1 - Rheology

Report
RHEOLOGY of Coatings
www.anton-paar.com
Overview
2
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
trowel test
- high-viscosity fluids: “thick“
- low-viscosity fluids: “thin“
e.g. for dispersions
finger test
- tacky:
- less tacky:
e.g. for paints,
offset-printing inks,
pigment pastes
3
“long“
“short“
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
4
1 Simple Test Methods
weight
Falling - rod Viscometers
printing ink
determination of the time
of the rod
to travel downwards
over a defined distance
falling rod
falling-rod viscometer,
e.g. type Laray
5
e.g. for testing
offset-printing inks (highly viscous)
and pastes
1 Simple Test Methods
Rotational Viscometers
for testing
„Low - shear Viscosity“ (LSV)
preset:
rotational speed
measurement: torque
(which is in fact not really low-shear)
Using the typical spindles
relative
viscosity values
are measured
- cylinders
- disks
- pins
- T-bars
6
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
7
1 Simple Test Methods
Cone & Plate Viscometers
for testing
„High - shear Viscosity“ (HSV)
preset:
measurement:
rotational speed
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
8
1 Simple Test Methods
helix 1
helix 2
blade
anchor
ball measuring
system
all these kinds of stirrers are
stirrer for
building materials
9
starch stirrer
relative
measuring systems
1 Rheometry
Measuring Systems for Absolute Values
Measuring Geometries for rotational and oscillatory rheometer
according to DIN 53019 and ISO 3219
Concentric Cylinders, CC
for low-viscosity liquids,
solvent-borne coatings
10
Cone & Plate, CP
Parallel - Plates, PP
for liquids;
for dispersions only with
a limitted particle size
(usually < 10 µm)
useful for
dispersions containing
coarse particles,
pastes,
offset-printing inks,
gel-like materials,
polymer melts
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
11
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:

12

Δv
0.5 m

 2500 s1
4
Δh 2  10 m  s
Brushing, Painting
at medium and
high shear rates
between 100 and 10,000 s-1
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
13
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
Spraying, Coating
at high shear rates
of 1000 to 10,000 s-1
seam sealing
filler
14
underbody spraying
base coat and clear coat
cavity conservation
2 Application (flow behavior)
Shear Rate Range
15
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)
Overview: Flow & Viscosity Curves
flow curves



1
2
3
16
yield point
viscosity curves
ideally viscous
shear-thinning
shear-thickening



(Newtonian)
(pseudoplastic)
(dilatant)
4
5
without a yield point
having a yield point
17
2 Application (flow behavior)
Flow Curves
1000
10
Water
mPa
mPas
10
lg 
1
lg 
1
0,1
constant viscosity,
ideally viscous
flow behavior
0,01

lg 
Double-gap measuring systems are special systems designed
for low - viscosity liquids.
1
18
0,1
DG 42
(double - gap)
T = +20°C
10
s-1
100
2 Application (flow behavior)
Flow Curves
Shear-thinning
flow behavior
0.5
150
Pas
Pa
0.4

100
0.3
aqueous methylcellulose
solution

T = +23°C
50
0.2
0.1
0
0
200
400
shear rate
19
Wall Paper Paste
600

s-1
1000
typical behavior
of polymer solutions:
continuosly shear-thinning
2 Application (flow behavior)
Shear-Thinning Behavior
material
at rest:
high viscosity
under shear:
decrease in viscosity
suspension
with
needle-shaped
or
platelet-shaped
particles
(e.g. flakes in
metallic-effect
automotive
coatings)
The particles are
suspended randomly
(if there are no
interaction forces).
The particles are
orientated in
flow direction.
consequence:
shear - thinning flow behavior,
decreasing viscosity
20
2 Application (flow behavior)
Effect of rheological additives (1)
1
2
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 scale
300
flow curves on a logarithmic scale
1,000
Pa
Pa
2
240
220
200
180
160
140
 120
100
80
60
40
20
0
100
1
50
Pa
45
40
35

10
30

lg
25

1
20

15
with yield point
10
1
2
5
0
0
0
5·10 1/s 10
1
0.1
0
100
200
300
400
500
 
Shear Rate
600
700
800
.
Gellant (Clay)
1/s
1,000
0.1
1
 Rate .
l Shear
10
100
1/s

Viscosifier (PUR)
g
Gellant (Clay)
Viscosifier (PUR)
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
1,000
2 Application (flow behavior)
Effect of rheological additives (2)
1,000


coating processes
Pa·s
100
 Brookfield 
 Krebs 
-Stormer
10
lg 
shear - thinning
flow behavior
 flow
cups 
viscosity
1
Coating 1
Coating 2
0.1
0.1
1
10
.
shearRate
rate
Shear
Gellant (Clay)
low - shear range
22
100
lg
1/s
Summary:
A single - point
viscosity
measurement
is not sufficient.
1,000

Viscosifier (PUR)
high - shear range
stirring, painting, rolling, spatters (?)
spray coating
2 Application (flow Behavior)
Effect of Rheobogical Additives (3)
(1)
Different rheological
additives as thickeners
(example:
water-based coatings)
(1) silica (clay,
inorganic gellant
(2)
left side:
at rest
right side:
when sheared
(3a)
(2) cellulose derivative,
polymer solution
(3a) unmodifiíed
polymer dispersion
(3b) polymer dispersion
with an associative
thickener
(3b)
(bar length: 100 nm = 0.1 µm)
For polymer dispersions: lower viscosity even though the higher molar mass of the polymer
23
2 Application (flow behavior)
Effect of Rheological Additives (4)
Viscosity functions of
pigmented
water-based coatings
lg

containing different
rheological additives
as thickeners, in principle:
1
(1) silica (clay),
inorganic gellant
2
3b
(2) cellulose derivative,
polymer solution
3a
low-shear
0.01
24
0.1
lg
1
10

high-shear
100
1000
10,000 s
-1
(3a) unmodifiíed
polymer dispersion
(3b) polymer dispersion
with an associative
thickener
3 Behavior after application
25
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)
3 Behavior after application
Levelling and Sagging
Application examples:
- brush coatings
- spray coatings
26
Requirements:
- Levelling without brush marks
or other flow defects
- controlled sagging
- desired layer thickness
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)
27
3 Behavior after application
Levelling and Sagging
Automotive
Coating:
High-rotational
atomizer
(bell),
electrostatically
supported
spray process
spray coating
problem:
sag control
Quelle: Fotos vom Daimler-Museum, Stuttgart
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
3 Behavior after application
Printing Process
Application examples:
- printing inks
Requirements:
- area printing:
without levelling problems
- halftone printing: dot sharpness
29
3 Behavior after application
Shear Rate Range
30
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 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
100
Comparison of two Formulations of Coatings :
Step Test with 3 Intervals

 = 0.1 s-1
= 0.1 s-1
Structure recovery
is faster
with the „gellant“
Pas
10
lg 
- less sagging,
- high wet-layer thickness,
- but maybe poor leveling

structure recovery
1
 = 100 s-1
0.1
0
100
200
time
32
300
t
400
500
s
600
Structure recovery
is slower
with the „thickener“
- good leveling,
- but maybe too much
700
sagging
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)
33
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)
34
3 After Coating
Step Tests (O-R-O): Structure Recovery
Comparison: 2 Spray Coatings, Step Tests in Oscillation / Rotation / Oscillation
 crossover G‘ = G‘‘
10
Structure recovery
3
1) Sprizlack
liquid,
B)G‘
as (mi
longAddiiv
as G‘‘ >
for leveling G'
Pa
1
G''
2) „gel - like“,
when
G‘ > G‘‘ 2
Sprizlack
sagging
is stopped
(mi Addiiv
A)
lg G'
0.1
G'
Analysis:
Time point
G'' of
crossover
Sprizlack
G‘ = G‘‘ 1
(one
can
be Addiiv)
optimized
G'
by rheological
additives.
lg G''
0.01
 = 0.2%
 = 0.2%
 = 15,000 s-1
100
35
200
300
time t
500
600
G''
s
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.
36
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
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
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
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.

40
1
ty

1 without a yield point
2 having a yield point y
4 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.
41
4 Storage Stability
Frequency Sweep: Long-term Behavior
Comparison of two Coatings: Dispersion Stability
10
G' > G''
Pa
1
lg G'
0.1
Long - term storage stability:
Evaluation at a low frequency
lg G''
G'' > G'
0.01
G' > G'' hence „gel - like“,
stable dispersion (Top Coat).
0.001
G'' > G' hence „liquid - like“,
unstable dispersion (Primer).
10
3
10
2
10
1
10
0
angular frequency lg 
42
10
1
rad/s 10
1%
T = +23°C
2
5 Curing Coatings
43
5
Curing (powder coatings, UV – coatings)
-
time - dependent and temperature - dependent melting and
curing
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
44
Foto: BASF Coatings
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
45
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,
TSG
46
gel formation, curing or chemical reaction
... sol /gel transition (when again G' = G'')
5 Curing Coatings
Oscillatory Tests
10
Comparison of two Powder Coatings
6
200
°C
180
Pa
10
160
5
140
Powder Coat 1
G'
120
G''
G'
10
4
100
G''
80
T
T
Powder Coat 2
G'
10
60
3
T
40
20
10
47
G''
2
0
100
200
300 400 500 600
time t
700
800
s
0
1,000

 0.1 %
ω = 10 rad/s
preset: T = T(t)

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