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Www.anton-paar.com RHEOLOGY of Coatings. 2 Overview 1Simple Test Methods, and Rheometry 2Flow behavior during the Application 3Behavior after the Application.

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Presentation on theme: "Www.anton-paar.com RHEOLOGY of Coatings. 2 Overview 1Simple Test Methods, and Rheometry 2Flow behavior during the Application 3Behavior after the Application."— Presentation transcript:

1 RHEOLOGY of Coatings

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

3 3 1 Simple Test Methods trowel test - high-viscosity fluids: “thick“ - low-viscosity fluids: “thin“ e.g. for dispersions finger test - tacky: “long“ - less tacky: “short“ e.g. for paints, offset-printing inks, pigment pastes

4 4 Flow Cups measurement of the flow time of low-viscosity liquids to determine the kinematic viscosity (weight-dependent viscosity !) Examples: oils, solvent-based coatings, gravure and flexo printing inks 1 Simple Test Methods

5 5 Falling - rod Viscometers determination of the time of the rod to travel downwards over a defined distance e.g. for testing offset-printing inks (highly viscous) and pastes weight printing ink falling rod falling-rod viscometer, e.g. type Laray 1 Simple Test Methods

6 6 Using the typical spindles relative viscosity values are measured - cylinders - disks - pins - T-bars preset: rotational speed measurement: torque Rotational Viscometers for testing „Low - shear Viscosity“ (LSV) (which is in fact not really low-shear) 1 Simple Test Methods

7 7 Rotational Viscometers for testing „Medium – shear Viscosity“ (MSV) originally preset: force (constant torque), using a freely falling weight (in grams), measurement: rotational speed of the rotational measuring system nowadays: preset of the speed, measurement of the torque Krebs spindles stirrer-like „paddles" relative viscosity values are measured here ; typically given in Krebs Units, KU 1 Simple Test Methods

8 8 Cone & Plate Viscometers for testing „High - shear Viscosity“ (HSV) preset:rotational speed measurement: torque Problem: Friction between cone and plate, since the tip of the cone is not truncated, sitting directly on the bottom plate. Consequence: Friction influences the measuring results 1 Simple Test Methods

9 9 all these kinds of stirrers are relative measuring systems helix 1helix 2 stirrer for building materials starch stirrer bladeanchorball measuring system 1 Simple Test Methods

10 10 Concentric Cylinders, CC for low-viscosity liquids, solvent-borne coatings Cone & Plate, CP for liquids; for dispersions only with a limitted particle size (usually < 10 µm) Parallel - Plates, PP useful for dispersions containing coarse particles, pastes, offset-printing inks, gel-like materials, polymer melts Measuring Geometries for rotational and oscillatory rheometer according to DIN and ISO Rheometry Measuring Systems for Absolute Values

11 11 2 Application (flow behavior) Flow behavior during the application - Application behavior in the flowing state when stirring, painting, brushing, rolling, spraying when pumping, dosing, blading, flatstream application, dip coating, pouring, using roboters or high-rotational disks or bells  Test method: Flow curves, at medium and high shear rates (rotation) Requirements: - ability to brush - limited coating force - no spatters - roller resistance

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

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

14 14 2 Application (flow behavior) Spraying of Automotive Coatings Spraying of Automotive Coatings car body degreasing & phosphatizing electro dip coating seam sealing underbody spraying filler base coat and clear coat cavity conservation Spraying, Coating at high shear rates of 1000 to 10,000 s -1 a) Plastisols: seam sealing and under-body sealing b) Coatings: dip coating, filler, base coat, clear coat c) Waxes: cavity conservation

15 15 ProcessShear Rates (s -1 ) sedimentation< to 0.01 surface levelling0.01 to 0.1 sagging0.01 to 1 dip coating1 to 100 pipe flow, pumping, filling into containers1 to 10,000 coating, painting, brushing100 to 10,000 spraying1000 to 10,000 (high - speed) coating, blade coating100,000 to 1 mio. 2 Application (flow behavior) Shear Rate Range

16 16 2 Application (flow behavior) Overview: Flow & Viscosity Curves flow curves viscosity curves yield point 1 ideally viscous (Newtonian) 4 without a yield point 2 shear-thinning (pseudoplastic) 5 having a yield point 3 shear-thickening (dilatant) 

17 17

18 18 0, mPas lg  0,01 0, mPa lg  s -1 DG 42 (double - gap) T = +20°C Water lg constant viscosity, ideally viscous flow behavior Double-gap measuring systems are special systems designed for low - viscosity liquids. 2 Application (flow behavior) Flow Curves

19 19      Pas      Pa   s -1 Wall Paper Paste aqueous methylcellulose solution T = +23°C shear rate   typical behavior of polymer solutions: continuosly shear-thinning 2 Application (flow behavior) Flow Curves Shear-thinning flow behavior

20 20 material at rest: under shear: high viscosity decrease in viscosity suspension with needle-shaped or platelet-shaped particles (e.g. flakes in metallic-effect automotive coatings) The particles are The particles are suspended randomly orientated in (if there are no flow direction. interaction forces). consequence: shear - thinning flow behavior, decreasing viscosity 2 Application (flow behavior) Shear-Thinning Behavior

21  lg  Application (flow behavior) Effect of rheological additives (1) lglg Example: comparison of flow behavior of a water-based dispersion with additive 1, a „gellant“ e.g. clay additive 2, a „viscosifier“ e.g. an associative thickener flow curves on a linear scaleflow curves on a logarithmic scale with yield point Summary: The gellant shows is effective especially in the low-shear range (or at rest, resp.), and the viscosifier in the high-shear range.

22 22 shear rate lg   Brookfield   Krebs  -Stormer  flow cups  low - shear range high - shear range stirring, painting, rolling, spatters (?) spray coating shear - thinning flow behavior Summary: A single - point viscosity measurement is not sufficient.  coating processes  Coating 1 Coating 2 viscosity lg 2 Application (flow behavior) Effect of rheological additives (2)

23 23 Different rheological additives as thickeners (example: water-based coatings) (1) silica (clay, inorganic gellant (2) cellulose derivative, polymer solution (3a) unmodifiíed polymer dispersion (3b) polymer dispersion with an associative thickener (bar length: 100 nm = 0.1 µm) left side: at rest right side: when sheared For polymer dispersions: lower viscosity even though the higher molar mass of the polymer (1) (2) (3b) (3a) 2 Application (flow Behavior) Effect of Rheobogical Additives (3)

24 24 Viscosity functions of pigmented water-based coatings containing different rheological additives as thickeners, in principle: (1) silica (clay), inorganic gellant (2) cellulose derivative, polymer solution (3a) unmodifiíed polymer dispersion (3b) polymer dispersion with an associative thickener 2 Application (flow behavior) Effect of Rheological Additives (4)

25 25 3 Behavior after application 3Behavior after the application - levelling, gloss, de-aeration - sagging, wet layer thickness, edge cover -structure recovery, time-dependent „thixotropic behavior“  Test method: step test, low – high – low shear (rotation or oscillation)

26 26 Application examples: - brush coatings - spray coatings Requirements: - Levelling without brush marks or other flow defects - controlled sagging - desired layer thickness 3 Behavior after application Levelling and Sagging

27 27 at very low shear rates between 0.01 and 1 s -1 (or at rest, respectively) Levelling, Brush Marks, Wet-layer Thickness, Sagging Example: Brush Paints 3 Behavior after application Levelling and Sagging

28 28 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  Quelle: Fotos vom Daimler-Museum, Stuttgart  Automotive Coating: High-rotational atomizer (bell), electrostatically supported spray process spray coating problem: sag control 3 Behavior after application Levelling and Sagging

29 29 Application examples: - printing inks Requirements: - area printing: without levelling problems - halftone printing: dot sharpness 3 Behavior after application Printing Process

30 30 ProcessShear Rates (s -1 ) sedimentation< to 0.01 surface levelling0.01 to 0.1 sagging0.01 to 1 dip coating1 to 100 pipe flow, pumping, filling into containers1 to 10,000 coating, painting, brushing100 to 10,000 spraying1000 to 10,000 (high - speed) coating, blade coating100,000 to 1 mio. 3 Behavior after application Shear Rate Range

31 31 a) rotation (3 intervals) Result: time - dependent viscosity Preset: three steps low / high / low shear rate 3 After Coating Step Tests (Rotation): Structure Recovery

32 32      Pas  s time t   structure recovery Comparison of two Formulations of Coatings : Step Test with 3 Intervals = 0.1 s -1 = 100 s -1 Structure recovery is faster with the „gellant“ - less sagging, - high wet-layer thickness, - but maybe poor leveling Structure recovery is slower with the „thickener“ - good leveling, - but maybe too much sagging lg  3 After Coating Step Tests (Rotation): Structure Recovery

33 33 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) 3 After Coating Step Tests (Oscillation): Structure Recovery

34 34 preset: 1 low-shear conditions (strain in the LVE-range, oscillation) 2 high-shear conditions (rotation) 3 low-shear conditions (strain in the LVE-range, oscillation) measuring result: 1 state of rest 2 structure decomposition 3 structure regeneration Step test with 3 intervals, as oscillation / rotation / oscillation (measuring „thixotropic behavior“) 2 nd test interval: liquid, at high shear rates 1 st & 3 rd test interval: G‘ > G‘‘ („gel-like structure“ at rest) 3 After Coating Step Tests (O-R-O): Structure Recovery

35 35             Comparison: 2 Spray Coatings, Step Tests in Oscillation / Rotation / Oscillation Structure recovery 1)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. 3 After Coating Step Tests (O-R-O): Structure Recovery

36 36 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 whole viscoelastic behavior is measured. 3 After Coating Step Tests: Structure Recovery

37 37 4 Storage Stability 4Long-term storage stability -settling (sedimentation), flotation -syneresis („blooding“), demixing -appearance after a time of rest („consistency“) -transport stability -gelation effects, fluidisation  Test method: frequency sweep (oscillation), low frequencies

38 38 Application examples:- emusion paints - coatings with metallic - effect Requirements: - no demixing - no sedimentation - no syneresis 4 Storage Stability Sedimentation

39 39 ProcessShear Rates (s -1 ) sedimentation< to 0.01 surface levelling0.01 to 0.1 sagging0.01 to 1 dip coating1 to 100 pipe flow, pumping, filling into containers1 to 10,000 coating, painting, brushing100 to 10,000 spraying1000 to 10,000 (high - speed) coating, blade coating100,000 to 1 mio. 4 Storage Stability Shear Rate Range

40 40 Controlled stress rotational tests: Flow Curves on a linear scale Yield Point as a limiting value of the shear stress 1 without a yield point 2 having a yield point  y   tyty Break of the structure - at - rest. Super - structure by a chemical - physical network via interactive forces.   4 Storage Stability Simple Method: Yield Point

41 41 Preset: constant amplitude, shear strain or shear stress (within the LVE - range) and variable frequency Precondition: First of all, the LVE - range has to be checked by an amplitude sweep. 4Storage Stability Frequency Sweep: Long-term Behavior

42 42   1 % T = +23°C Comparison of two Coatings: Dispersion Stability Long - term storage stability: Evaluation at a low frequency G' > G'' hence „gel - like“, stable dispersion (Top Coat). G'' > G' hence „liquid - like“, unstable dispersion (Primer). G '' > G ' G ' > G '' angular frequency lg  lg G' lg G''   4 Storage Stability Frequency Sweep: Long-term Behavior

43 43 5Curing (powder coatings, UV – coatings) -time - dependent and temperature - dependent melting and curing 5 Curing Coatings

44 44 Application examples: - powder coatings - UV – curing coatings Requirements: - melting - netting of the subtrate - good levelling Foto: BASF Coatings Foto: AlzChem Foto: DuPont Performance Coatings 5 Curing Coatings Examples

45 45 gel formation and curing preset: constant shear conditions (shear rate or shear stress) result: viscosity / temperature curve showing a viscosity minimum 5 Curing Coatings Rotational Tests

46 46 gel formation, hardening or curing process preset: constant shear conditions (amplitude and frequency) results: temperature-dependent G' and G'' curves T m... melting temperature (when G' = G'') T CR...temperature at the onset of the hardening process, gel formation, curing or chemical reaction T SG... sol /gel transition (when again G' = G'') 5 Curing Coatings Oscillatory Tests

47 47    0.1 % ω = 10 rad/s preset: T = T(t) Comparison of two Powder Coatings 5 Curing Coatings Oscillatory Tests


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