Strategies and analytical procedures for a sustainable plastic waste management. An application to poly (ethylene terephthalate) and polylactide in the packaging sector.

Abstract
The purpose of this Ph.D. Thesis was to assess the influence of different waste management processes such as material, energetic and biological valorisations on two key polyesters of the packaging industry, the current non-renewable poly (ethylene terephthalate) (PET) and the potential candidate to replace it in a near future, the bio-based polylactide (PLA). For that reason, several pilot plants were used to simulate the degradation conditions undergone by PET and PLA under mechanical recycling, pyrolysis, combustion and burial in soil. The changes were monitored by Differential Scanning Calorimetry (DSC), Dynamic-Mechanical-Thermal Analysis (DMTA), Thermogravimetric analysis (TGA), Fourier-Transform Infrared Spectrometry (FTIR), 2D-IR correlation spectroscopy for Evolved Gas Analysis (EGA), MALDI-TOF Mass Spectrometry, Scanning Electron Microscopy (SEM), Melt-Flow Rate (MFR), Tensile and Charpy testing, and Viscosimetry. Some strategies and analytical procedures were proposed, developed and applied to establish reliable specific parameters to be used as indicators of degradation and thus monitor the influence of each valorisation process on the quality of the material. The behaviour of mechanically recycled PET and PLA was assessed in terms of chemical, microstructural, thermal and mechanical properties. A general loss of performance was shown for PET and PLA reprocessed once and twice respectively. Afterwards, the properties of PLA recyclates were better in relative terms than those of PET recyclates. The thermal and thermo-oxidative decompositions caused by pyrolysis and combustion processes were evaluated concerning the thermal stability, evolved gases and kinetics of decomposition. The use of controlled combustion was stressed for both polymers since less energy was necessary to trigger the decomposition, and the mixture of evolved gases was more homogeneous. Finally, the use of combined thermal analysis techniques was proven as a reliable procedure to monitor the degradation subjected by PLA under burial in soil. The replacement of PET by PLA remains therefore as a suitable option from the point of view of the plastic waste management, since PLA offers better performance after reprocessing than PET, and its energetic valorisation - provided the biological valorisation cannot fulfil the demand - is possible and effective.