FOAM-ENHANCED DEVOLATILIZATION OF POLYSTYRENE MELT IN A VENTED EXTRUDER

Vented extruder devolatilization (DV) of PS melt containing 6, 000 ppm styrene was studied by scanning electron microscopy (SEM) and video p hotography. Vacuum DV of a polymer is accompanied by foaming, which st arts instantaneously upon supersaturation of the stretched melt and is enhanced at higher speeds of the vented extruder screw. As the volati les are removed from the melt, foaming gradually ceases, starting with the pushing flight of the screw. The experimental installation design allowed us to quench the polymer melt in the DV zone at various stage s of the process. Samples taken from four areas of the channel width w ere investigated by SEM. Bubble nucleation in the melt appears to take place mainly in the border area adjoining the gas phase. In the shear field caused by screw rotation, large bubbles become noticeably elong ated. Their surface, as well as the free surface of the melt, is cover ed with blisters, 1-100 mum in size. Microblisters are often concentra ted in areas subjected to stretching. Calculations of cooling due to v olatile evaporation and of heating due to viscous dissipation near a g rowing bubble shows that the process of foam-enhanced DV of a PS/styre ne system can be regarded isothermal if the initial volatile concentra tion does not exceed approximately 1%.