Applications of diffuse reflectance near-infrared spectroscopy and cure kinetics study by differential scanning calorimetry to selected thermosetting polymer-based systems of interest in the aviation industry

Abstract

Diffuse Reflectance Near-Infrared (near-IR) Spectroscopy combined with Partial Least Squares (PLS) regression was used to determine the moisture content and degree of cure in unfilled epoxy resins, carbon-reinforced epoxy resin and epoxy resin adhesives. PLS-generated calibration curves were consistently superior than the calibration curves based on Beer’s law. The same technique was also applied to polyimide-based prepregs to determine the degree of cure through the softening temperature, Ts. In both epoxy-based and polyimide-based systems, good correlations were established between the near-IR spectra and the properties of interest. The combination of the rapid, non-destructive technique coupled with a sophisticated chemometric software shows promise as a reliable technique that could be of great benefit to the aviation industry. Another aspect of this study was the evaluation of an epoxy resin prepreg as a candidate material to be used in repair scenarios in the aviation industry. The prepreg was partially characterized using mid-IR and 1H NMR spectroscopies indicating that it contains diglycidyl ether of bisphenol-A (DGEBA) and diamino diphenyl sulfone (DDS) as a curing agent. Ramp and soak experiments using the Differential Scanning Calorimetry (DSC) were performed to study the cure kinetics. A recommended cure cycle was determined to be a combination of a dynamic heating to 150-155 °C at 2 °C/min and an isothermal heating at the end temperature for 20 minutes.