Study On Dynamic Bubble Nucleation Mechanism In Cellular Plastics

In the foam process of cellular plastics, bubble nucleation has decisive effect on cell density and cell distribution, so it is the crucial factor to control the foam quality. Almost all the former work on bubble nucleation of cellular plastics is based on classical nucleation theory. Because classical nucleation theory supposes that bubble formation is occurred under static state, it doesn't reflect the effects of dynamic factors on bubble nucleation, such as shear and elongation. Recently, some experimental results show that shear flow has significant influences on bubble nucleation. However, it is still lack of reasonable theoretical analysis on bubble nucleation, and bubble nucleation mechanism made no essential progress. In order to improve the bubble nucleation theory to instruct practical appliance; following researches are done in this paper.By combining macromolecule orientation, size distribution of voids among molecule chains and bubble nucleation, the microcosmic mechanism of the effect of dynamic factors such as shear and elongation on bubble nucleation are put forward for the first time in this paper. Some experimental phenomena reflecting the effect of shear flow on bubble nucleation behavior, observed by Han or Lee, can be explained by the mechanism logically, as follow. (1) During the foam extrusion, macromolecules are oriented in the shear flow field. As the orientation degree increases, size difference of voids becomes smaller, so the gas supersaturates degree needing for bubble nucleation increases. Therefore, bubble nucleation is put off In the transaction direction of extrusion die, macromolecule orientation degree increases from the centerline to the die surface. Therefore, bubble formation occurred foremost at the centerline of the die, then gradually extend to the die surface. This can explain the experimental results observed by Han JH and Han CD. (2) As shear rate increases, molecule orientation degree increases while size difference of voids becomes smaller. Once the bubble nucleation begins, the number of voids acting as nucleating sites become larger. Therefore, bubble nucleation rate becomes larger. (3) At the centerline of straight die, molecule orientation is approximately omitted, so the critical pressure starting to nucleation is irrespective...