Wall slip of molten polymers

We measure the flow kinetics of a polyethylene extruded through the exit of a sapphire capillary tube in order to understand the nature of sharkskin, a surface roughness in the extruded material. Optical velocimetry shows that sharkskin can occur under a variety of polymer/wall boundary conditions; stick, slip, or oscillating stick/slip, demonstrating that the flow boundary condition is not the direct cause of sharkskin. Downstream of the exit, high-speed video microscopy reveals two distinct material failures during each sharkskin cycle, the first is cohesive and splits the material into two regions, the second one occurs at the polymer–wall interface. Upon modification of the surface with a polymer processing additive (PPA), we confirm strong slip at the wall and a suppression of sharkskin, but find that sharkskin does return at sufficiently high flow rates. The extensional strain rate at the onset of sharkskin is significantly higher in the case with PPA than that without. We then empirically define a ...

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Extensional deformation, cohesive failure, and boundary conditions during sharkskin melt fracture

The classical no-slip boundary condition of fluid mechanics is not always a valid assumption for the flow of several classes of complex fluids including polymer melts, their blends, polymer solutions, microgels, glasses, suspensions and pastes. In fact, it appears that slip effect in these systems is the rule and not the exemption. The occurrence of slip complicates the analysis of rheological data, although it provides new opportunities to understand their behavior in restricted environments delineating additional molecular mechanisms i.e. entropic restrictions due to limitations in the number of molecular conformations. This article discusses these complexities and provides future research opportunities.

Slip mechanisms in complex fluid flows

Spurt occurs in the flow of entangled melts in a capillary rheometer in which the driving pressure is controlled. As the driving pressure increases, at a critical value there is a sudden increase in flow rate, often called spurt. This discontinuity is followed by another regime of smoothly increasing flow rate. A phenomenon that may contribute to this instability is a curve of wall shear stress versus slip velocity that has a maximum followed by a minimum, and there are models that predict such behavior. We used a sliding plate rheometer (SPR) to study wall slip for a highly entangled, linear polybutadiene at 1atm and at 46MPa. By varying the plate speed, we were able to explore the entire curve of shear stress versus slip velocity. This curve exhibited a maximum and a minimum in the stress, providing support for theories predicting this behavior and an explanation for the spurt effect in capillary rheometry. The spurt flow of the same polymer was also observed, and slip velocities were estimated and compared with those determined using the SPR. The slip velocities obtained using the two instruments were in good agreement.Spurt occurs in the flow of entangled melts in a capillary rheometer in which the driving pressure is controlled. As the driving pressure increases, at a critical value there is a sudden increase in flow rate, often called spurt. This discontinuity is followed by another regime of smoothly increasing flow rate. A phenomenon that may contribute to this instability is a curve of wall shear stress versus slip velocity that has a maximum followed by a minimum, and there are models that predict such behavior. We used a sliding plate rheometer (SPR) to study wall slip for a highly entangled, linear polybutadiene at 1atm and at 46MPa. By varying the plate speed, we were able to explore the entire curve of shear stress versus slip velocity. This curve exhibited a maximum and a minimum in the stress, providing support for theories predicting this behavior and an explanation for the spurt effect in capillary rheometry. The spurt flow of the same polymer was also observed, and slip velocities were estimated and comp...

Wall slip and spurt flow of polybutadiene

In this paper we report rheo-optical and rheological observations made through a transparent slit die attached to a capillary rheometer. We find that the flow birefringence signal oscillates periodically near the die exit when sharkskin-like extrudate distortion is present. In contrast, steady behavior is observed in the die inland region. Specifically, the flow birefringence varies at the die exit with a period identical to that measured directly from the sharkskin extrudate. We also show that the exit flow instability leading to sharkskin can be observed directly through cross-polarizers in terms of the temporal change of the retardation order. We demonstrate that the same kind of interfacial flow instability can occur at a boundary discontinuity within the die land where the upper portion of a clean die wall meets the lower portion of a polysiloxane-coated die wall. Finally, stress relaxation upon the cessation of the slit die flow of two polybutadiene melts is studied through time-dependent flow birefringence measurements. The stress relaxation is then correlated with sharkskin time scales to describe the role of relaxation in sharkskin ridge formation.

Flow birefringence study of sharkskin and stress relaxation in polybutadiene melts

This paper demonstrates an adhesive mechanism for wall slip by showing experimental results obtained using low surface energy fluoropolymer-coated die walls A transparent slit die is constructed to allow direct flow visualization of wall slip By ‘coloring’ polybutadiene and polyethylene with carbon black and coating the lower half of the slit die wall with a fluoropolymer, we show by visual inspection that the blackened polymers stick to the clean half of the die wall and slip over the coated portion

Justin R Barone

Papers

Flow birefringence study of sharkskin and stress relaxation in polybutadiene melts

Adhesive wall slip on organic surfaces