...
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Slit in high shear measurement
 Slit geometry is comparable with blade coating geometry. With rigid blade the ang...
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Capillary and slit correlation to runnability
 Blade coating runnability was followed and ultrahigh shear viscosi...
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67
0 1 2 3 4 5 6 7 8 9 10
Brookfield
Haake
Hercules
Eklund
ACAV A4
ACAV A2
Shear Rate x 106 (1/s)
Practical shear ...
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Comparison of shearing times
Viscometer
Shearing
time
Compared to process
Brookfield 1 s Tens of thousand times
Ro...
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Rheology of Paper Coatings
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Viscosity of fine and coarse pigments
 Pigment type has effect on reachable viscosity and solids content of the p...
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Effect of aspect ratio on pigment slurry solids
 It is important to know the aspect ratios of clays to predict th...
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Gloss and surface strength
 Adding GCC with broad particle size distribution to high glossing clay increases
soli...
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Effective solids and ultrahigh shear viscosity
Picture: Omya
73
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Coater speed and clay content
 Share of clay in carbonate coating reduces solids content and maximum coating spee...
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Window for ideal solids
 Window for ideal solids is very narrow. Too high solids results in runnability problems
...
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Coarse and fine clay in topcoating color
 Viscosity of coarse and platy clay is considerably higher than with fin...
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Double coated papers
 There is less platy clay on the left coating formula in precoating. Solids content and
coat...
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Water retention principles
 Water retention is an important
factor in coating coverage.
 Two principles have eff...
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Water and coating penetration
 When speed is low, capillary penetration is high from application up to immobiliza...
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Filter cake formation and water retention
 Capillary absorption forces are working before and after the blade pre...
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Laboratory Study
Impact of new binders
GWR/HS viscosity; PCC/Clay 70/30, latex 12 pph
70
75
80
85
90
95
100
105
11...
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Origins of blade coating scratches
 Base paper
 Particles bigger than 20 µm. Stickies, pitch, white pitch, fille...
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Effect of solids and latex on color rheology
 GCC here is of NPSD-type or has narrow particle size distribution.
...
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Effect of latex particle size on rheology
84Picture: Omnova
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Effect of latex on ultrahigh shear viscosity
 Even if the latex in the formulation was only 12 parts (pigments 10...
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Relaxation-induced dilatancy
 The numerical simulations carried out suggest that the non-​hydrodynamic interactio...
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Detailed behavior of coating color
 Viscoelastic regions are subjected
during coating metering to high
compressiv...
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Coating holdout and binder migration
88
Picture: Somar
 On the left picture there is good coating holdout and no ...
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Working principle of rheology modifiers
89
Picture: Derrick Burrel, 2007 TAPPI Coating & Graphic Arts Conference
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Effects of associative rheology modifiers (RMs)
 Associative RMs thicken water phase with swelling and polymer ne...
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Practical effects of rheology modifiers
 It seems that HASE (associative
rheology modifier) is best in all
other ...
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Ultrahigh shear rate viscosity of rheology modifiers
92
Picture: Derrick Burrel, 2007 TAPPI Coating & Graphic Arts...
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Effect of elasticity in coating
 Lubricants may turn the balance of a
viscoelastic coating color more viscous and...
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Lubricants in coating
 Lubricants and release agents are used to
enhance smoothness and flow properties
of wet co...
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Pigment packing effect on absorption
95
Picture: Daniel Matte
Open structure – Low tortuosity
 To get good coatin...
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Ultra high shear viscosity vs. coating scratches
 Slit viscosity correlates very well with scratch count in blade...
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Conclusion on dispersant dose
Imerys researchers make the following conclusion:
 “Pilot coating data show a clear...
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Effect of dispersants and thickeners
 If the dispersant level is too high there can be problems in recirculating ...
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Special latex - biolatex behavior
 Ultra-high shear viscosity measurements show that deforming soft latex particl...
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Ultrahigh shear rate viscosity of three binders
100
Picture: Ecosynthetix
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Coating color development
 In coating developed the key question is to use relevant laboratory methods. The most
...
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102
Dilatant sand – Quicksand
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Rheology of Coating Colors 19 April

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Rheology of Coating Colors 19 April

  1. 1. Pele Oy Rheology in Paper and Paperboard Coating Pekka Komulainen Pekka.Komulainen@clarinet.fi 19 April, 2016
  2. 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. 3. Pele Oy Coated Paper and Board Grades
  4. 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. 5. Pele Oy Publishing paper grades in Europe  Standard newsprint, improved newsprint and SC magazine papers are uncoated grades. 5
  6. 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. 7. Pele Oy Coating rheology - important for paperboards 7 Table origin: www.zumbiel.com
  8. 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. 9. Pele Oy 9 Surface of paper coating Fine kaolin clay Ground Calcium Carbonate Pictures: SMI Precipitated Calcium Carbonate
  10. 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. 11. Pele Oy Introduction to Rheology Dilatancy
  12. 12. Pele Oy 12 Types of deformation Coating colors are viscoelastic Picture: Fapet
  13. 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. 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. 15. Pele Oy Pictures: www.viscopedia.com 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. 16. Pele Oy 16 General flow curve for suspensions Picture: Fapet
  17. 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. 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. 19. Pele Oy Particles in Coating Color
  20. 20. Pele Oy Conventional coating color compositions 20 Picture: SCG-DOW
  21. 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. 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: www.wernerblank.com
  23. 23. Pele Oy 23 Low shear rate rheology of GCC/Clay slurries Picture: Omya
  24. 24. Pele Oy 24 Gloss development of clay/GCC mixtures Picture: Omya
  25. 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. 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. 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. 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. 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: www.tulene.edu Picture: www.nature.com
  30. 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. 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. 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. 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: www.azom.com
  34. 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: www.azom.com
  35. 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: www.azom.com
  36. 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: www.azom.com
  37. 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. 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. 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. 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: www.rsc.org
  41. 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. 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. 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: www.malvern.com
  44. 44. Pele Oy Properties of Coating Colors
  45. 45. Pele Oy Main paper coating principles 45 Picture: Katarina Dimic-Misic
  46. 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. 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. 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. 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. 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. 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. 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 www.aca.fi
  53. 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. 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 www.aca.fi
  55. 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. 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 www.aca.fi
  57. 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. 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. 59. Pele Oy Low shear viscosity of coating components 59 Picture: Michel-Sanchez Enrique Albagloss is PCC
  60. 60. Pele Oy Example of flow patterns in blade coating 60 Picture: C.K.Aidun
  61. 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 1.5x106 1/s.  Minimum distances are much less than the average coating thickness and maximum local shear rate is higher than the calculated average.  Coating color should tolerate shear rates of some millions in blade coating.  The risk of dilatancy with shear thickening color is always present – even if the coater speed is much less than 30 m/s. 61 Base Paper Contacts CoatingBlade
  62. 62. Pele Oy Ultra high shear viscosity curves 62 Area Rheology Problems A High viscosity at low shear rates Pumping and startup problems B High viscosity at high shear rates Problems in screens and applicators C High viscosity at ultrahigh shear rates, dilatant Blade bleeding and streaking Problems maintaining target coat weight Web breaks D Low viscosity at ultrahigh shear rates Uneven coat weight E Low viscosity at all shear rates Water retention problems Picture: Valmet Modified from: www.aca.fi
  63. 63. Pele Oy Curtain coating and extensional viscosity  Instead of old air knife coating curtain coating is used when a contour coating is needed.  Extensional viscosity is a color’s resistance to accelerating flow. Extensional viscosity is most important in curtain coating but also in blade coating after blade.  Coating colors contain polymeric thickeners, which have considerable effect on extensional viscosity.  Extensional viscosity can be measured with ultrahigh shear rate slit viscometer. 63 Orientation and straightening in accelerating flow Picture: Voith Picture: www.aca.fi
  64. 64. Pele Oy Flow pattern in blade coating exit  Flow is contracted after the blade about 50% which means that the average flow speed after the blade is doubled. 64 Picture: Douglas W. Bousfield
  65. 65. Pele Oy Slit in high shear measurement  Slit geometry is comparable with blade coating geometry. With rigid blade the angle is about 45º and gap is 15-25 µm (contraction after blade about 50%).  Slit is just two blades against each other. Because the length is only 0,5 mm the delay time in the measurement is very short, similar to blade coating.  The problem with rotating viscometers is long measurement time and heating of the dispersion. Thixotropy has effect on the measurement. 65 Coating Color BLADE ~ 0.5 mm ~20µm Base paper www.aca.fi 0.5 mm SLIT 80µm
  66. 66. Pele Oy Capillary and slit correlation to runnability  Blade coating runnability was followed and ultrahigh shear viscosity measured. Correlation of slit viscosity to runnability was superior compared to capillary viscosity. 66 Capillary measurement, 0.5 Mill. 1/s and poor correlation Slit measurement, 3 Mill. 1/s and good correlation www.aca.fi
  67. 67. Pele Oy 67 0 1 2 3 4 5 6 7 8 9 10 Brookfield Haake Hercules Eklund ACAV A4 ACAV A2 Shear Rate x 106 (1/s) Practical shear rate ranges of viscometers www.aca.fi
  68. 68. Pele Oy Comparison of shearing times Viscometer Shearing time Compared to process Brookfield 1 s Tens of thousand times Rotational 1 s Tens of thousand times ACAV Capillar 1 ms About 30 times longer ACAV SLIT 30 µs Same as under blade 68 www.aca.fi
  69. 69. Pele Oy Rheology of Paper Coatings
  70. 70. Pele Oy Viscosity of fine and coarse pigments  Pigment type has effect on reachable viscosity and solids content of the pigment slurry and coating color.  Coarse and platy clay has lowest possible solids and GCC with broad particle size distribution highest. 70 Picture: Omya
  71. 71. Pele Oy Effect of aspect ratio on pigment slurry solids  It is important to know the aspect ratios of clays to predict the effect on possible solids content and gloss. 71 Picture: Omya
  72. 72. Pele Oy Gloss and surface strength  Adding GCC with broad particle size distribution to high glossing clay increases solids content and surface strength but decreases sheet gloss. 72 Picture: Omya
  73. 73. Pele Oy Effective solids and ultrahigh shear viscosity Picture: Omya 73
  74. 74. Pele Oy Coater speed and clay content  Share of clay in carbonate coating reduces solids content and maximum coating speed. Critical speed is defined with visual examination of stalagmites or coating scratches. 74 TAPPI 1999 Blade
  75. 75. Pele Oy Window for ideal solids  Window for ideal solids is very narrow. Too high solids results in runnability problems and too low solids in quality problems. Process variations should be minimized to be able to run close to the maximum solids. 75 Picture: Omya
  76. 76. Pele Oy Coarse and fine clay in topcoating color  Viscosity of coarse and platy clay is considerably higher than with fine clay. Reduced amount (15%) of coarse clay in the formulation gives almost same ultrahigh shear viscosity as 25% fine clay. The main pigment here is fine GCC.  Higher ultrahigh shear viscosity means that blade load in real coating is higher. 76 0 20 40 60 80 100 120 0 200000 400000 600000 800000 1000000 1200000 Viscosity,mPas Shear rate, 1/s Viscosity / Shear rate Coarse clay 25% Coarse clay 15% Fine clay 25%
  77. 77. Pele Oy Double coated papers  There is less platy clay on the left coating formula in precoating. Solids content and coating coverage are lower.  On the right, more platy particles can give relatively better water retention, smoothness, print gloss, coating coverage, ink holdout, print density, paper stiffness and bulk. 77 Fine clay Coarse clay Picture: Kenji Hirai
  78. 78. Pele Oy Water retention principles  Water retention is an important factor in coating coverage.  Two principles have effect: color viscosity and shape factor of particles.  Low shear viscosity has effect on capillary penetration and ultra- high shear viscosity under the blade or rod. 78 Low aspect ratio, fast penetration High aspect ratio, slow penetration Low viscosity, fast penetration High viscosity, slow penetration Picture: Kenji Hirai
  79. 79. Pele Oy Water and coating penetration  When speed is low, capillary penetration is high from application up to immobilization of coating. Platy particles decrease penetration.  When drying is slow and far from the blade, capillary penetration continues long way after the blade. 79 Blade
  80. 80. Pele Oy Filter cake formation and water retention  Capillary absorption forces are working before and after the blade pressure.  Pressure penetration is most effective under the blade.  Filter cake formation decreases the effective coating layer gap under the blade. This can increase shear rate (speed gradient) considerably. 80
  81. 81. Pele Oy Laboratory Study Impact of new binders GWR/HS viscosity; PCC/Clay 70/30, latex 12 pph 70 75 80 85 90 95 100 105 110 20 40 60 80 100 120 High shear viscosity @ 800 000 1/s , [mPas] Waterretentio,g/m2 Latex 1 Latex 2 Latex 3 Latex 4 6 65.1% 63.6% 66.0% 64.3% 62.5% 66.1% 64.3% 62.5% 66.9% 65.2% 63.2% Operating Window Runnability limits Qualitylimits Rheology and water retention 81 Picture: SMI
  82. 82. Pele Oy Origins of blade coating scratches  Base paper  Particles bigger than 20 µm. Stickies, pitch, white pitch, filler, fiber bundles, fiber vessels/ray cells, sand, coated broke, chemical agglomerates, wet end chemistry problems.  Loose fibers on top layer or filler layer, poor surface sizing, size agglomerates or retrogradation, paper pieces in size circulation, poor screening in size circulation.  Coating color  Screening of pigment/color  Hard pigment particles  Too high solids or low water retention, dilatancy in high shear rate  Poor dosage of dispersion agent or dispersion stability  Air in coating color leads to higher viscosity  Poor mixing of pigment/coating color  Fibers in color  Air in color 82
  83. 83. Pele Oy Effect of solids and latex on color rheology  GCC here is of NPSD-type or has narrow particle size distribution. 83 Pictures: Omya
  84. 84. Pele Oy Effect of latex particle size on rheology 84Picture: Omnova
  85. 85. Pele Oy Effect of latex on ultrahigh shear viscosity  Even if the latex in the formulation was only 12 parts (pigments 100 parts), it had great effect on viscosity when narrow particle size distribution PCC was included. PCC was 70% and fine clay 30% of the pigments. Conventional formulation was with GCC. 85 Picture: Dow
  86. 86. Pele Oy Relaxation-induced dilatancy  The numerical simulations carried out suggest that the non-​hydrodynamic interactions between particles are able to store energy when the coating layer is being compressed e.g. by the pressure pulse of a coating blade.  Relaxation of the stored energy has a potential to expand the system volume and induce dilatancy under conditions of water loss.  Correct formulation improves visco-elasticity of coating colors, and controls occurrence of bleeding as well as dynamic water retention under the blade. 86 Picture: Somar Good formulation – no bleeding
  87. 87. Pele Oy Detailed behavior of coating color  Viscoelastic regions are subjected during coating metering to high compressive forces. Dissolved material and fines are released from the elastic regions similar to the compression of a sponge.  The released fluid inclusive dissolved and dispersed matter is accelerated along the directional force of the pressure impulse, e.g. perpendicular to the pressure impulse.  Solid and liquid phase of the coating color are separated under pressure. After pressure decay the compressed regions expand again and pick up of what free liquid phase is still left in the wet film. 87 Picture: CTP
  88. 88. Pele Oy Coating holdout and binder migration 88 Picture: Somar  On the left picture there is good coating holdout and no binder migration.  When water retention is poor, binder can migrate to the base paper during water absorption and before immobilization point.  Other type of migration is during drying to the paper surface (skinning).  Both cause poor picking strength and mottled printing result.
  89. 89. Pele Oy Working principle of rheology modifiers 89 Picture: Derrick Burrel, 2007 TAPPI Coating & Graphic Arts Conference
  90. 90. Pele Oy Effects of associative rheology modifiers (RMs)  Associative RMs thicken water phase with swelling and polymer network in the suspension so that low shear viscosity increases but ultrahigh shear viscosity decreases through breaking down this polymer network between suspension particles.  RMs enhance orientation of high aspect ratio particles under ultrahigh shear rate and thus increase critical volume solids. RMs also reduce friction between particles so that particles can easier flow in a shear field over each other.  They improve static and dynamic water retention.  RMs also improve rheology after metering to reduce coating defects and to improve leveling before fast immobilization. 90 Picture: Derrick Burrel
  91. 91. Pele Oy Practical effects of rheology modifiers  It seems that HASE (associative rheology modifier) is best in all other properties than in water retention.  If water retention is reached with suitable pigments and binders, HASE seems to be the best rheology modifier.  ASE seems to be most balanced when improvement from cellulosic water retention additives are desired, 91 Picture: Derrick Burrel, 2007 TAPPI Coating & Graphic Arts Conference
  92. 92. Pele Oy Ultrahigh shear rate viscosity of rheology modifiers 92 Picture: Derrick Burrel, 2007 TAPPI Coating & Graphic Arts Conference
  93. 93. Pele Oy Effect of elasticity in coating  Lubricants may turn the balance of a viscoelastic coating color more viscous and less elastic and thus enable easier flow of a coating color still in very high coating solids. Together with higher coating solids and higher coating speeds also better paper quality may be achieved.  Lubricants may have ability to reduce friction and increase slip at the blade boundary thus preventing bleeding and dry stalagmite formation.  It must be remembered that base paper under the blade is compressed and behaves more elastic than wet coating layer. In addition, thickness of base paper is several times the thickness of coating layer.  Sometimes base paper is not calendered at all. This kind of base paper is much more compressible than calendered base paper. 93 Lubricants: Lower friction Both: Rheology modification Thickeners: Viscosity increase Coating Additives
  94. 94. Pele Oy Lubricants in coating  Lubricants and release agents are used to enhance smoothness and flow properties of wet coating as well as the plasticity and gloss of dried coating.  Lubricants can also effect on color rheology thus reducing build up of stalagmites in blade coating.  Lubricants are also used to reduce calender picking and dusting by reducing paper friction against calender rolls and enhancing deformation of paper surface in calender nips.  Lubricant products are mostly calcium and ammonium stearates. Wax emulsions are emulsions of paraffin waxes, microcrystalline waxes or polyethylene waxes. 94 Picture: Toshiharu Enomae Lubricants reduce COF of coated paper
  95. 95. Pele Oy Pigment packing effect on absorption 95 Picture: Daniel Matte Open structure – Low tortuosity  To get good coating coverage it is important how porous and tortuous the base is.  On the left picture base paper is not calendered or first coating is too open to get good water retention and coating coverage. The same is valid for printing ink and the term is normally ink hold out. High density and platy particles High tortuosity
  96. 96. Pele Oy Ultra high shear viscosity vs. coating scratches  Slit viscosity correlates very well with scratch count in blade coating. Scratches are a potential defect with high solids content and coarse platy pigments. 96 Results: TAPPI, 2001 Coating and Graphics Arts Conference and Trade Fair, pages 77 -86 Dr. Rajan R. Iyer, Ray Hollingsworth, Dr. David R. Skuse, Imerys, Sandersville,USA 1,60 % 1,60 % 0,80 % 0,80 % 0,40 % 0,40 % 0 %0 %
  97. 97. Pele Oy Conclusion on dispersant dose Imerys researchers make the following conclusion:  “Pilot coating data show a clear improvement in scratch reduction upon higher dispersant dose addition. There is a correlation between coating color viscosity and the scratch counts observed on the machine.  Better correlation between coating color viscosity and scratch counts is observed when the viscosity is measured with a slit attachment to a capillary viscometer.  This may indicate that the slit attachment measures viscosity of the coating under conditions similar to the converging geometry obtained in blade coating operations, or that the timescale of measurement in this device is closer to that of blade coating.  This implies that the elastic component of the color rheology is important and this should be measured in any future study.” 97 Real Brookfield Hercules
  98. 98. Pele Oy Effect of dispersants and thickeners  If the dispersant level is too high there can be problems in recirculating color stability (left lower curves).  For extensional viscosity interactive thickener seems to be most effective (right picture). 98 Pictures: Imerys
  99. 99. Pele Oy Special latex - biolatex behavior  Ultra-high shear viscosity measurements show that deforming soft latex particles have higher low shear viscosity but lower ultra-high shear viscosity.  Soft biolatex can deform, orientate and shrink under high shear. 99 Picture: Ecosynthetix
  100. 100. Pele Oy Ultrahigh shear rate viscosity of three binders 100 Picture: Ecosynthetix
  101. 101. Pele Oy Coating color development  In coating developed the key question is to use relevant laboratory methods. The most important ones are ultrahigh shear viscosity and dynamic water retention. The following conclusion is from Ecosynthetix presentation in PaperCon2012 Conference.  , PaperCon2012 101
  102. 102. Pele Oy 102 Dilatant sand – Quicksand

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