Is Recycled Polypropylene Suitable for Flame-Retarded Applications?

In this study, the performance of an intumescent flame-retardant (IFR) system in recycled polypropylene (PP) was investigated. To this aim, virgin and reprocessed PPs were melt-compounded with 21 wt % of an IFR consisting of piperazine pyrophosphate (PAPP) and melamine polyphosphate (MPP) (2:1 ratio), and the obtained materials were characterized in terms of morphology and rheological and combustion behavior. Cone calorimetry results showed that although both IFR-containing materials exhibited a similar reduction in heat release rate as compared to the unfilled matrices, the recycled PP-based system displayed a higher time to ignition and a pronounced delay in the second heat release rate peak. The observed behavior was explained by considering the formation of a more compact and denser char, likely promoted by the finer dispersion of IFR particles achieved in the low-viscosity recycled PP matrix. Furthermore, both materials achieved a V-0 rating in UL-94 testing and comparable LOI values. Finally, the assessment of the mechanical behavior of both systems demonstrated that the utilization of recycled PP does not compromise the tensile and flexural strength of the material, notwithstanding a slight decrease in ductility as compared to the virgin PP-based counterpart. Overall, these findings demonstrate that recycled PP can be effectively used in flame-retardant formulations, broadening its potential applications while reducing dependence on virgin materials and supporting the development of circular economy approaches.