Synergistic effects of the natural clays unexfoliated vermiculite (VMT), exfoliated vermiculate (EVMT), and montmorillonite (MMT) on the intumescent flame retardance of polypropylene were investigated systematically with the usual fire testing methods. The limiting oxygen index (LOI) of flame-retardant polypropylene (FRPP) filled with 30 wt % intumescent flame retardants (IFRs) composed of ammonium polyphosphate and pentaerythritol were increased from 30 to 33 vol % for VMT and MMT and to 36 vol % for EVMT when 1 wt % IFR was substituted for clay. The synergistic effectivities calculated on the basis of increases in the LOI values were 1.4 for VMT, 1.3 for MMT, and 1.6 for EVMT. Cone calorimetry also revealed the existence of a synergistic effect. EVMT had the best performance for lowering the peak values of the heat release rate and smoke production rate. The thermogravimetric analysis results show that EVMT had the best performance for increasing the char residue of FRPP higher than 650 degrees C compared with VMT and MMT. The high content of iron and the small particle size of EVMT may have been responsible for its high synergistic effect at a low filling level. No remarkable variations of the diffraction peaks were observed in the X-ray diffraction patterns of the original clay and the clay in FRPP. All of the formulations, with or without clay, exhibited small variations in the mechanical properties. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 120: 1225-1233, 2011