© 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

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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

.

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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

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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

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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

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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

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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

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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)

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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.

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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

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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

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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

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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

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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

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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: torsten.remmler@malvern.com