PREDICTION OF CELLULAR STRUCTURE IN FREE EXPANSION OF VISCOELASTIC MEDIA

A systematic model is presented for a free expansion polymer foaming p rocess that includes simultaneous nucleation and bubble growth. An inf luence volume approach, which couples nucleation and bubble growth, is used to account for the limited supply of dissolved gas. The melt rhe ology is described using the Larson viscoelastic model. The initial co nditions are obtained at the upper bound of critical cluster size unde r conditions of elastic deformation. The resulting set of equations ar e solved using a combination of numerical techniques. A parametric stu dy is conducted to examine the effects of key process variables on bub ble growth, nucleation, and final bubble size distribution. It shows t hat the factors influencing nucleation and growth affect the ultimate bubble sizes and their distribution. The Gibbs number, a dimensionless measure of the barrier to overcome for nucleation, has the strongest impact on the cellular structure of the foam. (C) 1998 John Wiley & So ns, Inc.