Melt strength and extensibility of talc-filled polypropylene

Polypropylene homopolymer melts containing up to 50 percent by weight of talc filler (24 vol%) were tested on a Gottfert Rheotens melt strength testing unit. Melt strength and drawability parameters were measured from these tests to evaluate changes in the extensional behavior of the melt induced by filler addition. It was found that the melt strength of the polypropylene was decreased for talc additions of less than 40 wt% (17.5 vol%) under all test conditions. Addition of 50 wt% talc improved the melt strength under all testing conditions. Drawability, defined as the stretch ratio at strand break, decreased with increased filler loading. The melt strength of all materials was found to decrease with increased temperature within the range 180degreesC to 210degreesC. The melt strength of all melts showed dependence on the test extrusion rate, and this dependence was well described by an exponential relationship. Highest dependencies were seen for the unfilled and 50 wt% talc-filled polymer. While it was found that addition of 50 wt% talc provided improvement in melt strength, high melt strength alone was a poor indicator of the melt processability, as drawability was also regarded as an important consideration.