Samples of commercial unsaturated polyester (UPE) resin, filled with phase-changeable fillers (PCMs), were prepared, and the thermal and mechanical properties of the cured samples were examined. Fillers chosen were paraffin and Wood's metal. Samples were prepared by making dispersions of these fillers in liquid unsaturated polyester followed by curing with methyl ethyl ketone peroxide (MEKP) and conaphtanate and rigid thermoset samples filled with PCM particles were obtained. The thermal and mechanical behaviors of such a filled composite around the melting points of fillers are very interesting. Effects of varying proportions of PCM on mechanical and thermal properties of final products were examined. The samples show thermal melting behavior without undergoing a change in physical state. Decreases in the maximum working temperature from 75 to 53 degrees C for metal-filled samples and from 75 to 43 degrees C for paraffin-filled samples were observed by using dynamic mechanical thermal analysis. Differential scanning calorimetry indicated that heat absorption of paraffin samples were higher than that of metal-filled samples. For paraffin-filled samples, heats of fusion were 3.44 cal/g for 10% filled sample and 6.35 cal/g for 20% filled sample. For Wood's metal-filled samples, heats of fusion were 1.18 cal/g for 10% metal-filled sample and 1.54 cal/g for 20% metal-filled sample. Surface hardness was tested with Shormeter D. Surface hardness of metal-filled composites varied from 86 to 34 shore D at 21 degrees C and 80.6 to 35 shore D at 80 degrees C. For paraffin-filled samples, surface hardness changed from 86 to 42 shore D at 21 degrees C and from 80.6 to 13 shore D. Morphology of the samples was determined by scanning electron microscopy, of the crack surfaces. (c) 2006 Wiley Periodicals, Inc.