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RHEOLOGY of Coatings.

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Presentation on theme: "RHEOLOGY of Coatings."— Presentation transcript:

1 RHEOLOGY of Coatings

2 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

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 1 Simple Test Methods 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

5 1 Simple Test Methods Falling - rod Viscometers
weight 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 printing ink falling rod falling-rod viscometer, e.g. type Laray

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

7 Rotational Viscometers
1 Simple Test Methods 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

8 „High - shear Viscosity“ (HSV)
1 Simple Test Methods 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

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

10 1 Rheometry Measuring Systems for Absolute Values
Measuring Geometries for rotational and oscillatory rheometer according to DIN and ISO 3219 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 Concentric Cylinders, CC for low-viscosity liquids, solvent-borne coatings

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

13 2 Application (flow behavior) 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 2 Application (flow behavior) Spraying of Automotive Coatings
a) Plastisols: seam sealing and under-body sealing b) Coatings: dip coating, filler, base coat, clear coat c) Waxes: cavity conservation 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

15 2 Application (flow behavior) Shear Rate Range
Process Shear Rates (s-1) sedimentation < to 0.01 surface levelling 0.01 to 0.1 sagging 0.01 to 1 dip coating 1 to 100 pipe flow, pumping, filling into containers 1 to 10,000 coating, painting, brushing 100 to 10,000 spraying 1000 to 10,000 (high - speed) coating, blade coating 100,000 to 1 mio.

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

17

18 2 Application (flow behavior) Flow Curves
10 1000 Water mPa mPas 10 lg h 1 lg t 1 constant viscosity, ideally viscous flow behavior DG 42 (double - gap) T = +20°C 0,1 0,1 0,01 1 10 s-1 100 lg Double-gap measuring systems are special systems designed for low - viscosity liquids.

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

20 2 Application (flow behavior) Shear-Thinning Behavior
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

21 2 Application (flow behavior) Effect of rheological additives (1)
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 1 2 flow curves on a linear scale flow curves on a logarithmic scale 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.

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

23 2 Application (flow Behavior) Effect of Rheobogical Additives (3)
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) (1) (2) right side: when sheared left side: at rest (3a) (3b) For polymer dispersions: lower viscosity even though the higher molar mass of the polymer

24 2 Application (flow behavior) Effect of Rheological Additives (4)
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

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

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

28 atomizer (bell), electrostatically
3 Behavior after application Levelling and Sagging Automotive Coating: 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

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

30 3 Behavior after application Shear Rate Range
Process Shear Rates (s-1) sedimentation < to 0.01 surface levelling 0.01 to 0.1 sagging 0.01 to 1 dip coating 1 to 100 pipe flow, pumping, filling into containers 1 to 10,000 coating, painting, brushing 100 to 10,000 spraying 1000 to 10,000 (high - speed) coating, blade coating 100,000 to 1 mio.

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

32 3 After Coating Step Tests (Rotation): Structure Recovery
Comparison of two Formulations of Coatings : Step Test with 3 Intervals 100 = 0.1 s-1 = 0.1 s-1 Structure recovery is faster with the „gellant“ - less sagging, - high wet-layer thickness, - but maybe poor leveling Pas 10 lg h  structure recovery Structure recovery is slower with the „thickener“ - good leveling, - but maybe too much sagging 1 = 100 s-1 0.1 100 200 300 400 500 600 700 s time t

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

34 3 After Coating Step Tests (O-R-O): Structure Recovery
Step test with 3 intervals, as oscillation / rotation / oscillation (measuring „thixotropic behavior“) 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 2nd test interval: liquid, at high shear rates 1st & 3rd test interval: G‘ > G‘‘ („gel-like structure“ at rest)

35 3 After Coating Step Tests (O-R-O): Structure Recovery
Comparison: 2 Spray Coatings, Step Tests in Oscillation / Rotation / Oscillation time t 0.01 0.1 1 10 Pa lg G' lg G'' 100 200 300 500 600 s  crossover G‘ = G‘‘ g = 0.2% = 15,000 s-1 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''

36 3 After Coating Step Tests: Structure Recovery
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.

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

39 4 Storage Stability Shear Rate Range Process Shear Rates (s-1)
sedimentation < to 0.01 surface levelling 0.01 to 0.1 sagging 0.01 to 1 dip coating 1 to 100 pipe flow, pumping, filling into containers 1 to 10,000 coating, painting, brushing 100 to 10,000 spraying 1000 to 10,000 (high - speed) coating, blade coating 100,000 to 1 mio.

40 Controlled stress rotational tests:
4 Storage Stability Simple Method: Yield Point Controlled stress rotational tests: Flow Curves on a linear scale Yield Point as a limiting value of the shear 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

41 Storage Stability Frequency Sweep: Long-term Behavior 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.

42 Frequency Sweep: Long-term Behavior
4 Storage Stability Frequency Sweep: Long-term Behavior Comparison of two Coatings: Dispersion Stability 0.001 0.1 10 Pa -3 -2 -1 1 2 rad/s G' > G'' 1 lg G' lg G'' 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' 0.01 g = 1 % T = +23°C angular frequency lg 

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

44 5 Curing Coatings 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

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

46 gel formation, hardening or curing process
5 Curing Coatings Oscillatory Tests gel formation, hardening or curing process preset: constant shear conditions (amplitude and frequency) results: temperature-dependent G' and G'' curves Tm melting temperature (when G' = G'') TCR temperature at the onset of the hardening process, gel formation, curing or chemical reaction TSG sol /gel transition (when again G' = G'')

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


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