Miscibility and co-continuous morphology of polypropylene-polyethylene blends

Polypropylene (PP) is immiscible in the melt, at low PP concentration, with high density (HDPE), low density (LDPE) and very low density (VLDPE) polyethylenes. When PP is the minor phase it forms liquid droplets in the polyethylene and these crystallise independently of the polyethylene. PP is miscible in some linear low density (LLDPE) polyethylene in the melt. When these blends are held at a temperature at which the PP can crystallise, but too high for the polyethylene (PE) to crystallise, the PP can crystallise from homogeneous solution. Even when it is the minor phase (20% mass) PP can crystallise and form a co-continuous phase in the PE. This type of crystallisation significantly changes the crystallisation kinetics. The PP crystals grow into the PE phase giving a diffuse morphology. The melt viscosity of the PE is important in determining the rate of growth of the PP crystals. This method has been used to detect miscibility of PP in the various PE's and to form blends with unique morphology.