An emulsion polymerization method was employed to prepare pulverized expandable graphite (pEG)-poly(methyl methacrylate-acrylic acid) copolymer [(PMA)] composite particles, and then the pEG-P(MA) particles were used for a flame retardant of the rigid polyurethane foam (RPUF). Fourier transform infrared (FTIR) spectroscopy data demonstrated the existence of P(MA) in the pEG-P(MA) particles, and the result of the thermogravimetric analysis (TGA) indicated that the content of P(MA) was 24.3 wt %. Morphological observation showed that the pEG particles were encapsulated by a layer of polymer coating to form typical core-shell composite particles. Due to the possible reaction between -COOH of pEG-P(MA) and R-CNO of isocyanate, the compatibility between the composite particles and the RPUF matrix was highly enhanced. In contrast to the pEG, the limiting oxygen index (LOI), the horizontal and vertical burning tests showed the pEG-P(MA) composite particles could improve the flame retardancy effectively. The improved flame retardancy of the RPUF matrix was attributed to the increased expansion volume ratio of pEG-P(MA) particles as exposed to fire. The dynamical mechanical analysis (DMA) showed that the incorporation of the core-shell particles could improve the storage modulus and tan delta of the RPUF composites. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 122: 932-941, 2011