Upcycling post-consumer PET bottles into low-density films and 3D-printed materials.

Recycled polyethylene terephthalate (rPET) from mixed-color bottle waste is typically downcycled due to its variable composition and reduced melt strength, limiting their contribution to circular economy strategies and high-value applications. In this work it is proved that gas dissolution foaming enables the upcycling of two industrial rPET, light-blue PET (PET_B) and color recycled PET (PET_C) into low-density microcellular films and 3D printed-parts. A comprehensive materials characterization allows to establish the optimum processing window for these materials. This way optimized microcellular structures were produced at mild saturation conditions (15 MPa, 50 °C). PET_B shows cell sizes close to 2 microns together with relative densities of 0.2 to 0.28, outperforming virgin PET. PET_C, despite containing several non-miscible contaminants, also leads to stable cellular materials with cell sizes from 5 to 18 microns. Finally, we demonstrate for the first time that rPET 3D-printed components can be foamed while preserving their overall geometry, undergoing a controlled volumetric expansion and achieving a 40 % density reduction with homogeneous microcellular structure. Overall, this study highlights the potential of gas dissolution foaming as a robust upcycling route for heterogeneous PET waste streams, enabling lightweight materials and supporting circular manufacturing strategies.