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Polymerization of an acrylate derivative of bisphenol-A by ionizing radiation in the presence of an organically modified montmorillonite was carried out in order to produce a flame-retardant composite. The measurement of the thermal effect during irradiation indicates that radiation curing of pure resin and resin-clay mixtures occurred in both cases. Moreover, differential scanning calorimetry and dynamic mechanical thermal analysis showed that a curing completion can be obtained by thermal curing after irradiation with a significant increase of the glass transition temperature. The morphology was investigated by X-ray diffraction and transmission electron microscopy. These have indicated that gamma irradiation leads to the formation of a microcomposite structure after radiation curing. The flame resistant characteristics were studied by means of thermal gravimetric analysis under nitrogen and air and by cone calorimeter tests. In particular, thermal gravimetric analysis in air showed that the presence of the dispersed clay in the polymer matrix increases the oxidation temperature. This is attributed to the formation of a more stable char, due to a protective skin of clay aggregates that builds up on the surface of the volatilizing polymer matrix during thermal degradation which protects the charred polymer from reactions with oxygen. Finally, the cone calorimeter tests show a reduction of heat release of the clay-polymer composite with respect to the neat resin. The contribution of the clay in reducing the rate of heat release is attributed to the dispersion of the filler that allows build-up of an inorganic surface layer at a rate which is sufficient for effectively compete with the feeding of the flame with combustible pyrolysis products. © 2006 Wiley Periodicals, Inc. Adv Polym Techn 25: 109–120, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/adv.20063