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 li near low density polyethylenes (LLDPE). The shear rate for the onset o f flow defects (fd) was found to decrease with increasing branch lengt h in the LLDPE copolymer, but the severity of fd was greater in butene - than in hexene- and octene-copolymers. Studies of partially fraction ated LLDPE showed that the critical shear rate for the onset of fd was closely related to the minimum molecular weight in the distribution o f molecular weights. In addition, evaluations of bond strengths in LLD PE/metal joints showed that the same molecular weight parameter signif icantly affected that performance criterion. In combination. the exper imental findings provided confirmation of sporadic adhesive failure (s lip/stick effect) as the mechanism responsible for the occurrence of o bserved flow defects. Its manifestation is attributable to the thermod ynamic drive that promotes the preferential localization of low molecu lar weight moieties at surfaces and at interfaces of polyolefins such as LLDPE.