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.
Mooney Slip Analysis
Meaning
Experimental evaluation of wall slip behavior in highly filled polymer melts or elastomers calculates the slip velocity as a function of wall shear stress. This rheological method, known as Mooney slip analysis, utilizes capillary dies of varying diameters while maintaining a constant length-to-diameter ratio to isolate wall slip effects. It enables accurate modeling of polymer flow in environments where the assumption of no slip at the wall is invalid.
Slip Measurement
Polymer compounds containing high filler loadings often exhibit slip at the metal walls of extrusion dies, which alters their flow behavior. Performing Mooney slip analysis requires measuring the apparent shear rate across at least three different capillary diameters. This systematic comparison allows engineers to determine if the flow rate is higher in smaller dies due to slip at the wall rather than viscous flow.
Mathematical Model
The calculation of slip velocity involves plotting the apparent shear rate against the reciprocal of the capillary radius for different shear stress levels. Through Mooney slip analysis, researchers can extract the slip velocity from the slope of these linear plots and use it to correct the shear rate calculations. This mathematical model provides the necessary boundary conditions for flow simulations of complex compounds like rubbers or PVC, which are prone to significant wall slip.
It transforms raw rheological data into actionable engineering parameters that are used to refine flow simulations and optimize die geometry.
Processing Impact
Designing extrusion dies for elastomers without accounting for slip leads to incorrect die dimensions and poor product quality. By incorporating the results of Mooney slip analysis, engineers can optimize die channel lengths to prevent melt distortion.