Development of β and α isotactic polypropylene polymorphs in injection molded structural foams

Structural foam moldings, composed of three co-axial cylinders differing in diameter (10 mm, 20 mm, and 30 mm) and length, were produced from isotactic polypropylene (PP) and 0.5 mass % 1,1′-azobisformamide on an in-line injection molding machine in a mould cavity pre-pressurized with nitrogen by the classical low-pressure process combined with egression of foamed melt from the core. Injection-molding conditions were as follows: melt temperature, 220°C, mold temperature, 20°C, cooling time, 5 min, gas-counter pressure, 0.5 MPa. The sprue gate was at the end of the smallest cylinder and its diameter was varied from 4 mm to 7 mm. To investigate the development of β -PP modification in terms of phenomena due to the phase change in the mould cavity (expansion), appropriate specimens (cross-sections) were cut from the middle of each cylinder in parallel and perpendicular orientation to the flow direction and were investigated by WAXS, DSC, and POM. As revealed by WAXS, β -PP is present in all cylinders, always concentrated in certain regions of the cross-section — mainly in the surface layers of the smallest cylinder (D1) and in the foamed core of the other two cylinders (D2 and D3). Its concentration was found to change with the sprue dimensions. High β -PP concentration is associated with a preferred orientation in the skin of the smallest cylinder and with better expansion conditions in larger cylinders. Presence of the β -phase in the surface layers and in the core of the moldings was proved by DSC and POM.