Quantifying Wall Shear Stress Thresholds Struck from Capillary Rheometry Data
Quantifying true wall shear stress requires Bagley end-loss and Rabinowitsch corrections on capillary data to prevent premature extrudate fracture.
Non Newtonian Index
Meaning
Numerical value representing the degree to which a fluid’s viscosity deviates from Newtonian behavior measures its shear sensitivity. This parameter, also called the non Newtonian index or power law index, determines whether a polymer melt exhibits shear thinning or shear thickening under flow conditions. It governs the velocity profile of the melt within extrusion channels and affects the calculation of pressure drops.
Flow Behavior
Newtonian fluids maintain a constant viscosity regardless of the shear rate applied, whereas polymer melts typically exhibit a decrease in viscosity as the shear rate increases. The non Newtonian index is the exponent in the power law fluid model, where a value of one represents a Newtonian fluid and values less than one indicate shear thinning. This index is a critical material property for designing processing equipment that operates under high shear conditions.
Power Law
Calculating the power law index involves plotting viscosity against shear rate on a logarithmic scale and determining the slope of the resulting linear region. In non Newtonian index determinations, the slope of this line corresponds to the index minus one, with steeper slopes indicating more pronounced shear thinning behavior. This mathematical relationship allows engineers to predict how viscosity will change as the polymer flows through the narrow channels of an extrusion die.
It simplifies the flow modeling of thermoplastic materials by replacing complex viscosity curves with a single, highly descriptive parameter.
Extrusion Tuning
Processors use this index to select appropriate processing speeds and avoid problems like melt fracture. A material with a low non Newtonian index will experience a sharp viscosity reduction under high shear, allowing for faster extrusion.