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RHEOLOGY of Coatings

RHEOLOGY of Coatings. Overview. 1 Simple Test Methods , and Rheometry 2 Flow behavior during the Application 3 Behavior after the Application 4 Long- term Storage Stability 5 Curing of Powder Coatings and UV – Coatings. 1 Simple Test Methods.

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RHEOLOGY of Coatings

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  1. RHEOLOGY of Coatings

  2. Overview • 1 Simple Test Methods, andRheometry • 2 Flow behaviorduringtheApplication • 3 Behavior after theApplication • 4 Long-term Storage Stability • 5 CuringofPowderCoatingsand UV – Coatings

  3. 1 Simple Test Methods trowel test- high-viscosity fluids: “thick“- low-viscosity fluids: “thin“e.g. for dispersions finger test- tacky: “long“ - lesstacky: “short“e.g. forpaints, offset-printinginks, pigmentpastes

  4. 1 Simple Test Methods Flow Cups measurementoftheflow timeoflow-viscosityliquids todeterminethekinematicviscosity (weight-dependentviscosity !) Examples: oils, solvent-basedcoatings, gravureandflexoprintinginks

  5. 1 Simple Test Methods weight Falling - rodViscometers determinationofthe timeoftherod totraveldownwards over a defineddistance e.g. fortesting offset-printinginks (highlyviscous) andpastes printing ink falling rod falling-rod viscometer, e.g. type Laray

  6. 1 Simple Test Methods preset: rotational speedmeasurement: torque RotationalViscometers fortesting „Low - shearViscosity“ (LSV) (whichis in fact not reallylow-shear) Using the typical spindles relative viscosity values are measured • cylinders • disks • pins • T-bars

  7. 1 Simple Test Methods RotationalViscometers fortesting „Medium – shearViscosity“ (MSV) originally preset: force(constanttorque), using a freelyfallingweight (in grams), measurement: rotationalspeedoftherotationalmeasuringsystem nowadays: presetofthespeed, measurementofthetorque Krebs spindles stirrer-like „paddles" relative viscosity valuesare measured here; typically given in Krebs Units, KU

  8. 1 Simple Test Methods Cone & Plate Viscometers fortesting „High - shearViscosity“ (HSV) preset: rotationalspeedmeasurement: torque Problem: Frictionbetweenconeandplate, sincethetipoftheconeis not truncated, sittingdirectly on thebottomplate. Consequence: Frictioninfluences themeasuringresults

  9. 1 Simple Test Methods helix 1 helix 2 blade anchor ball measuring system all these kinds of stirrers are relative measuring systems stirrerfor buildingmaterials starch stirrer

  10. 1 RheometryMeasuring Systems for Absolute Values Measuring Geometries forrotationalandoscillatoryrheometer accordingto DIN 53019 and ISO 3219 Cone & Plate, CP forliquids; fordispersionsonlywith a limittedparticlesize (usually < 10 µm) Parallel - Plates, PP usefulfordispersionscontaining coarseparticles, pastes, offset-printinginks, gel-likematerials, polymer melts ConcentricCylinders, CC forlow-viscosityliquids, solvent-bornecoatings

  11. 2 Application (flowbehavior) • Flow behaviorduringtheapplication • - Applicationbehavior in theflowingstatewhenstirring, painting, brushing, rolling, sprayingwhenpumping, dosing, blading,flatstreamapplication, dipcoating, pouring, • usingrobotersorhigh-rotationaldisksorbells • Test method: Flow curves, at medium andhigh shearrates (rotation) • Requirements: • - ability to brush • limited coating force • no spatters • roller resistance

  12. 2 Application (flow behavior) Coating, Painting, Brushing Application Example brush velocity (v = 0.5 m/s) wet layerthickness (h = 200 µm) calculation of the shear rate: Brushing, Paintingatmedium andhigh shear rates between 100 and 10,000 s-1

  13. 2 Application (flowbehavior) Industrial Spray Processes • Application examples : • Automotive coatings- spray roboters • high-rotational atomizers, electrostatically supported Requirements: - ability to pump - ability to spray Quelle: Fotos vom Daimler-Museum, Stuttgart

  14. car body degreasing & phosphatizing electro dip coating seam sealing underbody spraying filler base coat and clear coat cavity conservation 2 Application (flowbehavior) Sprayingof Automotive Coatings a) Plastisols: seamsealingandunder-bodysealingb) Coatings: dipcoating, filler, basecoat, clearcoatc) Waxes: cavityconservation Spraying, Coating at high shear ratesof 1000 to 10,000 s-1

  15. 2 Application (flow behavior) Shear Rate Range

  16. 2 Application (flowbehavior) Overview: Flow & ViscosityCurves flowcurves yieldpoint viscositycurves    1 ideallyviscous (Newtonian) 4 without a yieldpoint 2 shear-thinning (pseudoplastic) 5 having a yieldpoint 3 shear-thickening (dilatant)

  17. 2 Application (flow behavior) Flow Curves 1000 10 Water mPa mPas 10 lg h 1 lg t 1 DG 42 (double - gap) T = +20°C constant viscosity, ideally viscous flow behavior 0,1 0,1 0,01 1 10 s-1 100 lg Double-gap measuring systems are special systems designed forlow - viscosity liquids.

  18. shear rate 2 Application (flow behavior) Flow Curves Shear-thinning flow behavior 0.5 150 Wall Paper Paste aqueousmethylcellulosesolution T = +23°C Pas Pa 0.4 100 h t 0.3 50 0.2 typical behavior of polymer solutions: continuosly shear-thinning 0.1 0 0 200 400 600 s-1 1000

  19. 2 Application (flow behavior)Shear-Thinning Behavior material atrest:undershear:highviscositydecrease in viscosity suspension with needle-shaped or platelet-shapedparticles (e.g. flakes in metallic-effect automotive coatings) The particlesareThe particlesaresuspendedrandomlyorientated in(iftherearenoflowdirection.interactionforces). consequence: shear - thinningflowbehavior, decreasingviscosity

  20. 2 Application (flow behavior) Effect of rheological additives (1) Example: comparisonofflowbehaviorof a water-baseddispersionwith additive 1, a „gellant“ e.g. clay additive 2, a „viscosifier“ e.g. anassociativethickener 1 2 flowcurves on a linearscale flowcurves on a logarithmicscale 2 1  lg 1 with yield point 2 lg Summary: The gellant shows is effective especially in the low-shear range(or at rest, resp.), and the viscosifier in the high-shear range.

  21. 2 Application (flow behavior) Effect of rheological additives (2)  coating processes shear - thinning flowbehavior Summary: A single - point viscositymeasurement is not sufficient.  Brookfield   Krebs  -Stormer lg  flow cups  viscosity Coating 1Coating 2 lg shear rate low - shear rangehigh - shear range stirring,painting, rolling,spatters (?)spray coating

  22. 2 Application (flow Behavior) Effect of Rheobogical Additives (3) Different rheological additives asthickeners (example:water-basedcoatings) (1) silica (clay, inorganicgellant (2) cellulose derivative, polymer solution (3a) unmodifiíed polymer dispersion (3b) polymer dispersionwith an associativethickener(bar length: 100 nm = 0.1 µm) (1) (2) rightside:whensheared leftside:atrest (3a) (3b) For polymer dispersions: lowerviscosityeventhoughthehigher molar massofthe polymer

  23. 2 Application (flow behavior) Effect of Rheological Additives (4) Viscosityfunctionsofpigmentedwater-basedcoatings containing different rheological additives asthickeners, in principle: (1) silica (clay), inorganicgellant (2) cellulose derivative, polymer solution (3a) unmodifiíed polymer dispersion (3b) polymer dispersionwith an associativethickener

  24. 3 Behavior after application • 3 Behaviorafter theapplication • - levelling, gloss, de-aeration • - sagging, wetlayerthickness, edge cover • structurerecovery, time-dependent „thixotropicbehavior“ • Test method: steptest, low – high – lowshear (rotationoroscillation)

  25. 3 Behavior after application LevellingandSagging Application examples: - brush coatings- spray coatings Requirements:- Levelling without brush marks or other flow defects - controlled sagging - desired layer thickness

  26. 3 Behavior after application LevellingandSagging Levelling, Brush Marks, Wet-layer Thickness, SaggingExample: Brush Paints at very low shear rates between 0.01 and 1 s-1(or at rest, respectively)

  27. 3 Behavior after application LevellingandSagging AutomotiveCoating: High-rotational atomizer(bell), electrostatically supported spray process Example for surface treatment of cars: 1 car body mould metal sheet 2 kathodic dipping process, anti-corrosion protection 3 functional layer 4 water-base coat 5 clear coat spray coating problem: sag control Quelle: Fotos vom Daimler-Museum, Stuttgart

  28. 3 Behavior after application PrintingProcess Application examples: - printing inks Requirements: - area printing: without levelling problems - halftone printing: dot sharpness

  29. 3 Behavior after application Shear Rate Range

  30. 3 After Coating Step Tests (Rotation): Structure Recovery a) rotation (3 intervals) Preset: three steps low / high / low shear rate Result: time - dependent viscosity

  31. 3 After Coating Step Tests (Rotation): Structure Recovery ComparisonoftwoFormulationsofCoatings :Step Test with 3 Intervals 100 = 0.1 s-1 = 0.1 s-1 Structure recoveryis fasterwith the „gellant“ - less sagging, - high wet-layer thickness, - but maybe poor leveling Pas 10 lgh structure recovery Structure recoveryis slower with the „thickener“ - good leveling, - but maybe too much sagging 1 = 100 s-1 0.1 0 100 200 300 400 500 600 700 s time t

  32. 3 After CoatingStep Tests (Oscillation): StructureRecovery b) oscillation (3 intervals) Preset: three steps low / high / low strain amplitude Result: the two time-dependent functions of G'' (viscous) and G' (elastic behavior)

  33. 3 After CoatingStep Tests (O-R-O): StructureRecovery Steptestwith 3 intervals, asoscillation / rotation / oscillation(measuring „thixotropicbehavior“) preset: 1 low-shearconditions(strain in the LVE-range, oscillation) 2 high-shearconditions (rotation) 3 low-shearconditions(strain in the LVE-range, oscillation) measuringresult: 1 stateofrest 2 structuredecomposition 3 structureregeneration 2nd test interval:liquid, at high shear rates 1st & 3rd test interval:G‘ > G‘‘ („gel-like structure“ at rest)

  34. crossover G‘ = G‘‘ 10 Pa 1 lg G' 0.1 lg G'' 0.01 g= 0.2% g= 0.2% = 15,000 s-1 time t 100 200 300 500 600 s 3 After CoatingStep Tests (O-R-O): StructureRecovery Comparison: 2 Spray Coatings, Step Tests in Oscillation / Rotation / Oscillation • Structure recovery • liquid, • as long as G‘‘ > G‘ • for leveling • 2) „gel - like“, • when G‘ > G‘‘ • sagging is stopped • Analysis: • Time point of • crossover • G‘ = G‘‘ • can be optimized • by rheological • additives. Spritzlack 3(mit Additiv B) G' G'' Spritzlack 2(mit Additiv A) G' G'' Spritzlack 1(ohne Additiv) G' G''

  35. 3 After CoatingStep Tests: StructureRecovery a) rotation (3 intervals) result: time-dependent viscosity (here, the viscous behavior is measured only !) b) oscillation (3 intervals) result: two time-dependent functions G'' (viscous) and G' (elastic) here, the wholeviscoelastic behavior is measured.

  36. 4 Storage Stability • 4 Long-term storagestability • - settling(sedimentation), flotation • syneresis („blooding“), demixing • appearance after a time ofrest („consistency“) • transportstability • gelationeffects, fluidisation •  Test method:frequencysweep (oscillation), lowfrequencies

  37. 4 Storage Stability Sedimentation Application examples: - emusion paints - coatings with metallic - effect Requirements: - no demixing- no sedimentation - no syneresis

  38. 4 Storage Stability Shear Rate Range

  39. 4 Storage Stability Simple Method: Yield Point Controlled stress rotationaltests: Flow Curves on a linearscale Yield Point as a limitingvalueoftheshear stress  2 Break of the structure - at - rest. Super - structure by a chemical - physical network via interactive forces. 1 ty  1 without a yield point 2 having a yield point y

  40. Storage Stability • FrequencySweep: Long-termBehavior Preset: constantamplitude, shearstrainorshear stress (withinthe LVE - range)andvariable frequency Precondition: First of all, the LVE - rangehastobecheckedby an amplitudesweep.

  41. 10 Pa 0.1 0.001 -3 -2 -1 0 1 2 10 10 10 10 10 rad/s 10 4 Storage Stability FrequencySweep: Long-term Behavior ComparisonoftwoCoatings: Dispersion Stability G' > G'' 1 lg G' Long - term storage stability: Evaluation at a low frequencyG' > G'' hence „gel - like“,stable dispersion (Top Coat). G'' > G' hence „liquid - like“, unstable dispersion (Primer). lg G'' G'' > G' 0.01 g= 1 %T = +23°C angular frequencylg

  42. 5 CuringCoatings 5 Curing (powdercoatings, UV – coatings) - time - dependentandtemperature - dependentmeltingandcuring

  43. 5 CuringCoatings Examples Application examples: - powder coatings- UV – curing coatings Foto: AlzChem • Requirements: • melting • netting of the subtrate • good levelling Foto: DuPont Performance Coatings Foto: BASF Coatings

  44. 5 CuringCoatings Rotational Tests gel formation and curing preset: constant shear conditions (shear rate or shear stress)result: viscosity / temperature curve showing a viscosity minimum

  45. 5 CuringCoatings Oscillatory Tests gelformation, hardeningorcuringprocess preset: constantshearconditions (amplitudeandfrequency) results: temperature-dependent G' and G'' curves Tm... meltingtemperature(when G' = G'') TCR... temperatureattheonsetofthehardeningprocess, gelformation, curingorchemicalreaction TSG... sol /geltransition (whenagain G' = G'')

  46. 6 200 10 °C 180 Pa 160 5 10 140 Powder Coat 1 G' 120 G'' G' 4 100 T 10 T G'' Powder Coat 2 80 G' 60 G'' 3 10 T 40 20 2 0 10 0 100 200 300 400 500 600 700 800 s 1,000 time t 5 CuringCoatings Oscillatory Tests Comparison of two Powder Coatings g = 0.1 % ω = 10 rad/s preset: T = T(t)

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