Here, we propose a new method for estimating the extent of curing of thermosetting prepregs. In the proposed method, the extent of curing is estimated with the curing index (Ci), defined as the ratio of the glass-transition temperature (Tg) to the ultimate glass-transition temperature of the material. The advantages of this new method over the conventional degree of conversion (α) for estimating the extent of curing of thermosetting prepregs are discussed in detail. Ci and α of a toughened epoxy prepreg (977-2 unidirectional) were obtained for a wide range of isothermal curing temperatures with a differential scanning calorimeter. The ultimate heat of reaction varied inconsistently with decreasing curing temperature; this resulted in erratic behavior of α. However, Ci provided a more consistent estimate of the extent of curing because Tg, unlike α, did not need to be modified on the basis of the curing history of the material and was measured directly with the heat-flow data from differential scanning calorimetry. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011
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