Modelling of PET Quality Parameters for a Closed‐Loop Recycling System for Food Contact

Summary: A rigorous process model has been developed which describes a closed‐loop recycling system for PET beverage bottles. The reaction / mass transport model is aimed at the dominant quality parameters such as intrinsic viscosity, concentration of acetaldehyde, concentration of carboxylic end‐groups, and concentration of vinyl end‐groups, respectively. The model covers the main process steps being preform production (injection moulding), drying, solid‐state polycondensation, and melt filtration. The simulation reveals that after a single recycling loop all the relevant quality parameters achieve the specification, if certain temperatures, residence times, and surface areas for degassing are provided during the recycling process. Another simulation showed the evolution of quality parameters in PET being subjected to an “infinite” number of recycling loops in a closed system. In this case, the concentration of acetaldehyde and vinyl end‐groups decreases with the number of recycling loops, which is a desired effect. On the other hand, the concentration of carboxylic end‐groups increases with every completed recycling loop. Higher concentrations of carboxylic end‐groups make the polymer more susceptible to hydrolysis and increase the SSP process time needed to achieve the specified intrinsic viscosity for carbonated soft drink bottles. To overcome this problem, the recycled PET has to be blended with a certain amount of virgin PET in industrial processes.