The flame resistance properties of expandable polystyrene foams coated with a cheap and effective barrier layer

[Display omitted] •A cheap and effective flame retarded EPS system was developed.•A synergistic effect of flame resistance between fly ash and aluminum hydroxide was found.•Fly ash enhanced the compactness of char layer with improved flame resistance and smoke suppression properties.•The main flame resistance mechanism was the effective barrier effect of the char layer. A cheap and effective flame retarded EPS system was developed by using a high silica content based fly ash (FA) synergistic with aluminum hydroxide (ATH) in the thermosetting phenolic resin (PF) coating layer. In this study, scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analyses confirmed the formation of a barrier layer which uniformly encapsulated each EPS microsphere. The flammability of EPS-PF/ATH/FA foam with different compositions were characterized by the limited oxygen index (LOI) and UL-94 vertical burning tests, an evident flame retardant effect has been observed with up to 29.6% LOI value and UL-94 V-0 rating. The cone calorimetric analysis (CONE) indicated that the heat release rate (HRR), total heat release (THR) and smoke production rate (SPR) of the resulting EPS-PF/ATH/FA foam decreased obviously, and the foam maintained structural integrity after combustion. The thermal stability of EPS-PF/ATH/FA foams has been evaluated by the thermogravimetric analysis (TGA). It is obvious that the presence of PF-ATH/FA flame retarded binder has delayed the pyrolysis process of EPS matrix by about 10 °C. From analysis conducted on the combusted residues, it can be inferred that the dominant flame retardant action of EPS-PF/ATH/FA foam occurred in the compact char layer, which acted as an effective fire-proofing and heat shielding barrier in the condensed phase.