Gross melt fracture of polyethylene. II: Effects of molecular structure

Abstract

The effect of molecular structure (MW, MWD and LCB) on the critical tensile stress ((TJ for the onset of gross melt h c t u r e (OGMF), proposed in Part I (1) as a material-dependent criterion for hcture, was determined for a goup of polyethylenes varying in structure. These included linear low and high-density polyethylenes and several materials produced using metallocene and constrained geometry catalysts. It was found that the critical stress is independent of I & , for constant polydispersity but increases with increasing long chain branching and polydispersity. The addition of boron nitride particles had no effect on the (T, up to a level of 0.5% by weight.

Experimental study of the volume defects in polystyrene extrusion

“…Its value is in the range of 0.5-0.9 MPa, whatever the flow conditions. This is consistent with the range of 1.7-3.7 MPa, cited by Kim and Dealy [30] for the onset of gross melt fracture for a series of polyethylenes. The estimation of a critical elongational stress is obviously more difficult in the case of the slit geometry.…”

“…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].…”

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.…”

An experimental evaluation of the formation of an instability in monodisperse and polydisperse polystyrenes

“…There are a number of reviews on viscoelastic instabilities [1][2][3][4][5][6] and a recent review [6] highlighted three forms of polymer instability observed in extrusion which occur at increasing levels of flow deformation. These are the "Sharkskin" instability [7][8][9][10][11], the "stick-spurt instability" [12][13][14], and "upstream extrusion instabilities", also termed "gross melt fracture" or "volume instabilities" [15][16][17][18][19][20][21][22][23][24].…”