In this study, three different sizes of tripolyphosphate-modified expandable graphite (EG(p)) are prepared and incorporated into rigid polyurethane foam (RPUF) in order to obtain a flame retardant material with low density, good mechanical properties, and hydrophobicity. The influence of particle size on material combustibility, thermal stability, compression strength, pore cell structure, diathermancy, and hydrophobic property is investigated. Synergistic effect between the EG(p) and ammonium polyphosphate (APP II) is also examined. Results show the limiting oxygen index value and residue yield increase as the EG(p) size increases. On the contrary, the heat release rate and total heat release decrease as the EG(P) size increases. In addition, the chemical char formation effect of APP on RPUF is more important than the char formation of the graphite intercalation compounds. EG(p) and APP show synergistic effect in improving flame retardancy, thermal stability, and hydrophobicity. POLYM. ENG. SCI., 59:1381-1394 2019. (c) 2019 Society of Plastics Engineers