How to measure the shear viscosity properly?

How to measure the shear viscosity properly?

Torsten Remmler, Malvern Instruments

v

Capillary

p

testXpo Fachmesse für Prüftechnik 10.-13.10.2016

M

Rotation

Outline

• How is the Shear Viscosity defined?

• Principle of Operation: Rotational and Capillary Rheometer

• Choice of the Correct Geometry

• Steady State Condition

• Example for Steady State Shear Viscosity Curve

Basic Terms in Shear Rheometry

Tangential-force F

s

displacement u

a b

area = a · bGap = s

AFdtdsu

tan

.

strain []

Shear stress [Pa=N/m2]

Shear rate [1/s]

Typical Shear Rate Ranges

10-110-3

Sagging, Levelling

104101

Extrusion, Injection Moulding

100 102

Mixing, Blade Coating, Brushing

103 106

Roll Coating, Spraying

s-1

Rotational-Rheometer

High Pressure Capillary-RheometerSample: Water up to high viscousResults: Shear-Viscosity, Elongational-Viscosity, Wall Slip...

Sample: Water up to solidsResults: Shear-Viscosity, Yield Stesses, Visco-Elasticity, Relaxation...

Shear Viscosity

=.

Shear Rate. Shear Stress

Shear Viscosity

Resistance of a sample against the flow

Typical Shear Viscosities

Material

AirAcetonWaterOlive OilGlycerolMolten PolymersBitumenGlass at 500°CGlass at ambient

Shear-Viscosity (Pas)

10-6

10-4

10-3

10-1

100

103

108

1012

1040

Units: RememberPascal second Pas (SI) 1 Pas = 10 PPoise P (CGS) 1 mPas = 1 cP

Shear-Viscosity depends on…

• Physical-chemical structure of the sample • Temperature (up to 20% / K)• Pressure• Time• Shear Rate

p, t, ) =

.

.

Steady-State Flow Behaviour

.

Shear Rate

Silicon Oil, Suspension Inks, Paints Cornflower

Stre

ss

Shear Rate

Newtonian Shear Thinning Shear Thickening

Stre

ss

Shear Rate

Stre

ss

Shear Rate

Visc

osity

Visc

osity

Shear Rate

Visc

osity

Shear Rate

Principle of Operation: Rotational Rheometer

• The drive is situated above the sample, not below.

• The driven spindle is air bearing supported so torque can be measured.

• The separate torque transducer is eliminated!

Advantages:

• Wide Torque Range 10e-9 to 10e-1 Nm• Short Response times• Small inertia design• Direct Stress and Direct Strain

Sample

Upper Measuring Plate

Temperature Controller

Position sensor

Air bearing

Motor

Stress- and Strain Control

possible.

Choice of Geometry: Rotational Rheometry

R

M

Apply Torque /Measure Torque

Measure Displ.Apply Displacement

Parallel Plates Cup&Bob Solids Fixture

• the higher the viscosity,the smaller the geometry

• the higher the shear rate,the smaller the gap.

Rule of Thumbfor dispersions:

Gap Size > 10 * D90

Cone-Plate / Plate-Plate

0s-1 10s-1

10s-1 10s-1 10s-1

• Cone Adv: Const Shear Rate along the complete gap, easy cleaning, low sample volume, wide viscosity range

• Cone DisAdv: only for homogeneous samples, for disperse samples D90 < 10 x gap, solvent evaporation

• Plate Adv: flexible gap, auto-tension possible, low sample volume, often used for temperature dependent tests, good for disperse systems

• Plate DisAdv: shear rate dependency, solvent evaporation

Cup & Bob / Double Gap

• Cup&Bob Adv: large gap, works well for disperse systems, also for samples showing sedimentation, large surface area, nearly no evaporation effects, good for low viscous samples, less impact of loading errors

• Cup&Bob DisAdv: high moment of inertia limits oscillation and transient steps, high cleaning effort, large sample volumes (ca 2ml – 15ml)

• Double Gap Adv: highest sensitivity for low viscous samples, lower inertia compared to cup&bob, nearly no impact on loading errors

• Double Gap DisAdv: large sample volume (ca. 15ml – 30ml), difficult cleaning

Cup&Bob acc DIN53019

Double Gap

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

RH2000

Laminar Pipe Flow

Isothermal, stationary Flow of an incompressible fluid

Newtonian

-R R0

-R 0 R

0

Newtonian

Non-Newtonian

Q = Volume Flux, R= Die Radius, L= Die Length, P=Pressure Drop

3app

Q4R

.

2P. R

Lapp

Choice of Geometry: Capillary Rheometer

2-3 decades of shear rate can be achieved with a capillary die

3app

Q4R×

.

2P× R

×Lapp

Shear Rate Ranges:

2.0mm = ca. 0.1 bis 100 /s1.5mm = ca. 1 bis 1000 /s

1.0mm = ca. 10 bis 10000 /s0.5mm = ca. 100 bis 100.000 /s

0.3mm = ca. 1000 bis 1.000.000 /s

Q = Volume Flux, R= Die Radius, L= Die Length, P=Pressure Drop

Basic Viscometry: How to run a flow curve

CS-Mode: Steady state and non-steady state measurements

Steady state:

non-steady state:

t

t

Table

Ramp

t

.

No Time-Dependency: = ().

t

Time Dependency = (t).

Newton: =.

CR-Mode CS-Mode

. equivalentFlow Curve: = () = ()

. .

equivalentShear ViscosityCurve: = () = ()

.

Steady State Flow Properties

Steady State Condition: Rotational Rheometry

dLnJ/dLnt = 1 for pure viscous flow! Deviations show measurement errors!

Kinexus Rheometer

J

Steady State Condition: Capillary Rheometry

Equilibrium Pressure Drop is needed for Steady State Viscosity.

homogeneous inhomogeneous

Comparison Stress- and Rate Controlled Test

Shower Gel: Comparison CS □ und CR ∆ Shear Viscosity Curve

Normal Stress Difference N1

Fn

Ft

R

FN

Always watch the Normal Stress during a Shear Viscosity Measurement!

Example: Steady State Viscosity Curve

The maximum of relaxation spectrum H() corresponds to reciprocal cross-overFrequency from oscillatory shear experiment.

Xanthan Solution - measured with Cone Plate and Double Gap

0,001

0,01

0,1

1

10

100

1000

1,0E-04 1,0E-03 1,0E-02 1,0E-01 1,0E+00 1,0E+01 1,0E+02 1,0E+03

Shear Rate [1/s]

Shea

r vis

cosi

ty[P

as].

1%0.5%0.3%0.1% CP0.1% DG

1%

0.1%

0.3%

0.5% 3

310 h

AR

NM

ckc hsp 2

Mw= 2.400.000 g/mol

Conclusion

• Correct Geometry Choice is key for Viscosity Measurement

• Steady State Condition in both Rotational and Capillary Rheometry

• Monitoring N1

• Interpretation: Correct Stress / Shear Rate Range for Shear Viscosity Curve

Thank you for your attention.

Email: [email protected]