Functionalized bio-based intumescent flame retardant with ZIF-67 as an effective fire safe agent for thermoplastic polyurethane composites with enhanced mechanical properties

Exceptional fire safety, mechanical flexibility, ultraviolet (UV) resistance, and transparency are critical characteristics for thermoplastic polyurethane (TPU) materials in rapidly evolving industrial applications. Nevertheless, achieving a harmonious balance among these properties remains a significant challenge. In this paper, bio-based phytic acid/chitosan intumescent flame retardant (CaCS@PA) and cobalt metal-organic framework (ZIF-67) were incorporated into TPU, forming a densely crosslinked network through non-covalent interactions including hydrogen bonding and π-π stacking. The resulting TPU/ZIF-67-CaCS@PA composites show significant fire safety performance, with a 49.8% reduction in peak heat release rate and a 30.0% decrease in peak smoke production. Analysis of the condensed and gas phases indicated that the flame-retardant mechanism of ZIF-67-CaCS@PA primarily relies on the ability of ZIF-67 to influence the migration of intumescent flame retardant (IFR) gases and catalyze carbonization. This process accelerates the formation of a highly graphitized, thermally stable, and compact char layer, which acts as an effective protective barrier. Owing to the synergistic effect of supramolecular interactions, the tensile strength and elongation at break of the TPU composites were significantly enhanced to 39.6 MPa and 787.4%, representing increases of 155.6% and 27.7% prior to the pure TPU, respectively. Additionally, TPU/ZIF-67-CaCS@PA displays excellent transparency and UV resistance, effectively mitigating aging, yellowing, and strength degradation under UV exposure. This study successfully achieved a desirable balance between fire safety, mechanical performance, UV shielding, and transparency in TPU composites, offering valuable insights and potential advancements for both industrial applications and academic research.