Fire-safe and high-toughness PLA composites through Dual-Strategy: Ultralow-Loading PAPP@CoNi-LDH and uniaxial pre-stretching

In this work, we reported the first synthesis of an organic-inorganic hybrid flame retardant (PAPP@CoNi-LDH). This was achieved through the electrostatic self-assembly of piperazine pyrophosphate (PAPP) and cobalt–nickel layered double hydroxide (CoNi-LDH). Compared to PLA/5 wt% PAPP, the PLA/4.5 wt% PAPP/0.5 wt% PAPP@CoNi-LDH composite (FRPLA-0.5) achieved a superior Underwriters Laboratories Vertical Burning Test rating (UL-94, from V-2 to V-0) and higher Limiting Oxygen Index (LOI, from 22.5 % to 28.0 %), demonstrating the exceptional flame-retardant efficiency of PAPP@CoNi-LDH at minimal loading. Furthermore, the superior char-catalyzing capability reduced the peak heat release rate of FRPLA-0.5 from 556 to 490 kW/m 2 versus pure PLA. Moreover, uniaxial pre-stretching was applied to fabricate flame-retardant PLA composites with superior mechanical properties. For FRPLA-0.5 subjected to 2.0 × pre-stretching (pre-stretching ratio = 2.0), tensile strength surged from 54.7 MPa to 96.6 MPa, elongation at break increased from 6.1 % to 63.7 %, and impact strength rose remarkably from 0.7 kJ/m 2 to 87.2 kJ/m 2 . This dual-strategy design achieved high-performance PLA composites with simultaneously enhanced fire safety and mechanical robustness.