Synergetic Effect of α-ZrP Nanosheets and Nitrogen-Based Flame Retardants on Thermoplastic Polyurethane

A supramolecular self-assembly method was used to prepare melamine cyanurate/alpha-ZrP nanosheets (MCA@alpha-ZrP) as a novel hybrid flame retardant for thermoplastic polyurethane (TPU). Microstructure characterization showed a uniform dispersion with strong interfacial strength of the MCA@alpha-ZrP hybrid within the TPU matrix, leading to simultaneous enhancements in both mechanical and fire-safety properties. The TPU/MCA@alpha-ZrP nanocomposite exhibited 43.1 and 47.0% increments in tensile strength and fracture energy, respectively. Thanks to the platelike structure of alpha-ZrP coupled with the dilution effect of MCA (releasing nonflammable gases), the hybrid MCA@alpha-ZrP reduced the peak heat release rate of TPU by 49.7% in comparison with 15.8 and 35.4% for TPU/MCA and TPU/ alpha-ZrP composites, respectively. The fire performance index of TPU is significantly promoted by 90% upon adding the MCA@alpha-ZrP hybrid. Additionally, LOI and UL-94 tests showed high flame-retarding characteristics for the MCA@alpha-ZrP hybrid. For example, LOI increased from 20.0% for neat TPU to 25.5% for the MCA@alpha-ZrP hybrid system, and it was rated V-1 from the UL-94 test. Furthermore, the smoke production and pyrolysis products were significantly suppressed by adding the MCA@alpha-ZrP hybrid into TPU. Interfacial hydrogen bonding, the dilution effect of MCA, forming a "labyrinth" layer, and catalytic action of alpha-ZrP nanosheets synergistically improved both the mechanical performance and flame retardancy of TPU nanocomposites. This work provides a new example of integrating traditional flame retardants with functional nanosheets to develop polymeric nanocomposites with high mechanical and fire-safety properties.