Bubble nucleation and growth dynamics in polymer melts

The present work is concerned with identifying the important parameters in bubble nucleation and growth and determining their effects on foam structures obtained by free expansion foaming. This work involves extending classical nucleation theory to include elastic and non-ideal solution effects. Laboratory studies characterize gas solubility/diffusivity in polymers and surface tensions of polymer with dissolved gas. Free expansion foaming studies characterize property, structure, and operating condition effects. The experimental studies were conducted on LDPE/HDPE samples, and mixtures thereof, using N{dollar}sb2{dollar}, Ar, and CO{dollar}sb2{dollar} as the gas blowing agents. In addition, the effects of solid nucleating agents on the foaming process were also studied. With the cahn balance, the solubility/diffusivity data were obtained by thermogravimetric method and the surface tension by a Wilhelmy method over 250-350{dollar}spcirc{dollar}F and pressures upto 1800 psi. The free expansion foaming was conducted in a specially constructed high temperature/pressure cell.; The final foam structure is largely governed by the bubble nucleation events which are affected by gas solubility, surface tension, and the presence of agents to promote heterogeneous nucleation. Viscous and elastic effects and the non-ideal mixing effects of gas-polymer solution tend to decrease nucleation.; Highly non-ideal solubilities of gases in LDPE/HDPE increased with temperature unlike those observed in the low pressure Henry's law region. A modified Flory-Huggins theory represents the data quite accurately with a free parameter (interaction parameter). Gas diffusivity was insensitive to dissolved gas concentrations upto 120 ATM and increased slightly with temperature.; The Wilhelmy method on the Cahn balance was found to be an effective method for measuring surface tension of polymer melts with dissolved gas. The surface tension of the LDPE-N{dollar}sb2{dollar} decreases significantly with dissolved gas concentration. Such surface tension lowering is critically important to achieve efficient nucleation in foaming processes.; Low density foams can be obtained using a high solubility gas and allowing the bubble growth state to mature before quenching. Very fine structures can be achieved with high bubble nucleation system. Significant bubble-bubble coalescence occurred which broadened the cell size distributions.; In general these studies provide a number of insights into polymer foam processing mechanism.