Self-assembled double core-shell structured zeolitic imidazole framework-8 as an effective flame retardant and smoke suppression agent for thermoplastic polyurethane

The addition of traditional flame retardant additives can usually significantly reduce the flammability of thermoplastic polyurethane (TPU) but has a limited contribution to suppressing smoke release during combustion. To overcome this obstacle, a double core-shell structured zeolite imidazole framework-8 (ZIF-8) in combination with negatively charged phytic acid and positively charged hyperbranched polysiloxane (NH₂-HBPSi) was layer-by-layer self-assembled through the electrostatic method. The decorated NH₂-HBPSi layer effectively improved the interfacial compatibility between the fillers and the TPU matrix. The peak heat release rate (PHRR), total heat release (THR), total smoke production (TSP), and total CO production (TCOP) of the TPU composite containing 2.0 wt% ZIF-8@PA@NH₂-HBPSi were reduced by 35.2%, 25.2%, 65.5%, and 52.8%, respectively, as compared to those of pure TPU. The distinguished fire resistance and smoke suppression were ascribed to the flame retardant effects of Zn, N, P, and Si elements in the ZIF-8@PA@NH₂-HBPSi hybrid. In addition, the TPU/ZIF-8@PA@NH₂-HBPSi composite maintained a high elongation at break of 2178.2% and tensile strength of 46.87 MPa due to the steric effect of NH₂-HBPSi. This work presents a feasible strategy for the design and development of flame retardant fillers to improve fire resistance and inhibit smoke emission while maintaining the high ductility of TPU.