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www.anton-paar.com
RHEOLOGY of Coatings
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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
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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
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Flow Cups
measurement of theflow timeof 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
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Falling - rod Viscometers
determination of the timeof the rodto travel downwardsover 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
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Using the typical spindles
relative viscosity valuesare measured
- cylinders- disks- pins- T-bars
preset: rotational speedmeasurement: torque
Rotational Viscometersfor testing
„Low - shear Viscosity“ (LSV)(which is in fact not really low-shear)
1 Simple Test Methods
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Rotational Viscometers for testing
„Medium – shear Viscosity“ (MSV)
originallypreset: force (constant torque),using a freely falling weight (in grams),measurement: rotational speedof the rotational measuring system
nowadays: preset of the speed,measurement of the torque
Krebs spindles stirrer-like „paddles"
relative viscosity valuesare measured here; typically given in
Krebs Units, KU
1 Simple Test Methods
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Cone & Plate Viscometersfor testing
„High - shear Viscosity“ (HSV)
preset: rotational speedmeasurement: 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
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all these kinds of stirrers are relative
measuring systems
helix 1 helix 2
stirrer forbuilding materials
starch stirrer
blade anchor ball measuring system
1 Simple Test Methods
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Concentric Cylinders, CCfor low-viscosity liquids,solvent-borne coatings
Cone & Plate, CPfor liquids;
for dispersions only witha limitted particle size
(usually < 10 µm)
Parallel - Plates, PPuseful for
dispersions containing coarse particles,
pastes, offset-printing inks,gel-like materials,
polymer melts
Measuring Geometries for rotational and oscillatory rheometeraccording to DIN 53019 and ISO 3219
1 Rheometry Measuring Systems for Absolute Values
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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
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wet layerthickness (h = 200 µm)
brush velocity(v = 0.5 m/s)
calculation of the shear rate:
2 Application (flow behavior) Coating, Painting, Brushing
Brushing, Paintingat medium andhigh shear ratesbetween 100 and 10,000 s-1
14
s 2500sm102
m0.5
Δh
Δv
Application Example
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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
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2 Application (flow behavior) 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 ratesof 1000 to 10,000 s-1
a) Plastisols: seam sealing and under-body sealingb) Coatings: dip coating, filler, base coat, clear coatc) Waxes: cavity conservation
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Process Shear Rates (s-1)
sedimentation < 0.001 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.
2 Application (flow behavior) Shear Rate Range
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2 Application (flow behavior) Overview: Flow & Viscosity Curves
flow curves viscosity curves yield point
1 ideally viscous (Newtonian) 4 without a yield point2 shear-thinning (pseudoplastic) 5 having a yield point3 shear-thickening (dilatant)
- --
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0,1
1
10
mPas
lg h
0,01
0,1
1
10
1000
mPa
lg t
1 10 100s-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
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0.1
0.2
0.3
0.4
0.5
Pas
h
0
50
100
150
Pa
t
0 200 400 600 1000s-1
Wall Paper Paste
aqueous methylcellulosesolution
T = +23°C
shear rate
typical behavior
of polymer solutions: continuosly shear-thinning
2 Application (flow behavior) Flow Curves
Shear-thinning flow behavior
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material at rest: under shear: high viscosity decrease in viscosity
suspension with needle-shapedor platelet-shapedparticles(e.g. flakes inmetallic-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
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020406080
100120140160180200220240
300
Pa
0 100 200 300 400 500 600 700 800 1,0001/s
Shear Rate .
Gellant (Clay) Viscosifier (PUR)
0
5
10
15
20
25
30
35
40
45
50
Pa
0 5·100
101
1/s
0.1
1
10
100
1,000
Pa
0.1 1 10 100 1,0001/s
Shear Rate .
Gellant (Clay) Viscosifier (PUR)
2
2
1
1
- lg -
2
1
2 Application (flow behavior) Effect of rheological additives (1)
lg
Example: comparison of flow behavior of a water-based dispersion withadditive 1, a „gellant“ e.g. clayadditive 2, a „viscosifier“ e.g. an associative thickener
flow curves on a linear scale flow 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.
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0.1
1
10
100
1,000
Pa·s
0.1 1 10 100 1,0001/s
Shear Rate .
Gellant (Clay) Viscosifier (PUR)
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)
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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)
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3a
3b
2
1
lg
0.01 0.1 1 10 100 1000 10,000 s-1
lg
low-shear high-shear
Viscosity functions of pigmented
water-based coatingscontaining 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)
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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)
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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
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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
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Example for surface treatment of cars:1 car body mould metal sheet2 kathodic dipping process, anti-corrosion protection3 functional layer4 water-base coat5 clear coat
Quelle: Fotos vom Daimler-Museum, Stuttgart
Automotive
Coating:High-rotational
atomizer (bell),
electrostaticallysupported
spray process
spray coatingproblem:
sag control
3 Behavior after application Levelling and Sagging
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Application examples:
- printing inks
Requirements:- area printing: without levelling problems- halftone printing: dot sharpness
3 Behavior after application Printing Process
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Process Shear Rates (s-1)
sedimentation < 0.001 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.
3 Behavior after application Shear Rate Range
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a) rotation (3 intervals)
Result:time - dependent viscosity
Preset:three steps low / high / low shear rate
3 After Coating Step Tests (Rotation): Structure Recovery
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0.1
1
10
100
Pas
0 100 200 300 400 500 600 700s
time t
- structure recovery
Comparison of two Formulations of Coatings :Step Test with 3 Intervals
= 0.1 s-1 = 0.1 s-1
= 100 s-1
Structure recoveryis fasterwith the „gellant“ - less sagging,- high wet-layer thickness,- but maybe poor leveling
Structure recoveryis slower with the „thickener“ - good leveling,- but maybe too much sagging
lg h
3 After Coating Step Tests (Rotation): Structure Recovery
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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
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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 rest2 structure decomposition3 structure regeneration
Step test with 3 intervals, as oscillation / rotation / oscillation (measuring „thixotropic behavior“)
2nd test interval:liquid, at high shear rates
1st & 3rd test interval:G‘ > G‘‘ („gel-like structure“ at rest)
3 After Coating Step Tests (O-R-O): Structure Recovery
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3Spritzlack( )mit Additiv B
G'
G''
2Spritzlack( )mit Additiv A
G'
G''
1Spritzlack( )ohne Additiv
G'
G''
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%
g = 0.2%
= 15,000 s-1
Structure recovery
1) liquid, as long as G‘‘ > G‘ for leveling
2) „gel - like“, when G‘ > G‘‘ sagging is stopped
Analysis: Time point of
crossoverG‘ = G‘‘
can be optimizedby rheological
additives.
3 After Coating Step Tests (O-R-O): Structure Recovery
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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
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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
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Application examples: - emusion paints- coatings with metallic - effect
Requirements:- no demixing- no sedimentation- no syneresis
4 Storage Stability Sedimentation
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Process Shear Rates (s-1)
sedimentation < 0.001 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.
4 Storage Stability Shear Rate Range
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Controlled stress rotational tests:
Flow Curves on a
linear scale Yield Point as a limiting value of the shear stress
1 without a yield point2 having a yield point -y
2
1
ty
Break of the structure - at - rest.Super - structure by a
chemical - physical network via interactive forces.
-
-
4 Storage Stability Simple Method: Yield Point
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Preset:constant amplitude, shear strain or shear stress (within the LVE - range)andvariable frequency
Precondition:First of all, the LVE - range has to be checkedby an amplitude sweep.
4 Storage Stability Frequency Sweep: Long-term Behavior
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g = 1 %T = +23°C
0.001
0.1
10
Pa
10-3
10-2
10-1
100
101
102
rad/s
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''
1
0.01
4 Storage Stability Frequency Sweep: Long-term Behavior
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5 Curing (powder coatings, UV – coatings) - time - dependent and temperature - dependent melting and
curing
5 Curing Coatings
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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
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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
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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 reactionTSG ... sol /gel transition (when again G' = G'')
5 Curing Coatings Oscillatory Tests
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102
103
104
105
106
Pa
G'
G''
0
20
40
60
80
100
120
140
160
180
200°C
T
0 100 200 300 400 500 600 700 800 1,000stime t
Powder Coat 1G'
G''
T
Powder Coat 2G'
G''
T
g = 0.1 %
ω = 10 rad/s preset: T = T(t)
Comparison of two Powder Coatings
5 Curing Coatings
Oscillatory Tests