The glass transition temperature (tg) as a parameter for monitoring the cure of an amine/epoxy system at constant heating rates



A continuous heating transformation (CHT) cure diagram displays the time and temperature of events that a material encounters during the course of continuous heating at different heating rates. For thermosetting systems, these events include gelation, vitrification, and devitrification. Analysis of isothermal data for a diglycidyl-type epoxy/tetrafunctional aliphatic amine system, which is initially liquid at room temperature, indicated that kinetic control applied prior to vitrification; i.e., for Tg<Tcure (temperature of reaction). The parameters so obtained (i.e., order of reaction, Arrhenius pre-exponential term, and activation energy) together with an experimental relationship between Tg and extent of reaction were used as a basis for modeling the onset of vitrification and devitrification in the CHT diagram. For heating scans from below Tgo (glass transition temperature of the reactants), initial devitrification occurs when Tcure first passes through the Tg of the initial reacting material, except for very low heating rates for which inital devitrification is not encountered. Vitrification is identified as the point at which Tg becomes equal to the increasing curing temperature. Diffusion control retards the reaction rate beyond vitrification. In the limiting case of complete retardation of the rate after vitrification, the reaction can proceed along the Tg = Tcure path as long as the additional reaction with increasing temperature is sufficient to increase Tg at least at the same rate as the heating temperature; devitrification occurs thereafter. The critical heating rates at which the material does not devitrify in the vicinity of Tgo, and that at which no vitrification is encountered, are calculated.