Recycled PET foaming: Supercritical carbon dioxide assisted extrusion with real-time quality monitoring

Foaming of recycled poly(ethylene terephthalate) (rPET) was performed by supercritical carbon dioxide (sc-CO2) assisted extrusion. The intrinsic viscosity (IV) of rPET was increased from 0.62 dl/g to 0.87 dl/g using an epoxy-functional chain extender, which provided adequate rheological properties for cell stabilization so that an apparent density of less than 0.15 g/cm3 became achievable. Homogeneous and talc induced heterogeneous crystal and cell nucleation, subsequent cell growth and stabilization processes were examined using differential scanning calorimetry (DSC) and scanning electron microscopy (SEM), respectively. It was found that using talc the crystallization temperature increases which results in smaller cell size distribution. A strong correlation was evinced between the apparent density and the Fourier transform near-infrared (NIR) spectrum of the foamed rPET samples enabling quick and non-destructive characterization. Accordingly, NIR spectroscopy is demonstrated as a suitable method for in-line quality monitoring during extrusion foaming of recycled PET, being especially prone to quality fluctuations