Effects of different metal ions in phosphonitrile-modified organometallic complexes on flame retardancy of epoxy resin

Two organometallic (Zn and Fe) coordination compounds, modified by phosphonitrile, were prepared by solvothermal method. The introduction of the two organometallic coordination compounds significantly improved the flame retardancy, smoke suppression and thermal stability of epoxy resins (EP). Especially, zinc-containing coordination compounds exhibited higher efficiency than iron-containing ones in flame-retardant EP. LOI values of Zn-PDH/EP and Fe-PDH/EP were further enhanced and reached up to 29.8% and 28.9%, respectively. Organometallic coordination compounds modified by phosphonitrile mainly exerted the flame-retardant action in condense phase by the combined effect of the catalysis charring effect of metal ions and flame-retardant effect of phosphonitrile, and thus a high-temperature stable graphitized carbon layer with P- and N-containing crosslinking structures formed. This high-quality carbon layer was a vital factor in improving the heat/smoke suppression properties of EP thermosets. Further, due to the weaker catalytic char-forming effect of iron ions and the formation of fewer cross-linked structures, the strength and barrier properties of the Fe-PDH/EP char layer were relatively lower than those of Zn-PDH/EP. Consequently, Zn coordination compounds presented higher efficiency in suppressing heat/smoke release of EP matrix than Fe hybrids. Finally, the gas-phase function of organometallic coordination compounds should not be overlooked due to releasing phosphorus-oxygen free radicals and N-containing fragments during pyrolysis, exhibiting flame inhibition and dilution effect in gas phase, especially for 6%Zn-PDH/EP.