Advances in Bio-Based Intumescent Flame Retardancy for Natural Fibre Thermoplastic Composites

This work investigates fully bio-based flame-retardant (bioFRs) systems for polypropylene
(PP) composites reinforced with flax fabrics, addressing the inherent flammability of natural fibre-reinforced thermoplastics. A dual-phase strategy combining bio-based additives in the matrix with chemically treated fabrics was implemented. Two bio-based flame retardants,
phosphorylated carbonized cellulose (PCC) and a phytic acid - piperazine salt (PA–PPN), were prepared, evaluated and compared with a conventional ammonium
polyphosphate/pentaerythritol (APP/PER) system. Fire performance was assessed by mass loss cone calorimetry to examine the formation of protective intumescent char layers relevant for thermal shielding applications. Treated fabrics significantly influenced fire behaviour, promoting earlier heat release while improving char cohesion and structural integrity. Among the formulations, PA–PPN showed the highest efficiency, enabling the formation of a cohesive and expanded char. When combined with pentaerythritol and melamine, PA–PPN-based system achieved fire performance comparable to APP/PER, with a peak heat release rate of ~230 kW·m⁻², despite a much lower phosphorus content in the blend (2.8 wt.% vs. 9.45 wt.%).
These findings highlight the strong potential of phytic acid-derived systems as sustainable and efficient flame-retardant solutions for natural fabric-reinforced thermoplastic composites.