The evolution of properties of recycled poly(ethylene terephthalate) as function of chain extenders, the extrusion cycle and heat treatment

Abstract

Poly(ethylene terephthalate) (PET) is the most widely used plastic in beverages packaging. It is also the most recycled plastic in the world. It is estimated that 6 million tons of PET are recycled (rPET) each year worldwide. Recycling of this material by melt processing has been the subject of many studies, in order to limit the degradation processes that lead to a significant decrease in the molecular weight (viscosity). Two key points are highlighted: The former is the presence of impurities like adhesive, glue and Poly Vinyl Chloride etc. The latter is the presence of water. These were therefore the main factors of the degradation of rPET. The impurities can be eliminated by a selective recovery and the moisture by a suitable drying combined with the addition of chain extenders namely Caprolactam (CAP) and/or Trimellitic anhydride (TMA). This combination has proved to be very promising since extruded mixtures (rPET/TMA or CAP) have quite acceptable rheological properties especially in terms of intrinsic viscosity, dynamic viscosity and melt flow index (MFI) at low concentration of chain extender. Rheological and FTIR analysis showed that the degradation of rPET becomes more significant from the second extrusion cycle. Finally, DSC analysis showed that T g were not affected by extrusion cycle number; However, cold crystallization temperature T cc2 were significantly affected by heat treatment. The DSC analysis showed also that from the 2nd extrusion cycle, a conversion of heating crystallization temperature (T c) which appeared during the first heating (1st scan) to a melting temperature (T m1) that appeared during the second heating (3rd scan) occurred due to the change of the decomposition mechanism environment (from oxygen environment to that of nitrogen).