Assessment of LDPE melt strength by use of rheotens mastercurves

In a Rheotens rest, the tensile force needed for elongation of an extruded filament is measured as a function of the draw ratio. In this way, the melt strength can be assessed which is nn important property in many polymer pr ocesses like fibre spinning,film blowing and blots moulding. Several LDPE g rades with different melt index and polymerised by both tubular ann autocla ve reactor technology were investigated. Ir is demonstrated that to assess melt strength, Rheotens tests have to be compared at constant extension pre ssure, while comparison at constant mass flow rate can be highly misleading . Ar constant extrusion pressure,Rheotens curves are invariant nor only wit h respect to temperature brit also with respect to average molar mass, and polymer grades with similar branching structure and molar mass distribution fall onto a common mastercurve. Two distinct ann different Rheotens master curves were found for grades produced by tubular versus autoclave reactor. technology, indicating distinct differences in the branching str structure. Irrespective of the processing conditions, all LDPE melts investigated fh il by brittle fracture, and a true rupture stress in the range of 1 to 2 MP a was found.