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Synergistic effects of intumescent flame retardant and nano-CaCO3 on foamability and flame retardant property of polypropylene composites and their foams were carefully investigated. The differential scanning calorimetry results showed that the intumescent flame retardant played a plasticizing effect on the polypropylene/intumescent flame-retardant composites and accelerated the crystallization rate. The rheological properties and supercritical CO2-assisted molding foaming behaviors of the polypropylene/intumescent flame retardant/nano-CaCO3 composites showed that the nano-CaCO3 could enhance their foamability. Scanning electron microscopy pictures and mechanical properties of the polypropylene/intumescent flame-retardant composites foams indicated that the agglomeration of intumescent flame retardant would reduce the cell uniformity and even cause the cell collapse. Furthermore, the stress concentration, caused by the agglomeration, could reduce the mechanical properties of the PP composites foams. The synergistic effect of the nano-CaCO3 could improve the cell uniformity and reduce the stress concentration so that the mechanical properties of the polypropylene/intumescent flame retardant /nano-CaCO3 composites foams were improved. Moreover, the polypropylene/intumescent flame retardant/nano-CaCO3 composites foams had the higher limit oxygen index values than the polypropylene/intumescent flame-retardant foams. TGA results also showed that the nano-CaCO3 could improve the thermal stability of the polypropylene composites foams by forming compact carbon layer. The experimental results indicated that the foamability of the polypropylene composites and the flame-retardant property of their foams could be improved by the synergistic effects of intumescent flame retardant and nano-CaCO3. [ABSTRACT FROM AUTHOR]