Polyphosphazene based core-shell structure using MXene as a hard template: Improving the toughness and fire safety performance of UPR composites

• Constructed Ti₃C₂Tₓ@PPZ using MXene as a hard template with polyphosphazene-based COFs encapsulation. • Reduced PHRR by 54.2 % and THR by 25.5 % in UPR composites. • Achieved simultaneous 179 % increase in impact strength for UPR composites. • Cleverly designed to balance flame retardancy and mechanical properties. • Multivariate coupling fire prevention mechanism. Unsaturated polyester resin (UPR) offers distinct advantages in processability and versatility. However, its practical implementation in demanding scenarios is frequently hampered by high flammability and inherent brittleness. Developing efficient multifunctional flame retardants that simultaneously enhance fire resistance and toughness remains a critical need in material science. Addressing this challenge, this study employs the design concepts of “synergistic flame retardancy enhancement” and “nanotechnology toughening” to ingeniously construct a novel multifunctional core-shell flame retardant, Ti 3 C 2 Tx@PPZ. The core consists of carbonized titanium nanosheets (Ti 3 C 2 Tx), meticulously encapsulated within a shell of polyphosphazene-based covalent organic frameworks (PPZ-COFs), achieving synergistic performance optimization and complementarity. Upon integrating Ti 3 C 2 Tx@PPZ into UPR, the resulting composites exhibit remarkable concurrent improvements in both fire resistance and mechanical toughness. This enhancement is primarily attributed to the synergistic interaction between Ti 3 C 2 Tx and PPZ. At a loading of 2 wt% Ti 3 C 2 Tx@PPZ, the composite achieved a significant 54.2% reduction in peak heat release rate (PHRR), with total CO production (COP) showing a similar downward trend. Furthermore, a minimal loading of 0.5 wt% yielded the highest impact strength (10.9 MPa), representing a 179% increase compared to pure UPR. Crucially, this work pioneers the integration of COFs design principles into flame retardant architecture. Detailed analysis reveals the underlying flame-retardant and toughening mechanisms driven by multiple synergistic effects, providing valuable insights for the development of next-generation high-efficiency multifunctional flame retardants.