Capillary Rheometry Data Corrections for Polymer Processing Accuracy
Correcting raw capillary rheometry data for entry losses, shear gradients, and wall slip prevents costly tooling rework and protects gross margins.
Mooney Wall Slip
Meaning
The relative velocity discrepancy between a flowing polymer melt and the solid wall of a processing channel or capillary rheometer characterizes the loss of fluid adhesion at the boundary. Material scientists evaluate Mooney wall slip to correct viscosity measurements and predict flow behavior in high-shear extrusion dies. This slip alters the velocity profile of the melt, making it transition from a parabolic to a plug-like flow.
Failing to account for this boundary behavior results in underestimating the pressure drop through the die.
Rheological Measurement
Capillary rheometry tests of varying die diameters are conducted to isolate slip effects from bulk flow. By evaluating Mooney wall slip, the practitioner plots the apparent shear rate against the reciprocal of the capillary radius at constant wall shear stress. This graphical analysis yields the slip velocity of the material.
Processing Outcome
High slip velocities can prevent the onset of melt fracture, which is a surface distortion that ruins the cosmetic appearance of extruded products. When Mooney wall slip occurs, it reduces the extreme shear stresses concentrated near the metal wall. This reduction allows for higher throughput rates without triggering surface defects such as sharkskin.
However, excessive slip can lead to flow instability and output fluctuations if the polymer slips and sticks unpredictably. Extrusion operators must monitor the temperature and pressure profiles to maintain a stable slip regime.
Boundary Condition
Surface roughness and tool material affect the onset and magnitude of the sliding behavior. Changing from a steel die to one coated with a fluoropolymer can alter the interaction between the melt and the wall. This dependency implies that slip values obtained from one tool cannot be directly applied to another without calibration.