Foaming Visualization of Thermoplastic Polyolefin (TPO) Blends with N ₂ 2007-01-0572

Polymers are often blended to create compounds with new or enhanced properties in order to compensate for an individual polymer's weakness or lack of inherent properties. In the field of polymer foaming, polymer blends are also used to generate fine-cell structures via heterogeneous nucleation. Recently, an interest in physical blowing agents, such CO₂ and N₂, has increased because of their low impact on the environment. It has thus become additionally important to pursue research on the foaming of polymer blends employing these particular physical blowing agents in an effort to keep up with the demand for environmentally friendly products. In this study, thermoplastic polyolefin (TPO) blends were prepared with polypropylene (PP) and a metallocene-based polyolefin elastomer (POE) using twin-screw extruders and a batch mixer. A batch foaming simulation experiment with N₂ was performed using a newly developed high-pressure cell featuring two sapphire windows; with the help of a high-speed charge-coupled device (CCD) camera, the windows permitted the observation of the early stage of bubble nucleation as well as the growth behavior of the polymer blends. This technology enabled us to see that phase morphology is directly affected by elastomer content. We also prepared TPO blends with the same composition of PP and POE by different compounding conditions. Even with the same composition, the morphology of the blends was changed by the compounding conditions. Based on the batch foaming simulation results, the effects of elastomer content and blend morphology on cell nucleation and expansion behaviors were investigated and compared. A foam with a high cell density and a uniform cell structure was obtained when the TPO blend was used; it demonstrated a continuous PP matrix with smaller dispersed elastomer droplets.