Flow defects in linear low density polyethylene processing: Instrumental detection and molecular weight dependence

An elongational rheometer coupled with a primary extrusion device has been used for the detection of flow limitations in the extrusion of linear low density polyethylenes (LLDPE). The shear rate for the onset of flow defects (fd) was found to decrease with increasing branch length in the LLDPE copolymer, but the severity of fd was greater in butene- than in hexene- and octene-copolymers. Studies of partially fractionated LLDPE showed that the critical shear rate for the onset of fd was closely related to the minimum molecular weight in the distribution of molecular weights. In addition, evaluations of bond strengths in LLDPE/metal joints showed that the same molecular weight parameter significantly affected that performance criterion. In combination, the experimental findings provided confirmation of sporadic adhesive failure (slip/stick effect) as the mechanism responsible for the occurrence of observed flow defects. Its manifestation is attributable to the thermodynamic drive that promotes the preferential localization of low molecular weight moieties at surfaces and at interfaces of polyolefins such as LLDPE.