Role of slip and fracture in the oscillating flow of HDPE in a capillary

Abstract

Certain polymers exhibit two distinct branches in their capillary flow curves (wall shear stress versus apparent wall shear rate). This gives rise to oscillatory flow in constant-piston-speed rheometers and to flow curve hysteresis in controlled-pressure rheometers. These curious phenomena have attracted considerable interest over a period of many years, but their basic mechanisms are still the subject of debate. Building on previous work we have developed a model that predicts all the essential features of the curves of pressure and flow rate versus time in the oscillatory flow regime. Fluid compressibility and the second branch of the flow curve are necessary features of the model, but fluid elasticity is found not to be an essential element. While our macroscopic measurements do not prove it conclusively, our data lead us to believe that on the high-flow-rate branch of the flow curve there is slip along a cylindrical fracture surface near the wall. The jump to the high-flow branch occurs when this fracture occurs, at an upper critical value of the shear stress, while the jump back to the low-flow branch occurs when adhesion is established at the fracture surface at a lower critical shear stress.

A review of nonlinear oscillatory shear tests: Analysis and application of large amplitude oscillatory shear (LAOS)

“…Wall slip is expected to be one of the main reasons for the occurrence of even harmonic contributions [22,132]. Graham [73] demonstrated the occurrence and growth of even harmonics in a dynamic wall slip model.…”

Model for Melt Fracture Instabilities in the Capillary Flow of Polymer Melts

“…1); such a curve has been prevalent in the recent literature [1,2]. When the die is coupled to a large reservoir in which the polymer can be compressed, we find that the multivalued flow curve gives rise to oscillatory spurt flow as has been discussed previously [2]. More significantly, and even in the absence of the reservoir, we find that the elastic nature of the fluid can give rise to periodic oscillations, chaotic behavior, and large-scale spatial structures in the die, which we conjecture is responsible for sharkskin.…”

“…Here ᐉ is a constant and c is a dimensionless quantity defined locally at each point x along the walls at time t. c 0 gives stick, while c !g ives total slip. Experiments show that the slipping length jumps sharply at a critical value of the shear [4,5], while hysteresis is observed on the multivalued flow curves seen in extrusion [1,2]. Together, these two features suggest the change in slipping length is the nonequilibrium analog of a first-order phase transition, presumably the result of a transition in the local conformational state of the polymer at the wall.…”

“…In various regimes, the model exhibits steady flow, periodic oscillations, and more complicated spatiotemporal structures, which can be observed experimentally. [S0031-9007(96) The capillary flow of molten polymers has received much attention in the plastics and chemical engineering communities [1,2] because at higher flow rates the extrusion of the polymer melt is commonly accompanied by instabilities which manifest themselves as surface distortions, called "melt fracture," in the final plastic product. A standard experiment has the melt pushed from a large reservoir into the capillary ("die") and extruded out the other end.…”

Experimental Investigation of Die Drool and Slip-stick Phenomena during HDPE Polymer Melt Extrusion

“…This instability appears at very narrow processing window (depending on temperature and flow rate for given polymer melt) in which pressure in extrusion die oscillates between two extreme values, although the imposed flow rate is kept constant. A lot of works focused on this instability have been already published [14][15][16][17] however, only one small notice about correlation between slip-stick and die drool phenomena can be found in [18].…”