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)

18. 2 Application (flow behavior) Flow Curves10
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)