The processing–structure relationships in thin microcellular PET sheet prepared by compression molding

The microcellular foaming of a thin (100–250 μm) poly(ethylene terephthalate) (PET) sheet by compression molding is the focus of our investigation. A microcellular PET sheet can be successfully prepared by first preheating with a blowing agent matrix, then saturating the PET sheet by gas generated from the blowing agent decomposition, and lastly, applying a simultaneous pressure and temperature quench. Seven influencing microcellular structure factors were investigated systematically, namely the preheat time, saturation time, saturation pressure, the upper-plate temperature, the lower-plate temperature, blowing agent content, and PET sheet thickness. The relative importance of the individual processing parameters was determined. The results showed that saturation time, saturation pressure, and PET sheet thickness had a greater effect on the cell size and cell density, whereas saturation time, saturation pressure, and the upper-plate temperature were more important for the relative density. Also, the effects of saturation pressure and the upper-plate temperature on the microcellular structure in the microcellular PET sheet were explained by classical nucleation theory, and the effect of the PET sheet thickness on the microcellular structure was explained by the loss control of gas diffusion.