Capillary Rheometry Shear Rate Correction Protocols for Bimodal High Density Polyethylene
Correcting capillary rheometry data via Bagley and Rabinowitsch protocols is mandatory to prevent up to 50 percent viscosity errors in bimodal HDPE die design.
Mooney Correction
Meaning
A mathematical procedure in capillary rheometry calculates the true shear rate of a polymer melt by correcting for wall slip. By measuring flow rates through capillaries of different diameters but identical length-to-diameter ratios, the Mooney correction allows researchers to isolate slip velocity from shearing deformation. This calculation ensures that viscosity measurements remain accurate for materials that exhibit slip at the capillary wall.
It is applied when characterizing highly filled compounds or linear polymers that slip during processing.
Wall Slip
Many high molecular weight polymers do not adhere perfectly to the die wall during capillary flow. Instead, they exhibit slip, which leads to an underestimation of melt viscosity. Applying the correction decouples this boundary velocity from the deformation occurring within the flowing polymer melt.
Experimental Procedure
Evaluating the material requires running it through at least three different capillary dies of varying diameters while keeping the shear stress constant. Plotting the apparent shear rate against the reciprocal of the capillary radius yields a straight line where the slope represents the slip velocity. This slope is then subtracted from the apparent shear rate to find the actual shear rate, which provides the true shear viscosity curve needed for high-shear flow analysis.
Viscosity Accuracy
Correcting for slip is necessary for generating reliable material datasheets and simulation models. Without the Mooney correction, the reported viscosity values would be artificially low, which could lead to incorrect tooling design and injection moulding simulation errors. Reliable rheological data prevents processing defects during production runs.