Tuning High and Low Temperature Foaming Behavior of Linear and Long-Chain Branched Polypropylene via Partial and Complete Melting

While existing foam studies have identified processing parameters, such as high-pressure drop rate, and engineering measures, such as high melt strength, as key factors for improving foamability, there is a conspicuous absence of studies that directly relate foamability to material properties obtained from fundamental characterization. To bridge this gap, this work presents batch foaming studies on one linear and two long-chain branched polypropylene (PP) resins to investigate how foamability is affected by partial melting (Method 1) and complete melting followed by undercooling (Method 2). At temperatures above the melting point, similar expansion was obtained using both foaming procedures within each resin, while the PP with the highest strain hardening ratio (13) exhibited the highest expansion ratio (45 ± 3). At low temperatures, the foamability of all resins was dramatically improved using Method 2 compared to Method 1, due to access to lower foaming temperatures (<150 °C) near the crystallization onset. Furthermore, Method 2 resulted in a more uniform cellular structure over a wider temperature range (120–170 °C compared to 155–175 °C). Overall, strong extensional hardening and low onset of crystallization were shown to give rise to foamability at high and low temperatures, respectively, suggesting that both characteristics can be appropriately used to tune the foamability of PP in industrial foaming applications.

Microcellular injection molding of polypropylene and glass fiber composites with supercritical nitrogen

Microcellular injection molding of polypropylene and glass fiber composites (PP-1684/GF-950) was performed using supercritical nitrogen as the physical blowing agent. Based on design of experiment matrices, the influences of glass fiber content and operating conditions on cell structure, glass fiber orientation and mechanical properties of molded samples were studied systematically. The results showed the cell morphology and glass fiber orientation of foaming parts were definitely influenced by the cooling and shear effects. The mechanical properties of foamed polypropylene–glass fiber composites could be effectively enhanced by improving the cell morphology, dispersion state and orientation of the glass fiber at optimal weight percentage [Formula: see text]. And the optimal conditions for injection molding were obtained by analyzing the signal-to-noise ratio analysis of the mechanical properties of the molded samples, which were a shot size of 36 mm, a supercritical N2 weight percentage of 0.4%, an injection speed of 60%, a melt temperature of 190℃ and a mold temperature of 70℃. The molded specimens of polypropylene–glass fiber composites, produced under those optimal conditions, exhibited very uniform fiber dispersion and microcellular structures with an average cell size less than 30 µm. And the mechanical properties normalized by weight ratio of the microcellular samples were increased significantly, especially the impact strength.