The curing of an epoxy prepolymer based on the diglycidyl ether of bisphenol A (DGEBA) with dicyandiamide (DDA) as the hardener and imidazole as the catalyst agent was analyzed using microdielectrometry, differential scanning calorimetry, viscosity measurements, and insolubles in THF for gel‐point detection. Interpreting dielectric data with respect to chemorheology continues to be the subject of scientific discussion. The focus of this issue is to give an industrial point of view on the collected on‐line dielectric measurements during an epoxy/fiber glass composite cure. Hence, isothermal polymerizations of DGEBA/DDA/imidazole resin were examined and dielectric properties such as ionic conductivity were related to the cure kinetics by conversion through an experimentally established equation. This mathematical model was used to predict reaction advancement of epoxy processing under nonisothermal cure conditions. This model is shown to be able to forecast both isothermal and nonisothermal cure data of unaged resin. According to these results, cure monitoring was carried out on prepregs. Whereas some deviations of the law were observed at the time of the last stage of the cure, good correlation was obtained for the reaction rate during the in‐mold process curing time.

The curing of an epoxy prepolymer based on diglycidyl ether of bisphenol A (DGEBA) with isophorone diamine (IPD) as a hardener was analysed using differential scanning calorimetry, rheological measurements, microdielectrometry, and insolubles in THF for gel point detection. The effects of the initial hydroxyl concentration of the DGEBA prepolymer, the molecular features of which are different, were determined on the cure kinetics of epoxy networks Chemical reaction kinetics of this DGEBA/IPD system during isothermal conditions was studied in the 60–120 °C temperature range and a kinetic model relation based on assumption of an autocatalytic mechanism has been proposed. Gelation and vitrification phenomena of this reactive mixture were studied and allowed us to establish the TTT diagram. Furthermore, dielectric data and viscosity measurements have been interpreted with respect to kinetics. Indeed, it was shown that the modified percolation law equation suggested by Macosko et al can be used to describe the chemoviscosity as a function of temperature and extent of reaction by using a temperature‐ and conversion‐dependent critical exponent. In other respects, dielectric properties such as ionic conductivity were related on one hand to viscosity through a Stokes‐based equation in the 0 to 0.5 conversion range, and on the other hand to conversion through an experimental equation. © 1999 Society of Chemical Industry

Ranee and 2Awcirbat 8 rue des Saussaies 75008 Paris Gdex, FranceThe curing of a fiberglass epoxy composite based on diglycidyl ether of bisphenol A (DGEBA) with dicyanodiamide (DDA) as the hardener and imidazole as the catalyst agent was analyzed using microdielectrometry. The curing behavior of thick epoxy composite parts was examined in a production environment for compression molding process. The particular focus of this paper is to present the method used to collect on-line real-time conversion measurements during an epoxy/fiberglass composite cure. For this purpose, temperature and ionic conductivity profiles during industrial moldings of a thick epoxy part were recorded. Corresponding conversion profiles were deducted from a previous empirically established correlation and discussed in terms of cure gradients as a function of the through-the-thickness location and of the cure cycle time.