Cell Nucleation and Growth Study of PP Foaming with CO ₂ in a Batch-Simulation System 2006-01-0507

TPO is being used to make automotive parts for its number of advantages: i) low temperature flexibility and ductility, ii) excellent impact/stiffness/flow balance, iii) excellent weatherability, and iv) free-flowing pellet form for easy processing, storage, and handling. However, by foaming TPO, due to its higher rigidity-to-weigh ratio, it would offer additional advantages over the solid counterparts in terms of reduced weight, reduced material cost, and decreased fuel usage without compromising their performance. Since a major component in TPO is polypropylene (PP), understanding PP foaming behaviours is an important step towards understanding TPO foaming. For foam materials, cell density and cell size are two significant parameters that affect their material properties. In this research, we observed the cell nucleation and initial growth behaviours of PP foams blown with CO₂ under various experimental conditions in a batch foaming simulation system. We found that the nucleating agent content, initial temperature, pressure (i.e., gas content), and pressure drop rate during foaming have a significant effect on cell density and cell growth. Based on the experimental results, optimum foaming conditions and effective processing strategies for PP-CO₂ system are suggested.