TTT‐diagram for epoxy film adhesives using quasi‐isothermal scans with initial fast ramps

Abstract

ABSTRACTThe characterization of film adhesives is challenging because they required freezer storage, contain an inseparable filler—thermoplastic knit or fiber‐reinforcement, and are heat activated systems with a pre‐cure and unknown chemistry. A testing protocol that eliminates these sources of error is proposed. This study presents a method to generate time–temperature‐transformation (TTT) diagrams of epoxy film adhesives via differential scanning calorimetry (DSC). Non‐isothermal and isothermal DSC scans are used to capture the reaction and the glass transition temperature. The use of an initial fast ramp—up to 500 K/min—in the isothermal scans is explored for the first time. This technique shows the potential to produce a quasi‐isothermal cycle, eliminating the loss of data in the initial stage of the reaction. The total heat released, the activation energy, and the fractional kinetic parameter, are estimated via model‐free methods. The Kamal–Sourour model and the formal kinetic model are fit to model the rate of cure. The simplest model that accurately captures the reaction, a parallel two‐step model, A , is outlined. The glass transition temperature is modeled via DiBenedetto's equation to include the diffusion‐controlled mechanism. The TTT‐diagrams of two commercial adhesives, DA 408 and DA 409, are shown with an analysis of processing optimization. The use of quasi‐isothermal scans with initial fast ramps combined with the correction for filler, moisture, and pre‐curing history can be applied to characterize fast curing thermosets, complex B‐stage resins, and thermosetting composites. The modeling results can also be used in numerical studies of residual stresses and dimensional stability in the manufacturing of thermosetting composites. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 45791.