Porous liquids assisted in-situ generated Co-LDH@MOF heterostructure with abundant defects for flame retardant and mechanical enhancement in polyurea composites

Preventing aggregation and inducing homogeneous dispersion of flame retardant nanofillers is critical to enhance the fire safety and mechanical properties of nanocomposites. Although chemical modifications are commonly used to improve the filler's compatibility with the polymer chain, such methods are often cumbersome and limited. Herein, porous liquids (PLs) with flame retardant function were constructed in which porous defect-Co-LDH@ZIF-67 (d-LDH@ZIF) heterostructure particles were converted into liquid materials by electrostatic interaction with a large-volume solvent. This is also the first report of PLs in the field of flame retardant. Specifically, the d-LDH@ZIF heterostructure was designed by defect engineering and in situ growth strategy to compensate for the low catalytic activity and specific surface area of Co-LDH, and to serve as a rigid porous framework for PLs. Numerous cobalt/oxygen vacancies and lattice defects in the d-LDH@ZIF heterostructure have also been proven to confer higher flame retardancy relative to Co-LDH. d-LDH@ZIF porous liquids (PLs-d-L@Z) presents favorable liquid characteristics and achieves a monodisperse state in the polyurea matrix. The limiting oxygen index of polyurea composites blended with 20 wt% PLs-d-L@Z (3 wt% d-LDH@ZIF content) can be increased to 24.2 % and pass the V-0 rating in the UL-94 test. Moreover, the peak of heat release rate, total heat release, and total smoke production are reduced by around 40.4, 27.0, and 39.9 %, respectively, compared to neat polyurea. Emphatically, the PLs-modified polyurea composites exhibit significantly improved mechanical and impact resistance properties, as well as favorable chemical resistance. This strategy of liquefying solid flame retardant fillers will open an avenue to the design of functional nanomaterials for fire safety and other potential applications.