Microcellular PET Foams Produced by the Solid State Process

Abstract

The basic procedure to produce microcellular foam is a two stage process. In the first stage, a polymer sample is placed in a high pressure, nonreactive gas environment at a temperature below the glass transition temperature and allowed to absorb gas. In the second stage, the pressure is released, forming a supersaturated solution of gas and polymer. The specimen is then heated to or above the glass transition temperature of the original polymer. This causes bubbles to nucleate and grow. The majority of work on microcellular foams to date has been conducted on the polystyrene-nitrogen system. This chapter studies the resultant foam parameters (density, average cell diameter, and cell nucleation density) in terms of process control parameters (saturation pressure (PSAT), saturation time (tSAT), saturation temperature (TSAT), foaming pressure (PF), foaming time (tF), and foaming temperature (TF). Additionally, the role of crystallinity is studied. Crystallinity is important to the microcellular process because crystallites provide nucleation sites for bubble growth. Crystallization increases in PET during the sorption of CO2 but the two processes are not coupled. The carbon dioxide-induced crystallinity is the dominant factor affecting bubble nucleation and growth in PET foams produced by the solid state process. The crystallized PET, when foamed, shows the highest bubble nucleation densities achieved in this system. High-nucleation densities are usually coupled with small bubble sizes. Thus the carbon dioxide induced crystallization can be employed to obtain smaller bubbles than are seen in low crystallinity or amorphous PET foams.