Enhancement of Poly(Lactic Acid) Fire Retardancy Through the Incorporation of Sludge Residue as a Synergistic Additive

The escalating global challenge of waste production underscores the urgency for innovative waste management solutions. Sewage sludge, a byproduct derived from anaerobic digesters of wastewater treatment, was investigated as a flame-retardant synergist in Poly(Lactic Acid) (PLA). Micronized sludge was combined with ammonium polyphosphate (APP) at different ratios. The formulation containing (4:1) APP:Sludge exhibited enhanced flame retardancy compared to APP alone, achieving higher Limiting Oxygen Index (LOI) values and a V-0 rating in the UL-94 test. Cone calorimeter analysis further confirmed that the sludge contributed to reducing heat release and smoke generation. SEM–EDS analysis indicated that microcrystals, mainly composed of phosphorus and calcium oxides from APP and sludge, likely acted as protective barriers against heat transfer. In addition, filament extrusion demonstrated that sludge incorporation is compatible with 3D printing. This approach preserved structural integrity, sustainably utilized sewage sludge, and reduced reliance on commercial flame retardants. Integrating sludge as a synergist offers a promising solution for waste management and safer, more sustainable flame-retardant materials, supporting a circular economy.