Study on the definition, mechanism and controllability of secondary bubbles based on the bubble nucleation model in injection foaming polypropylene

The process of nucleation and growth of polypropylene foam was observed by using visualizations of the mold-opening foam injection molding (MOFIM) and free foaming (FREEF). The fitting of the mathematical model formula was used to supplement the judgment conditions of the secondary bubbles to explore the generation process and formation conditions of the secondary bubbles. The results of changes in blowing agent content and melt temperature proved the rationality of the judgment basis and the appearance of secondary bubbles started from the late stage of balanced-foaming. Then, the combined action of several nucleation mechanisms led to the emergence of secondary bubbles, which was observed utilizing glass fibers as nucleating agents and tracers. The data for the two foaming modes indicated that the formation of secondary bubbles is closely related to temperature and pressure drop. The bubble nucleation model was amended and validated by regulating the temperature variation in the mold cavity to control the number of secondary bubbles, which enabled the nucleation process of secondary bubbles to be fitted to an "S-shaped" curve. Finally, a controllable number of secondary bubbles was obtained from the bimodal bubble structure. Herein, this study enriches our understanding of the formation process of secondary bubbles, and provides theoretical guidance for fabricating high density, small size foam materials.

Autoclave foaming and steam-chest molding of polypropylene/polybutene-1 blend bead foams and their crystallization and mechanical properties

Expanded polypropylene (EPP) foams have showed wide applications in our daily life, such as automotive and packaging. Usually, autoclave foaming combined with steam-chest molding is the main artwork to prepare the high-precision EPP foam products. However, the foaming behavior of EPP and the excessive pressure required for molding still need to be further improved, which is great significance for energy saving and cost saving, etc. Herein, this study finds that adding a certain amount of polybutene-1 (PB-1) into the PP can help to reduce the temperature and pressure required for foaming/molding, and to broaden the foaming temperature. For example, in order to make the foam beads bonding well and with the expansion ratio of 20, the molding pressure should be higher than 2.7 bar for Neat PP foams, but just 1.5 bar for PP/PB-1 mixtures. Moreover, the effects of PB-1 content on the crystallization properties and foaming/molding behaviors of the PP/PB-1 bead foams are illustrated, and then the mechanical properties are also studied. Furthermore, the low-pressure foaming strategy presented here is beneficial for reducing the barriers of energy consumption and promoting the development of new bead foam materials.