When dealing with applications where plastics could be subjected to combustion and fire, the addition of flame-retardant (FR) additives into the polymers becomes a mandatory requirement. While FR systems are designed on purpose depending on several aspects, such as the specific application, the type of polymer, and the various fire risk scenario, their effectiveness in recycled polymers has not yet been systematically elucidated. On the other hand, considering that FR additives are abundantly present in numerous plastic waste streams, their presence can interfere with the mechanical recycling processes, posing significant challenges for the management of end-of-life FR-plastic parts.

In this study, two different approaches were followed in order to assess (i) the effectiveness of an intumescent FR system as flame-retardant for recycled polypropylene (PP); (ii) the possible modification of the combustion behavior of PP/FR during the reprocessing.

To this aim, virgin and reprocessed PP were melt compounded with an intumescent FR composed of piperazine pyrophosphate (PPAP) and melamine pyrophosphate (MPP) and characterised in terms of combustion, thermal and rheological behaviour. Furthermore, PP/FR was subjected to five reprocessing cycles in a twin-screw extruder (trying to simulate a mechanical recycling process) and characterised after each cycle.

Despite the different combustion behavior of reprocessed PP/FR as compared to the virgin one, the intumescent FR still provided satisfactory flame retardancy properties. In fact, the flame retardancy properties remain largely unaffected by repeated processing cycles, resulting in no change in UL94 classification even if the material is reprocessed up to five times.

These results would pave the way for a greater variety of applications for recycled PP, reducing the reliance on virgin plastic and advancing the circular economy of PP-based waste.