Characterization of low‐molecular‐weight oligomers in recycled poly(ethylene terephthalate)

Low‐molecular‐weight compounds extracted from recycled poly(ethylene terephthalate) (PET) from post‐consumer soft‐drink bottles, treated at 150–220°C in vacuo or extruded at several temperatures and two different mass flow rates, were characterized by high performance liquid chromatography (HPLC), size exclusion chromatography (SEC) and desorption chemical ionization/mass spectrometry (DCI/MS). Among these products, cyclic and linear oligomers (polymerization degree 2–6) were identified, and their distribution was determined. As a consequence of vacuum treatment of PET, the overall oligomer content decreases with increasing temperatures. Conversely, the extrusion at crescent temperatures causes an increase of the oligomer concentration; the same effect was observed by increasing the residence time. A further effect was represented by the enlargement of molecular weight distributions, i.e. larger rings and longer chains were formed in greater amounts at higher temperatures and residence times; a significant fraction of medium‐molecular‐weight oligomers (M̄ n = 1400–1900) was also formed.