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© 2017 Malvern Instruments Limited
Optimizing rheology for paint and coating applications
Torsten Remmler, Malvern Instruments GmbH
Zwick testXpo 2017
© 2017 Malvern Instruments Limited
Outline
› Typical Process Conditions for Paints and Coatings
› How to measure the Viscosity?
› Impact of Particle Properties: Size, Volume, Polydispersity
› Storage Stability
› Summary
© 2017 Malvern Instruments Limited
Shear Flow Properties: Overview of Basic Terms
› Temperature› Pressure› Shear Rate (Speed)› Shear Stress (Force)› Time
Shear Force F
h
Displacement u
a b
AFdtd
tan
.
Shear stress [Pa=N/m2]
Shear rate [1/s] p, t, ) =
.
.
Dynamic Shear Viscosity*:
Unit: [] = 1 Pas
* Isothermal, isobaric, steady state dynamic shear viscosity of an incompressible, isotropic fluid
Area A = a bStrain = u / h
.
© 2017 Malvern Instruments Limited
Typical Shear Rate Ranges for Paints and Coatings
Shear Rate (s-1)
She
ar V
isco
sity
10-6 106
Sedimentation
Sagging
Levelling DippingPouring
MixingExtrusionPumpingBrushing
Spraying
1
Storage
Viscometer range
Rotational Rheometer
Capillary Rheometer
© 2017 Malvern Instruments Limited
How to calculate the Shear Rates?
Blade Coating,Brushing
Slot Die Coating Spraying
Q = Volume Flux, R= Die Radius, L= Die Length, b= Slot Width h=Slot Height, v=Velocity, h= Wet Layer Thickness
© 2017 Malvern Instruments Limited
Principle of Operation: Rotational Rheometer
Sample
Upper Measuring Plate
Temperature Controller
Position Sensor
Air Bearing
Motor
Apply TorqueMeasure Torque
Measure Displacement.Apply Displacement
Stress- and Rate Controlled
© 2017 Malvern Instruments Limited
Given quantity: piston speed wall shear rateMeasured quantity: pressure drop wall shear stress
Bore
L
Fully developed flow region
entry
P1
Pw
00 L
Z
P
v
2R
Entrance pressure drop
Shear pressure drop
Full pressure drop
=
+
Principle of Operation: Capillary Rheometer
Malvern RH2000
© 2017 Malvern Instruments Limited
Interpretation of Shear Viscosity Curves of Coatings
Low Shear Data needed for Stability Analysis High Shear Limit: Stress must not drop with increasing Shear Rate
Upper ShearRate Limit
Impact on Storage Stability and Leveling: Does a Zero ShearViscosity exist?
Blade Coating
Slot Die Coating
© 2017 Malvern Instruments Limited
Optimizing Rheology of Coatings:1. Impact of Adding Coarse or Fine Particles
• Small particles increase the low shear viscosity, as they have more surface area which gives more electrostatic and inter-particle forces.
• Note that the viscosity is almost independent of particle size at higher shear rates, as here hydrodynamic forces dominate
0% talc
29% coarse talc (D50 = 19µm
29% fine talc (D50 = 5 µm)
0 % talc
40% coarse talc (D50 = 19µm)
29% coarse talc (D50 = 19µm)
© 2017 Malvern Instruments Limited
Optimizing Rheology for Coatings: 2. Impact of Particle Loading
› Changing the volume fraction of the particles….Lo
g Z
ero
She
ar V
isco
sity
1.0m
5.01.0 m
5.0m
Volume Fraction of Particles
Newtonian Shear Thinning Shear Thickening m
mmedium
1
Krieger-Dougherty:
Norman J. Wagner, J. B. (2009). Shear thickening in colloidal dispersions. Physics Today , 27-32.
© 2017 Malvern Instruments Limited
Example: Shear Thickening of a Spray Coating
Shear Thickening at High Shear Rates: Critical for Spray Process Strong Shear Thinning at Low Shear Rates: Defines the Structure Build-Up after Spraying
© 2017 Malvern Instruments Limited
We keep the volume fraction () constantBut changing polydispersity…
What happens to the viscosity?
Particle Size Distribution
0.1 1 10 100 1000 3000 Particle Size (µm)
0
5
10
15
20
Vol
ume
(%)
Optimizing Rheology for Coatings: 3. Impact of Polydispersity
© 2017 Malvern Instruments Limited
Impact of Polydispersity on Flow Behaviour
Increasing amount of small particles
Zero
She
ar V
isco
sity 100%
small particles
100% large
particles
Increasing amount large particles
0%
100%
100%
0%
If you want to increase the solid content of the sample but keep the viscosity the same, increase the particle size distribution (polydispersity) as well. Conversely, narrow the particle size distribution to increase the viscosity.
m
mmedium
1
Krieger-Dougherty
© 2017 Malvern Instruments Limited
Further Factors affecting Coating Rheology
Volume fraction Particle size Particle size distribution
Particle shapeElectrostatic interactions
Vs
- - - - - -+ + + +
+ + + +
Steric Hindrance
Laser Diffraction
Digital Microscopy
Light Scattering
Size and Zeta
Spray ParticleAnalyzer
Wet
Dry
© 2017 Malvern Instruments Limited
Storage Stability: Importance of Zero Shear Viscosity
3
434
21
2
321
21
grr
gr
AgV
Agm
AF
v = settling speed, r= particle radius, ρ1 = density of the particleρ2 = density of the fluid, g = gravitational acceleration = dynamic shear viscosity
Shear Stress by Gravity:
Settling Speed:
Buoyancy
Gravity
© 2017 Malvern Instruments Limited
Summary
• Rotational and Capillary Rheometry cover the typical Processing Conditions for Paints and Coatings
• Shear Viscosity Function related to Particle Properties (Size, Polydispersity, Volume)
• Storage Stability: Importance of Zero Shear Viscosity
Any Questions?
Email: [email protected]