Time–temperature–transformation (TTT) cure diagram: Modeling the cure behavior of thermosets

Abstract

The times to gelation and to vitrification for the isothermal cure of an amine‐cured epoxy (Epon 828/PACM‐20) have been measured on macroscopic and molecular levels by dynamic mechanical spectrometry (torsional braid analysis and Rheometrics dynamic spectrometer), infrared spectroscopy, and gel fraction experiments. The relationships between the extents of conversion at gelation and at vitrification and the isothermal cure temperature form the basis of a theoretical model of the time–temperature–transformation (TTT) cure diagram, in which the times to gelation and to vitrification during isothermal cure versus temperature are predicted. The model demonstrates that the “S” shape of the vitrification curve depends on the reaction kinetics, as well as on the physical parameters of the system, i.e., the glass transition temperatures of the uncured resin (Tg 0), the fully cured resin (Tg∞), and the gel (gelTg). The bulk viscosity of a reactive system prior to gelation and/or vitrification is also described.