How to measure the shear viscosity properly?

Transcription

1 testxpo Fachmesse für Prüftechnik How to measure the shear viscosity properly? M p v Rotation Capillary Torsten Remmler, Malvern Instruments

2 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

3 Tangential-force F Basic Terms in Shear Rheometry displacement u s area = a b Gap = s. u s d dt Ftan A a b strain [] Shear rate [1/s] Shear stress [Pa=N/m 2 ]

4 Typical Shear Rate Ranges Sagging, Levelling Extrusion, Injection Moulding Mixing, Blade Coating, Brushing Roll Coating, Spraying s -1 Rotational-Rheometer Sample: Water up to solids Results: Shear-Viscosity, Yield Stesses, Visco-Elasticity, Relaxation... High Pressure Capillary-Rheometer Sample: Water up to high viscous Results: Shear-Viscosity, Elongational-Viscosity, Wall Slip...

5 Shear Viscosity Resistance of a sample against the flow =. Shear Viscosity Shear Stress. Shear Rate

6 Typical Shear Viscosities Material Shear-Viscosity (Pas) Air Aceton Water Olive Oil Glycerol Molten Polymers Bitumen Glass at 500 C Glass at ambient Units: Remember Pascal second Pas (SI) 1 Pas = 10 P Poise P (CGS) 1 mpas = 1 cp

7 Shear-Viscosity depends on. p, t, ) =. Physical-chemical structure of the sample Temperature (up to 20% / K) Pressure Time Shear Rate

8 Steady-State Flow Behaviour Newtonian Shear Thinning Shear Thickening Stress Stress Stress Shear Rate Shear Rate Shear Rate Silicon Oil, Suspension Inks, Paints Cornflower Viscosity Viscosity. Viscosity Shear Rate Shear Rate Shear Rate

9 Principle of Operation: Rotational Rheometer Stress- and Strain Control possible. Motor Air bearing Position sensor 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! Upper Measuring Plate Advantages: Sample Temperature Controller Wide Torque Range 10e-9 to 10e-1 Nm Short Response times Small inertia design Direct Stress and Direct Strain

10 Choice of Geometry: Rotational Rheometry Apply Torque / Measure Torque M Measure Displ. Apply Displacement Rule of Thumb for dispersions: Gap Size > 10 * D90 R the higher the viscosity, the smaller the geometry the higher the shear rate, the smaller the gap. Parallel Plates Cup&Bob Solids Fixture

11 Cone-Plate / Plate-Plate 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 0s -1 10s -1 10s -1 10s -1 10s -1

12 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 acc DIN53019 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 Double Gap DisAdv: large sample volume (ca. 15ml 30ml), difficult cleaning

13 Principle of Operation: Capillary Rheometer Given quantity: piston speed wall shear rate Measured quantity: pressure drop wall shear stress Bore v Full pressure drop L P = Entrance pressure drop P w P 1 Fully developed flow region 2R + Shear pressure drop entry 0 0 L Z RH2000

14 Laminar Pipe Flow Isothermal, stationary Flow of an incompressible fluid Newtonian Newtonian. app 4 R Q 3 -R 0 R Non- Newtonian app R. P 2 L -R 0 R Q = Volume Flux, R= Die Radius, L= Die Length, P=Pressure Drop 0

15 Choice of Geometry: Capillary Rheometer. app app 4 Q 3 R R P 2 L Shear Rate Ranges: 2.0mm = ca. 0.1 bis 100 /s 1.5mm = ca. 1 bis 1000 /s 1.0mm = ca. 10 bis /s 0.5mm = ca. 100 bis /s 0.3mm = ca bis /s 2-3 decades of shear rate can be achieved with a capillary die Q = Volume Flux, R= Die Radius, L= Die Length, P=Pressure Drop

16 Basic Viscometry: How to run a flow curve CS-Mode: Steady state and non-steady state measurements Steady state: Table.. No Time-Dependency: = () t t non-steady state: Ramp. Time Dependency = (t) t t

17 Steady State Flow Properties Newton: =.. equivalent.. Flow Curve: = () = () CR-Mode CS-Mode Shear Viscosity. equivalent Curve: = () = ()

18 Steady State Condition: Rotational Rheometry Kinexus Rheometer J dlnj/dlnt = 1 for pure viscous flow! Deviations show measurement errors!

19 Steady State Condition: Capillary Rheometry homogeneous inhomogeneous Equilibrium Pressure Drop is needed for Steady State Viscosity.

20 Comparison Stress- and Rate Controlled Test Shower Gel: Comparison CS und CR Shear Viscosity Curve

21 Normal Stress Difference N1 F t F n F N R Always watch the Normal Stress during a Shear Viscosity Measurement!

22 Example: Steady State Viscosity Curve Shear viscosity[pas] ,1 Xanthan Solution - measured with Cone Plate and Double Gap The maximum of relaxation spectrum H() corresponds to reciprocal cross-over Frequency from oscillatory shear experiment. Mw= g/mol 1% sp kh 2 c c A 0.5% 3 0.3% 0.1% M 3 10 R h N 1% 0.5% 0.3% 0.1% CP 0.1% DG 0,01 0,001 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]

23 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

24 Thank you for your attention.