Melt compatibility between polyolefins: Evaluation and reliability of interfacial/surface tensions obtained by various techniques

The interfacial tension in the melt state represents a key parameter to quantify the compatibility of polymer blends processed by extrusion techniques. Its evaluation is still challenging for polyolefin-based blends and, here, two complementary techniques (i.e. rheological and optical methods) were tested to evaluate the PP/PE compatibility in the melt state. In particular, interfacial/surface tensions at 200 °C between several types of isotactic polypropylene (iPP) and polyethylene (PE) (i.e. Ziegler Natta- and metallocene-catalyzed grades) are specifically addressed to detect slight modifications of melt compatibility and to potentially highlight the role of polymer structure induced by the used catalyst. Classical dynamic rheology experiments coupled with morphological analyses were first attempted and several trends are clearly observed in term of iPP/PE interfacial tensions. iPP/metallocene PE associations display the lowest interfacial tension (0.9–1.2 mN/m, relative standard deviation ≈ 20%) and as-selected metallocene PE clearly gave rise to an enhanced melt compatibility with iPP matrices. Interfacial tensions could not be directly and precisely evaluated by optical methods using the polymer-in-polymer pendant drop method due a poor precision arising from an ultra-low iPP/PE melt density difference. However, surface tensions in the melt state obtained by the pendant drop method under nitrogen atmosphere represent the most reliable tool with an excellent sensitivity to as-selected polyolefin catalysts. An indirect but precise insight on polyolefin compatibility was found possible using surface tensions with a full discrimination between as-selected polyolefins in agreement with enhanced melt compatibilities in metallocene iPP/metallocene PE blends.