Influence of processing aids on the uniaxial extensional behavior of metallocene polyethylenes

摘要

It has been reported in the literature that boron nitride powders with hexagonal crystal structure in combination with fluoroelastomers at small concentrations (typically 500-1000 ppm) can be used as suitable processing aids in melt extrusion processes to overcome flow instabilities such as sharkskin and gross melt fracture. They essentially act as energy dissipaters and suppress the unbound increase of extensional stresses, which are responsible for triggering gross melt fracture. This paper reports on the mechanism by which the presence of a small amount of boron nitride changes the behavior of metallocene polyethylenes in uniaxial elongation. Samples modified with different processing aids were tested in solid and molten state using uniaxial elongation and other rheological tests. The stress-strain behavior is described by the BDEW (Ball-Doi-Edwards-Warner; Ball et al., Polymer, 22, 1010, (1981)) network theory. This theory predicts a reduction of the entanglement density (number of "slip-links") in the presence of boron nitride particles that explains the decrease in extensional stresses. Thus, it can be argued that the presence of boron nitride particles dissipates the release of elastic energy at lower extensional stress levels due to the decrease in the entanglement density, thus preventing catastrophic failures such as gross melt fracture.