Beyond processing methods: the impact of the drying technique on PET performance

Plastic materials are essential in modern societies, and their recycling is a very important step in the transition towards a circular economy. PET (polyethylene terephthalate) is currently considered one of the plastics with the greatest potential for recycling due to its good properties and ability to be processed by industrial methods. However, those processes may deteriorate PET properties, since they involve high temperatures and shear stresses that together with the presence of moisture (due to the strong hydrophilic nature of PET) can provoke hydrolysis of the polymer with a corresponding loss in molecular weight. When working with recycled PET the excess moisture contained in the material may be higher. It is evident the huge importance of the drying stage in the processing of this resin. However, it has not been yet studied the influence of different techniques on the final mechanical properties of processed parts and on the processing cycle (time/costs). In this work, two drying techniques were applied: a convection oven—widely used in the industry—and an infrared oven. The aim was to study the influence of this part of the processing cycle on the final mechanical properties of PET parts obtained from both virgin material and waste soda bottles. Results confirmed that drying by infrared technology reduces drying time by 80% which implies a drastic reduction in total processing time for both types of PET (virgin and recycled). Even though no significant differences were found in the conventional and non-conventional mechanical properties, differences in the propagation mode under fracture were noticeable. These were found to be due to unintentional esterification reactions induced by IR radiation in thick materials, probably due to heat concentration, that should be taken into account when this drying technique is intended to be used.