Melamine–formaldehyde resin was modified by ethylene glycol to decrease the amount of free formaldehyde and extend the storage time. The modified resin (EMF) was further used to prepare microencapsulated ammonium polyphosphate (MCAPP). The structures of both EMF and MCAPP were well characterized. Afterward, EMF and MCAPP were mixed and coated on the surface of pre‐expanded polystyrene particles to prepare flame‐retardant expandable polystyrene foams (EPS). Both water resistance and impact strength were enhanced by the presence of MCAPP, and the flammability of the samples was also significantly improved. For the sample containing 75 phr MCAPP, the limiting oxygen index value was increased to 31.4% with a V‐0 rating in the UL‐94 vertical burning test. Cone calorimeter tests showed that the peak heat release rate of the sample declined sharply to 172.7 kW/m2, which is 81.6% lower than that of neat EPS. The smoke production of EPS foams during combustion was suppressed by the presence of MCAPP, and the thermal stability was also improved. Scanning electron microscopy showed that the char layer of the flame‐retardant sample after combustion became compact with negligible voids or cracks, which could further form an isolation barrier to prevent both heat and flame transfer. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46471.

Flame retardant expandable polystyrene (EPS) foams were prepared by coating method. Red phosphorus (RP) and expandable graphite (EG) were chosen as the flame retardants to be coated on the surface of expanded PS beads. By the presence of 33 phr RP/EG with a mass ratio of 1:1, the limiting oxygen index of EPS foam could reach up to 26.9%, with V‐0 rating obtained in UL‐94 test. The peak heat release rate could also decrease to 180.67 kW/m2, which was 72.9% lower than that of neat EPS sample. Thermogravimetric analysis revealed an obvious increase of thermal stability and residue char amount by the presence of RP and EG. From the observation and analysis of char residue, it could be proposed that there existed a significant synergistic effect between RP and EG. RP could be oxidized and further react with graphite by the presence of oxygen at high temperature, forming isolated char layer and releasing nonflammable gases. Moreover, P radicals were generated at high temperature and could capture the radicals formed during the combustion of polystyrene and eliminate the burning chain reactions.

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