Rabinowitsch Shear Rate Corrections Integrated with Bagley Wall Stress Regressions
Integrating Rabinowitsch shear rate corrections with Bagley wall stress regressions converts raw capillary metrics into true viscosity, protecting yield margins.
Flow Behavior Index
Meaning
Rheological metrics characterize the degree of non-Newtonian behavior exhibited by a fluid under shear. This dimensionless number, represented as the exponent in the power-law viscosity model, indicates whether a material thins or thickens as the flow rate increases. The flow behavior index dictates how a polymer melt behaves during processing, with values below unity indicating shear-thinning behavior.
Understanding this value is necessary for predicting how the melt will distribute inside an injection mold or an extrusion die at high shear rates.
Processing Behavior
Low values of the index signify that the melt viscosity decreases rapidly under shear. This property allows highly shear-thinning polymers to flow into complex mold cavities with minimal resistance during the high-speed injection phase. In contrast, materials with an index close to unity require higher pressures to fill the same mold volume, restricting their use in intricate geometries.
Structural Influence
Molecular weight distribution and polymer branching govern the sensitivity of the index to changes in shear rate. Broad molecular weight distributions typically yield lower flow behavior index values, providing better processability in conventional extrusion lines. Linear polymers exhibit less shear-thinning behavior, which limits their processing window during high-throughput operations.
Testing Requirement
Capillary rheometers measure the shear stress across a range of shear rates to plot the viscosity curve. The flow behavior index is determined from the slope of this curve in the power-law region. If the test is conducted at too low a shear rate, the polymer remains in its Newtonian plateau, producing a false reading of unity.