Improving of cell structure of microcellular foams based on polypropylene/high-density polyethylene blends

Polypropylene/high-density polyethylene (PP/HDPE) blends and their nanocomposites are melt compounded using twin-screw extruder. The as-extruded samples are then batch foamed with supercritical carbon dioxide. It is demonstrated that the blend with a viscosity ratio close to unity produces a microcellular foam with very uniformly distributed finer cell structure, whereas nonuniformly distributed cells may be obtained in the foamed blend with a viscosity ratio greater than unity. Then three methods are tried to improve the cellular structure of the latter. The results show that the increase of HDPE content improves the uniformity of cellular size and distribution, but results in the increase of cell diameter and the decrease of cell density. Properly lowering the foaming temperature reduces the cell diameter and improves the cellular distribution. Incorporating nanoparticles, including nano-calcium carbonate and nanoclay, into the blend leads to a significantly smaller cell size and more homogeneous cell distribution. A critical content of nanoparticles exists to obtain dramatically improved cell structure. Moreover, the results show that most cells in foamed blends are nearly reticulated. So open-celled foams are achieved in this work.