Synthesis of metal-containing Schiff based cyclomatrix polyphosphazene microspheres based on polyoxometalates and application in flame retardance for epoxy resin

A novel kind of polyoxometalate (POM) -containing Schiff based polyphosphazene microspheres (PSPMs) were successfully prepared through the polycondensation between POM-containing polyaldehyde and 4,4′-diaminodiphenylmethane. The PSPMs attained excellent thermal stability and demonstrated good monodispersity with a diameter of about 400 nm. In this work, the fire resistance of PSPMs was investigated in the epoxy resin (EP) matrices. Notably, the PSPMs displayed good distribution in EP matrices. Therefore, the dynamic mechanical properties of EP/PSPMs composite were enhanced. After the addition of 4 wt% PSPMs, the thermogravimetric analysis showed that the char yield at 750°C of the EP/PSPMs composite increased from 12.9 wt% (EP) to 29.0 wt% in nitrogen. And the composite attained a limiting oxygen index of 27.5% and passed the UL-94V-1 rating. In comparison with that of pure EP, the peak value of the heat release rate and total smoke production decreased dramatically. Meanwhile, the toxic CO gas was largely reduced due to the reduction reaction of POM during the combustion process. To further illuminate the flame-retardant mechanism, the char residues and gaseous pyrolysis products of EP/PSPMs were examined. This work may provide a new perspective for the synthesis and application of functional metal-containing polymer microspheres.