Morphology, thermomechanical and barrier properties of polypropylene ethylene vinyl alcohol blends

Processing as well as physical properties of polypropylene (PP)/ethylene vinyl alcohol (EVOH) and PP/EVOH/high density polyethylene (HDPE) blends have been investigated. Blends were prepared both on a batch mixer and a twin-screw extruder. Extrusion was followed by drawing between two roll mills. Minor phase deformation during stretching was analysed using shape relaxation theory. When PP/EVOH blends were prepared on the internal mixer, low aspect ratio EVOH particles were obtained. This did not lead to appreciable barrier properties improvement. In contrast, an EVOH fibrillar and lamellar morphology obtained by extrusion end drawing was found to induce a significant decrease (85%) in O-2 permeability For only 20 vol.% EVOH content. The barrier properties of the PP/EVOH extruded blends were predicted using a hybrid diffusion model that takes into account the morphology of the blends. When PP.g.Maleic-Anhydride is added to PP/EVOH blends and for high draw ratios, elongation at break remains important, even at 20 vol.% EVOH. This result, together with the large gain in barrier properties, makes the PP/PP.g.MA-EVOH blends a proper material for applications such as food packaging. The use of HDPE/EVOH blend with PP intended to keep the EVOH well dispersed in the system did not give the expected barrier properties, due to the poor adhesion at the PP-HDPE interface. (C) 1997 Elsevier Science Ltd.