Retrieving Equivalent Shear Viscosity for Molten Polymers from 3-D Nonisothermal Capillary Flow Simulation.

Area of Science:

• Polymer Rheology
• Non-isothermal Fluid Dynamics

Background:

• Viscous heating causes significant temperature rises in highly viscous polymer melts.
• These temperature rises are a major source of error in capillary rheometry viscosity measurements.

Purpose of the Study:

• To develop and validate a method for correcting shear viscosity measurements affected by viscous heating.
• To improve the accuracy of viscosity measurements for polymer melts in capillary rheometry.

Main Methods:

• Theoretical derivation and 3-D non-isothermal flow simulations were employed.
• Analysis of capillary pressure losses using a viscous Cross model.
• Experimental validation using amorphous, crystalline, and filler-reinforced thermoplastic melts.

Main Results:

• An approach was developed to retrieve equivalent shear viscosity using average fluid temperature at the wall.
• The methodology accurately predicts experimental pressure drops for various polymer types.
• A material-independent scaling relation (Θ ~ Na^0.72) was found for temperature rise.

Conclusions:

• Applying a viscous heating correction is crucial for accurate shear viscosity determination.
• Using average wall temperature is a more accurate parameter than bulk temperature for correction.
• The developed method and scaling relation enable prediction of temperature rise and viscosity changes.