The synthesis and properties of an intumescent zinc-chelated organometallic flame retardant

Organometallic flame retardants (OFRs) are currently attracting intensive attention. Here, we synthesized a new zinc-chelated OFR, ZnDSi, which was incorporated with phosphorus, silicon, and nitrogen functional moieties. The resulting ZnDSi exhibited strong intumescent properties upon heating and demonstrated high thermal stability, with a decomposition temperature (T_{d, 5 %}) of 349 °C, making it suitable for the melt-processing of most engineering plastics. Comparative experiments demonstrated that the chelated zinc converted the properties of the resultant OFR, including its intumescence, thermostability and hydrophobicity. The ZnDSi was tested in polyamide 66 (PA66) for the flame-retardant properties. Incorporating 10 wt% of ZnDSi into PA66 attained a limiting oxygen index of 29.1 %, accompanied by the reduction of the peak heat release rate by 35.0 % and total heat release by 28.2 % when compared to the pure PA66 sample, according to the cone calorimeter tests. Mechanism investigation was conducted to probe into the interactions between zinc and organic components through a custom apparatus for pyrolytic gas analysis, and it was found that zinc could reduce the soot release by suppressing the formation of the soot precursors during the pyrolysis. This work contributes to a deeper understanding of how metal elements function in OFR systems, which may conduce to developing OFRs with advanced performance.