Quantification of Colour Formation in PET Depending on SSP Residence Time, Temperature, and Oxygen Concentration

A systematic approach was carried out to quantify the discolouration of different poly(ethylene terephthalate) (PET) samples such as virgin bottle-grade resins, bottle-to-bottle recycled chips, and bottle-to-bottle recycled flakes. The PET samples were treated in a fixed bed reactor in the solid- state. The possible influencing parameters such as residence time, temperature, and oxygen partial pressure were studied by analysing the CIELAB b*-coordinate (axis - yellow to blue) with a spectrophotometer at the end of the experiment. The PET samples were analysed once as chips respectively flakes retained from the experiments and again as plates obtained from a following standardised melting procedure. The combination of experiments and literature review results in the proposal of different discolouration mechanisms, which are influenced by temperature, residence time, and oxygen concentration. Using the design of experiments (DoE) approach, a mathematical model was compiled that can be used to predict the yellowing of the different PET grades.