Influence of Cooling Rate on Physical Foaming Behavior of Molten Isotactic Polypropylene Systems

Abstract

Physical foaming of polymers is widely used to produce lightweight materials with improved thermal and mechanical properties. This study investigated the effect of cooling rate on the foaming behavior of two isotactic polypropylene (iPP) systems–a linear homopolymer (H1) and a long-chain branched homopolymer (Homo-LCB, L2). Polymer samples were saturated with nitrogen gas at 7 MPa and 200°C for 1 h, then cooled at controlled rates of 24, 40, and 52°C/min under constant pressure and depressurized at T_d = 110°C–200°C to induce bubble nucleation. Bubble nucleation was monitored using high-resolution visualization system and X-ray computed tomography (X-ray CT) imaging. The results showed that number of bubbles increased around T_d ∼130°C, reached a maximum at 120°C for cooling rates of 40 and 52°C/min, and at 115°C for 24°C/min. Higher cooling rates resulted in the formation of more bubbles. Additionally, the L2 samples consistently exhibited higher bubble counts compared to H1. Differential scanning calorimetry (DSC), small-angle X-ray scattering (SAXS), and wide-angle X-ray diffraction (WAXD) analyses revealed that increased cooling rates led to a reduction in crystallinity. Upon cooling, increasing crystallinity initially enhanced bubble nucleation, then hindered it. These findings support polymer foam design for diverse applications.