Anomalous behavior of cured epoxy resins: Density at room temperature versus time and temperature of cure

The room temperature density (ρ RT ) of a difunctional aromatic epoxy resin cured with a tetrafunctional aromatic amine passes through a maximum value in the vicinity of gelation with increasing conversion. For a given cooling rate cure resutls in a unique value of ρ RT for each conversion as long as the material does not vitrify on cure. The occurrence of vitrification during cure eliminates the one‐to‐one relationship because of the nonequilibrium nature of the glass transition region and of the glassy state. In the glass transition region there is competition between physical aging which increases the density and chemical aging which, after gelation, decreases ρ RT . After gelation, prolonged isothermal cure and physical aging to well beyond vitrification result in limiting values of ρ RT which decrease with increasing temperature of cure. The maximum in the ρ RT vs. conversion relationship is discussed in terms of the effects of shrinkage due to cure, the corresponding nonlinear increase in the glass transition temperature with increasing conversion after gelation, and longer relaxation times in the glass transition region with increasing crosslink density. Other factors which affect room temperature density are discussed.