Tpa Rheology Presentation

Mark C. Edwards ● Gammadot Rheology Testing & Consultancy Services ● www.gammadot.com

Rheology - A Practical Approach To Quality ControlA presentation to the Telford Polymer Association November 25th 2008

Why Rheology?The single most important parameter in the processing of a molten polymer is the viscosity function of the material – if it doesn't flow you can't process it! This parameter is highly sensitive to material structure. Rheological measurements can detect discrete changes in structure through flow behaviour, and can be fast & accurate methods of providing quality control for all aspects of the process.




Discriminating batch to batch variance

Excellent in:




‘Mapping’ processing problems

Excellent in:




‘Mapping’ processing problems

Aiding failure analysis

Excellent in:


Which Rheometer?

Melt Flow Indexer (Rate)


Brookfield Viscometer


‘LV’ ‘RV’ ‘HB’ Low Viscosity Medium Viscosity High Viscosity

Capillary Extrusion Rheometer


Controlled Stress / Rate Rotational Rheometer


Which Rheometer??


Rule of thumb: if structure sensitive measurements are required such as batch variance, melt stability, degradation studies, etc, then a rotational rheometer in oscillation mode. Investigations required at process-relevant shear rates call for the capillary rheometer.

Oscillatory Rheometry

The oscillation technique involves applying a sinusoidal oscillating stress () wave to the sample and measuring the resulting strain wave ().



A purely elastic (Hookean solid) material will retain all of the deformation energy applied to it and will therefore have a phase difference (phase angle) of 0 degrees. Conversely, a purely viscous (Newtonian) fluid dissapates all of the applied energy and will have a phase angle of 90 degrees. Polymeric materials are Visco-elastic and as such their responses fall somewhere between these two extremes.


Hookean Solid Response Newtonian Fluid Response Visco-Elastic Response

Phase Angle = 0 Phase Angle = 90 Phase Angle = 0<>90


Measurements are normally made isothermally at constant strain amplitude, across a range of frequencies. A wealth of information relating to structural responses can be gained in oscillation mode.


100.0

1000.0

10000.0

100000.0

Frequency (Hz)

Com

plex

Vis

cosi

ty (

Pa.

s)

Mo

dulu

s (P

a)

G'G"

h*

Narrow MwD

Broad MwD

High Mw Low Mw

Modulus Cross Over Point


Relationship of G’ / G” cross-over point to average molecular weight for HDPE.


HDPE Molecular Weight Average Vs Modulus Cross Over Frequency

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1000000

0.1 1 10 100 1000 10000Frequency (Hz)

Mol

ecul

ar W

eigh

t (M

w)

Mw 120k

Mw 98k

Mw 145k

Mw 90k

Mw 60k

Mw 75k

Mw 340k

Mw 40k

Mw 8k


Extremely accurate, reproducible results – calibrated instruments should give comparable data.


Reologica Viscotech Controlled Stress Rheometer DataControl PP @ 220°C, 5 millistrain

10

100

1000

10000

0.01 0.10 1.00 10.00 100.00

Frequency (Hz)

Co

mp

lex

Vis

cosi

ty (

Pa.

s)

Nov.04 (StressTech)

Jun.08 (ViscoTech)

Oct.08 (ViscoTech)


Simple ‘Go / No Go’ quality control programme set up for an automotive component manufacturer suffering from batch variance of incoming materials.


Reologica ViscoTech Controlled Stress RheometerTemperature: 220°C, Strain: 5 millistrain

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0.01 0.10 1.00 10.00 100.00

Frequency (Hz)

Co

mp

lex

Vis

cosi

ty (

Pa

.s)

Short Shot


Invaluable in most failure analysis projects – oscillatory rheometry can usually give first indications of failure cause or at least point the investigation in the right direction.


Reologica ViscoTech Controlled Stress Rheometer DataTemperature: 255°C, Strain: 2 millistrain

0.01

0.1

1

10

100

0.1 1 10

Frequency (Hz)

Co

mp

lex

Vis

cosi

ty (

Pa.

s)

PBT Virgin Granules

0 Hrs Drying

1 Hrs Drying

2 Hrs Drying

3 Hrs Drying

'Good' Component

Failed Component

‘Good’ Component

‘Failed’ Component


Individual test programs can be run as a ‘macro’ to give tailored test profiles.


Reologica ViscoTech Controlled Stress Rheometer DataExotherm Tracker Programme. Frequency: 1Hz, Strain: 5 millistrain

1.0

10.0

100.0

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100000.0

1000000.0

10000000.0

100000000.0

0.001 0.01 0.1 1 10 100

Time (Hours)

Sto

rag

e M

od

ulu

s (P

a)

Blue Curve: Thermal Ramp (20°C to 130°C @ 20°C/min Heating)Red Curve: 1 hour 130°C Isothermal (simulate adiabatic heating)Green Curve: 18 Hr Thermal Ramp (130°C to 20°C @ -0.102°C/min cooling)

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For further information on rheological theory & measurement techniques check out the relevant sections on our website:


Tpa Rheology Presentation

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