Growth and coalescence of bubbles during late stages of polymer foaming processes

A framework has been established for describing growth of the deformed bubbles and thinning of the interstitial films during the late stages of the foaming process of a viscoelastic foam cell. A two-dimensional cell model depicts the geometrical constraints imposed by the existence of neighboring growing bubbles. Larson's constitutive model represents the viscoelastic behavior of the polymer melt. The effect of the intermolecular forces on the stability of the foam and the coalescence of the films is addressed as are shifts in the thermal environment and its influence upon the process dynamics and the final foam characteristics.; An analysis of the results indicates the competing effects of polymer elasticity and the long range intermolecular forces on the thin film hydrodynamic stability. Van der Waals intermolecular forces act to destabilize the thin film between bubbles and cause coalescence, whereas elasticity combats this instability and increases film rupture time. Compared to Newtonian foams, viscoelastic systems exhibit more resistance to deformation and higher stability during disturbances.; The developed computer simulation can be used to study the effects of the process operating conditions and base material properties on the dynamics of the foaming process and the characteristics of the final product. It provides a basis for predicting the lower limits of foam density possible with given polymer systems. Model predictions when compared to experimental data from the literature exhibited excellent matches for light foams (dry foams).