Blister-Promoted Bubble Growth in Viscous Polymer Melts
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BLISTER-PROMOTED BUBBLE GROWTH IN VISCOUS POLYMER MELTS RAMON J. ALBALAK*, ZEHEV TADMOR, AND YESHAYAHU TALMON Department of Chemical Engineering, Technion - Israel Institute of Technology, Haifa 32000, Israel *Current address: Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139
ABSTRACT Residual 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. INTRODUCTION In the manufacturing of most polymers unreacted monomer, solvents and other low molecular weight volatile components are frequently removed in a devolatilization step. Industrially this step is carried out in equipment belonging to one of two major types. The first type consists of rotating machinery which is usually a vented modification of classical polymer processing equipment such as ,single- and twin-screw extruders. The second type of devolatilizing equipment. -consists of so-called falling-strand devolatilizers in which polymer melt containing volatiles to be removed is extruded as thin strands into a vacuum tank. As the strands fall towards the bottom of the tank their volatile content is reduced [1,2]. It is now recognized that devolatilization is a complex process involving boiling, foaming, and foam breaking. Early work [3,4] treated devolatilization of polymer melts as a simple surface renewal and diffusion process in spite of large deviations between diffusion coefficients calculated from experimental data and those that could be reasonably expected for the styrene-polystyrene system examined. Later Newman and Simon 15] modeled the process of falling-strand d
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