Transcript 1 - Rheology
RHEOLOGY of Coatings
www.anton-paar.com
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: - less tacky: e.g. for paints, offset-printing inks, pigment pastes
“long“ “short“
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
1 Simple Test Methods weight printing ink 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
5 falling rod
falling-rod viscometer, e.g. type Laray
6 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 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 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
1 Simple Test Methods helix 1 helix 2 blade anchor ball measuring system 9 stirrer for building materials starch stirrer
all these kinds of stirrers are
relative measuring systems
1 Rheometry Measuring Systems for Absolute Values Measuring Geometries for rotational and oscillatory rheometer
according to DIN 53019 and ISO 3219
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
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
2 Application (flow behavior) Coating, Painting, Brushing Application Example
brush velocity (v = 0.5 m/s)
12
wet layer thickness (h = 200 µm)
calculation of the shear rate:
Δv Δh 0.5
2 10 4 m m s 2500 s 1
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 13
Quelle: Fotos vom Daimler-Museum, Stuttgart
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 underbody spraying
14
filler base coat and clear coat cavity conservation
15 2 Application (flow behavior) Shear Rate Range
Process
sedimentation surface levelling sagging
dip coating pipe flow, pumping, filling into containers coating, painting, brushing spraying
(high - speed) coating, blade coating
Shear Rates (s -1 )
< 0.001 to 0.01
0.01 to 0.1
0.01 to 1
1 to 100 1 to 10,000 100 to 10,000 1000 to 10,000
100,000 to 1 mio.
2 Application (flow behavior) Overview: Flow & Viscosity Curves flow curves
viscosity curves
yield point
1 ideally viscous 2 shear-thinning 3 shear-thickening (Newtonian) (pseudoplastic) (dilatant) 4 without a yield point 5 having a yield point
16
17
2 Application (flow behavior) Flow Curves 18
10 1000
Water
mPa mPas lg 1 0,1 10 lg 1
constant viscosity, ideally viscous flow behavior
0,1 DG 42 (double - gap) T = +20 °C 0,01 1 10 s -1 100 lg
Double-gap measuring systems
are special systems designed
for low - viscosity liquids
.
2 Application (flow behavior) Flow Curves 19 Shear-thinning flow behavior
0.5
Pas 0.4
0.3
0.2
0.1
0 200 400 shear rate 600 s -1 1000 0 150 Pa 100 50
Wall Paper Paste
aqueous methylcellulose solution T = +23 °C typical behavior of polymer solutions: continuosly shear-thinning
20 2 Application (flow behavior) Shear-Thinning Behavior material
at rest: high viscosity
suspension
with needle-shaped or platelet-shaped particles (e.g. flakes in metallic-effect automotive coatings) under shear: decrease in viscosity 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
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 flow curves on a
logarithmic
scale 300
Pa
240 220 200 180 160 140 120 100 80 60 40 20 0 0 1,000
Pa
2 100 50
Pa
45 1 40 35 10 1 30 25 20 15 lg 1 with yield point 10 5 2 0 0 5 ·10 0
1/s
10 1 0.1
100 200 300 400
Shear Rate
.
500 600 700 800
1/s
1,000 0.1
1 l
Shear Rate
.
10 100
1/s
Gellant (Clay) Viscosifier (PUR) Summary: The
gellant
g Viscosifier (PUR) shows is effective especially in the
low-shear range
(or at rest, resp.), and the
viscosifier
in the
high-shear range
. 1,000
21
2 Application (flow behavior) Effect of rheological additives (2) 22
1,000
Pa·s
100 10 viscosity 1 0.1
0.1
Coating 1 Coating 2
1
coating processes
Brookfield Krebs -Stormer flow cups
shear - thinning flow behavior
Summary: A single - point viscosity measurement is not sufficient.
shear rate
.
10 100 lg
1/s
1,000 Viscosifier (PUR)
high - shear range stirring, painting, rolling,
spatters (?)
spray coating
2 Application (flow Behavior) Effect of Rheobogical Additives (3)
left side: at rest
(1) (2)
right side: when sheared
(3a) (3b)
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)
23
For polymer dispersions: lower viscosity even though the higher molar mass of the polymer
2 Application (flow behavior) Effect of Rheological Additives (4) lg
Viscosity functions of pigmented
water-based coatings
containing different rheological additives as thickeners, in principle:
1 2 (1) silica (clay), inorganic gellant 3b (2) cellulose derivative, polymer solution 3a
low-shear
lg
high-shear 0.01 0.1 1 10 100 1000 10,000 s -1
(3a) unmodifiíed polymer dispersion (3b) polymer dispersion with an associative thickener 24
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
3 Behavior after application Levelling and Sagging Levelling, Brush Marks, Wet-layer Thickness, Sagging
Example:
Brush Paints 27
at very low shear rates between 0.01 and 1 s -1 (or at rest, respectively)
3 Behavior after application Levelling and Sagging Automotive Coating: High-rotational atomizer (bell), electrostatically supported spray process
spray coating problem: sag control
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
28
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
sedimentation
surface levelling sagging
dip coating pipe flow, pumping, filling into containers coating, painting, brushing spraying (high - speed) coating, blade coating
Shear Rates (s -1 )
< 0.001 to 0.01
0.01 to 0.1
0.01 to 1
1 to 100 1 to 10,000 100 to 10,000 1000 to 10,000 100,000 to 1 mio.
3 After Coating Step Tests (Rotation): Structure Recovery a) rotation (3 intervals) 31
Preset: three steps low / high / low shear rate Result: time - dependent viscosity
32 3 After Coating Step Tests (Rotation): Structure Recovery
lg 100
Comparison of two Formulations of Coatings : Step Test with 3 Intervals
= 0.1 s
-1
= 0.1 s
-1
Pas
Structure recovery is faster with the „gellant“
10 1
structure recovery
- less sagging, - high wet-layer thickness, - but maybe poor leveling
Structure recovery is slower with the „thickener“
= 100 s
-1
- good leveling, 0.1
0 100 200 300 400 500 s 600 700 sagging time
t
3 After Coating Step Tests (Oscillation): Structure Recovery b) oscillation (3 intervals)
Preset: three steps low / high / low strain amplitude
33
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
2 nd test interval: liquid, at high shear rates 1 st & 3 rd 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
10 Pa 1 crossover G‘ = G‘‘
Structure recovery 1)
Sprizlack 3 (mi Addiiv B) for leveling G'
lg G' lg G''
0.1
0.01
= 0.2% = 15,000 s
-1
100 200 300 time
t
= 0.2% 500 600
s 2) „gel - like“, when G‘ > G‘‘
(mi Addiiv A)
Analysis:
G'' Sprizlack 1 (one Addiiv) by rheological
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
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
sedimentation
surface levelling sagging dip coating pipe flow, pumping, filling into containers coating, painting, brushing spraying (high - speed) coating, blade coating
Shear Rates (s -1 )
< 0.001 to 0.01
0.01 to 0.1
0.01 to 1 1 to 100 1 to 10,000 100 to 10,000 1000 to 10,000 100,000 to 1 mio.
4 Storage Stability Simple Method: Yield Point 40 Yield Point as a
limiting value of the shear stress
Break of the structure - at - rest.
Super - structure by a chemical - physical network via interactive forces.
Controlled stress rotational tests: Flow Curves on a
linear
scale
2 1
t y
1
without
2
having
a yield point a yield point
y
4 Storage Stability Frequency Sweep: Long-term Behavior 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.
42 4 Storage Stability Frequency Sweep: Long-term Behavior Comparison of two Coatings: Dispersion Stability
10 Pa 1
G ' > G ''
lg G' lg G'' 0.01
0.1
G '' > G '
0.001
10 3 10 2
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).
10 1 10 0 angular frequency lg 10 1 rad/s 10 2 1 % T = +23 °C
43 5 Curing Coatings
-
5 Curing (powder coatings, UV – coatings)
time - dependent and temperature - dependent melting and curing
5 Curing Coatings Examples
Foto: AlzChem
44
Foto: DuPont Performance Coatings Foto: BASF Coatings Application examples:
- powder coatings - UV – curing coatings
Requirements:
melting netting of the subtrate good levelling
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
5 Curing Coatings Oscillatory Tests
gel formation, hardening or curing process
preset
: constant shear conditions (amplitude
and
frequency)
46 results
: temperature-dependent G' and G'' curves
T m T CR T SG
... ...
melting temperature (when G' = G'')
... temperature at the onset of the hardening process, gel formation, curing or chemical reaction
sol /gel transition (when again G' = G'')
47 5 Curing Coatings Oscillatory Tests
10 6
Pa
10 5
G'
10 4
G''
10 3 10 2 0 Comparison of two Powder Coatings 100 200 300 400
time t
500 600 700 800 200
°C
180 160 140 120 100
T
80 60 40 20
s
0 1,000
Powder Coat 1
G' G'' T
Powder Coat 2
G' G'' T 0.1 % ω = 10 rad/s preset: T = T(t)