Non-Linear Melt Viscosity Calibration in Polymer Processing Operations
Non-linear viscosity calibration requires multi-point shear rheometry and Cross model fitting to prevent extreme die sizing and molding pressure errors.
Rabinowitsch Adjustment
Meaning
Analytical scaling factor applied in capillary rheometry converts the apparent shear rate of a fluid at the capillary wall into the true shear rate. The rabinowitsch adjustment accounts for the non-parabolic velocity profile of non-Newtonian fluids during flow through a tube or slit. Applying this correction is necessary because the standard velocity profile equations assume Newtonian behaviour, which underestimates the true shear rate for shear-thinning materials.
Correction Formula
Calculation of the true shear rate involves determining the derivative of the flow rate with respect to the wall shear stress. This rabinowitsch adjustment uses the slope of the log-log plot of flow rate against pressure drop to calculate a correction factor. For a Newtonian fluid, this slope is unity, and the correction factor is zero, whereas for polymer melts, the factor is larger, indicating the flatter velocity profile in the centre of the capillary.
Data Accuracy
Failure to apply this correction leads to large errors in the calculated viscosity, which can compromise the design of processing equipment such as extrusion dies. True viscosity data are required to predict pressure drops and shear heating in high-rate manufacturing environments.
Rheological Standards
Industry testing protocols mandate this adjustment when processing high-volume polymer grades. It ensures that measurements taken on different rheometers are directly comparable.