| In polymer extrusion, a melt instability called gross melt fracture is often observed when the extrusion rate is increased beyond a critical value. Because gross melt fracture limits the production rate, much attention has been devoted to predicting when it will occur. However, there is still no general understanding of the origin of this phenomenon. Several criteria have been proposed for the onset of gross melt fracture (OGMF), but these depend on the procedure used for measuring them and thus are not material properties. The use of a critical extensional stress (CES) is proposed here as a criterion for OGMF. This stress is estimated using the entrance pressure drop analyses proposed by Cogswell and by Binding.;Orifice dies were used to eliminate the complication arising from the presence of a capillary. The entrance angle and contraction ratio were varied to study the effect of die geometry. Various materials including conventional linear low and high density polyethylene and new polyethylenes produced by metallocene and constrained geometry catalysts were used in a study of the effect of molecular structure on OGMF. The latter materials are of special interest, because they have very well-defined molecular structure in terms of molecular weight (MW), molecular weight distribution (MWD), and long chain branching (LCB). In addition, the effect of particulate additives (boron nitride and carbon black) was investigated.;It was found that CES is independent of the die, contraction ratio, and entrance angle, as long as the angle is equal to or greater than 90 degrees. This result suggests that CES is, indeed, a material property. The CES was found to be independent of additive content up to a level of 0.5 wt%. Further, it was found that while CES is independent of MW, increasing levels of LCB and broadening MWD increase CES. Finally, brittle fracture at the entrance of the die is proposed as the cause of gross melt fracture. |
