Gross melt fracture of polyethylene. I: A criterion based on tensile stress

Abstract

We propose the use of a critical tensile stress as a criterion for the onset of gross melt fracture (OGMF). This stress is estimated from the entrance pressure drop using the method of Cogswell. Carbon black tracer was used to verify that what we deemed to be GMF, based on the inspection of extrudate, was indeed the result of a brittle rupture. An orifice die was used to eliminate the complications arising from the presence of a capillary. Polyethylenes having a variety of molecular structures were used to evaluate the usefulness of the uitical tensile stress for characterizing polymers. The critical stress was found to be independent of temperature and contraction ratio. It is also independent of entrance angle, as long as this is equal to or greater than 90 degrees. The critical stress is thus a material property and can be used to rate the tendency of polymers to exhiiit gross melt fracture.

Experimental study of the volume defects in polystyrene extrusion

“…It is now well admitted, and it is largely confirmed by our results, that the periodic volume instability is initiated at the die entry [10][11][12][13][14][15][16][17][18][19][20][21]. An explanation proposed in the literature concerns the destabilization of entry vortices [5,10,20].…”

“…The elongational stresses and deformations developing in the convergent have often been associated with the onset of the upstream instability [12,21,30]. Using capillary rheometry experiments, we can derive approximate values of elongational stresses from entrance pressure losses, according to the Cogswell method [29].…”

“…However, applications to molten polymers remain less common. Kim and Dealy [21] have recently suggested a critical tensile stress as a criterion for the onset of gross melt fracture. They argue that gross melt fracture would involve the true fracture (rupture) of the melt, specifying that it is not clear whether this is the result of chain scission or sudden disentanglement and chain pull-out.…”

Slip flows of Newtonian and viscoelastic fluids in a 4:1 contraction

“…The development of vortices in these geometries affects the smoothness of the flow and promotes the appearance of instabilities, as reported by Kim and Dealy [1] and Miller et al [2]. The flow through a 4:1 planar sudden contraction under conditions of creeping flow and no wall slip is a long standing classic benchmark problem in computational rheology [3].…”

Anomalous rheological behavior of polyethylene melts in the gross melt fracture regime in the capillary extrusion: Effect of long‐chain branching

“…GMF is reported to occur when the extensional stress at the entrance of a die exceeds a critical condition that seems to depend only on the polymeric fluid. 2 Excellent reviews on the sharkskin 3,4 and GMF 2,5 are found elsewhere.…”