Blister-Promoted Bubble Growth in Viscous Polymer Melts

ABSTRACTResidual monomer and other low molecular weight volatile components are removed from polymer melts in a devolatilization step involving bubble formation and growth. Polymer strands containing residual volatiles were extruded into a heated and evacuated devolatilization tank and were then frozen by the flow of cooling water. They were subsequently fractured in liquid nitrogen to reveal their cross-sections and examined in a scanning electron microscope (SEM).SEM observations revealed a previously unknown growth phenomenon in which devolatilization was seen to proceed through a ‘blistering’ mechanism. We discovered that volatile bubbles growing in the melt are fed by the formation of blisters on their inner surfaces. These blisters are formed by the coalescence of a growing bubble and the many satellite micro-bubbles formed around it as it expands. We propose a general mechanism for bubble growth in which we have shown that heterogeneous bubble nucleation in the core, which is governed by the degree of superheat, plays a major role in determining the overall rate of devolatilization. Tensile stresses accompanying bubble growth may result in a local increase in superheat by reducing the local pressure in the melt. This additional superheat combined with the possible accumulation of impurities on the macrobubble surface may be sufficient to increase the nucleation rate of microbubbles in the melt adjacent to the growing bubble, resulting in the large number of blisters formed on the bubble surface.