Hierarchical cyclic phosphates@MIL-101 toward improving thermal stability, fire retardancy, UV shielding, and mechanical performance of thermoplastic polyurethane based on nanoconfinement engineering

To address the challenge of balancing fire safety and mechanical properties in thermoplastic polyurethanes (TPU), this study developed a CP@MIL-101 hybrid by confining liquid cyclic phosphates within the pore structure of MIL-101 using a hierarchical design strategy. The results demonstrated that the introduction of CP@MIL-101 slightly improved the thermal stability of TPU. Notably, the residual char yield increased significantly from 0.1 % to 9.3 % with only 0.5 % loading. With just 2 wt% loading of CP@MIL-101, the TPU composite achieved a UL-94 V-0 rating (3 mm) and exhibited a 20.4 % increase in the limiting oxygen index (LOI), reaching 26.0 %. Furthermore, significant reductions in peak heat release rate, total heat release, and fire growth rate index were observed, indicating enhanced fire safety of the TPU composites. Characterization analyses suggested that the improved fire retardancy resulted from the synergistic effects of MIL-101 and cyclic phosphates in both the condensed and gas phases, through the formation of a protective char layer and free radical quenching. Additionally, incorporating 2 wt% CP@MIL-101 endowed the TPU matrix with exceptional ultraviolet shielding properties due to unique electron transitions absorption. Simultaneously, the tensile strength of the TPU increased by 15.85 % compared to neat TPU, demonstrating mechanical reinforcement. This work establishes a strategic approach for developing multifunctional flame retardants and fire-safe TPU composites, paving the way for their application in high fire-safety environments.