Surface characterization of polypropylene/(ethylene-propylene) copolymer blends (PP/EP): Application to injection-moulded systems

Polypropylene/ethylene - propylene copolymer blends (PP/EP) were characteri zed by time-of-flight (ToF), SIMS, by dynamic wetting and by atomic force m icroscopy and force modulation spectroscopy (AFM-FMM), In a first step, mod el systems of compression-moulded PP/EP physical blends were prepared. With these materials, ToF-SIMS provides linear relationships between the select ed peak and the EP content in the material. This indicates that both phases are present at the surface in the same concentrations as in the bulk. Howe ver, the EP signal is weak. In dynamic wetting measurements with water, for each tested blend composition the advancing contact angle corresponds to t hat of PP and the receding contact angle corresponds to that of EP, This me thod is thus suitable for detecting the presence of the two phases at the s urface even for low concentrations of one consituent, The force modulation image gives elastic contrasts between EP and PP and is thus able to reveal the surface morphology, In a second step, these methods were used to charac terize injection-moulded PP/EP reactor blends. Both AFM-FMM and ToF-SIMS su ggest that a PP skin covers EP nodules at the surface. This is confirmed by electron microscopy examinations of cross-sections. This surface phase seg regation leads to the presence of PP in the first 100 mm of the surface and could be explained by preferential wetting of the mould by the phase with the lower viscosity, PP in this case. However, dynamic wetting reveals that the PP skin is not complete. Thus, a low EP concentration may be found at the surface, in the form either of EP nodules or of EP molecules having mig rated through the PP skin, Copyright (C) 1999 John Wiley & Sons, Ltd.