Upcycling PET waste into Al-MOFs with one-step hydrothermal method and its application in flame retardant of PA66

Abstract

We developed a sustainable closed-loop strategy to convert post-consumer PET waste into functional aluminum-based metal-organic frameworks (Al-MOFs) via a one-step hydrothermal method. This process uses PET-derived terephthalic acid as the organic linker. The resulting rod-like Al-MOFs (5–10 μm, MIL-53 structure) exhibit high thermal stability and were incorporated into polyamide 66 (PA66) as flame-retardant additives. At an optimal 4 wt% loading, the PA66/Al composite achieves a 40 % increase in limiting oxygen index (from 25 % to 35 %) and reduces peak heat release rate by 33.04 % (from 727.61 kW·m⁻² to 487.24 kW·m⁻²), while enhancing tensile strength by 19.74 % (from 57.42 MPa to 68.76 MPa). Mechanistic studies confirm that Al-MOFs function through a condensed-phase flame-retardant mechanism: catalytic carbonization promotes continuous char formation, evidenced by a 47.71 % increase in experimental char residue (4.895 %) over the theoretical value (3.314 %) under N₂. Concurrently, in situ-generated Al₂O₃ (XPS Al 2p at 74.6 eV) reinforces the char barrier, suppressing heat/oxygen transfer and volatile release. This effect further reduces peak smoke production rate by 56.38 % (0.1442 to 0.0629 m²·s⁻¹) and decreases CO/CO₂ evolution. The combined flame suppression and mechanical enhancement stem from interfacial interactions between Al-MOFs carboxyl groups and PA66 amide bonds. This work provides a scalable approach to transform plastic waste into high-performance polymer additives that simultaneously impart flame retardancy and reinforcement without compromising material properties.