Solid-state NMR characterization of multi-component intumescent flame retardant polybutylene succinate formulations

In this work, a series of multi-component intumescent PBS formulations were elaborated by the combination of intumescent flame retardant (IFR) system (consisting of ethylenediamine phosphate and melamine poly(aluminum phosphate), 7:3, wt:wt) and synergists. Three boron/zinc-containing compounds (i.e., melamine borate, zinc oxide, zinc carbonate) were examined as potential synergists to improve the fire behavior and fire retardancy of PBS. The MLC results showed that the incorporation of all of these compounds enhanced the fire retardancy of PBS to some extent. Among them, zinc oxide exhibited the best fire retardant performances in terms of reduction of the peak heat release rate (pHRR, -61%), fire growth rate index (FIGRA, -19%), maximum average rate of heat emission (MARHE, -55%), and increase of the time to pHRR (+370 s) and to flameout (+650 s) as compared to PBS/IFR10%. Furthermore, the effect of zinc oxide loading on the fire retardancy of PBS was also investigated, the results showed that 2 wt.% loading exhibited the best performances. Solid-state NMR characterization indicated that the presence of ZnO resulted in the formation of a crack-free and dense intumescent char layer reinforced by complex aluminum-zinc-phosphates glassy species. The intumescent char layer provided a protective physical barrier with improved flexibility and cohesion, which effectively limits the heat and mass transfer between condensed and gas phases and hence, improving the fire retardancy of PBS.