Fiberoptic intrinsic fluorescence for in‐situ cure monitoring of amine cured epoxy and composites

Abstract

The cure reactions of epoxy‐diamine and its composites are monitored in‐situ using the intrinsic fluorescence of the aromatic diamine, diaminodiphenyl sulfone (DDS). With a fiberoptic fluorimeter, in‐situ cure monitoring was performed via a single fiber, distal‐end probe, in neat epoxy as well as in commercial grade prepregs containing graphite fibers and DDS curing agent. The prepregs were investigated during multiply lamination in an oven. The fluorescence excitation spectra were obtained by emitting at 420 nm with a scan range of 320 to 400 nm, and the DDS peak position was determined as a function of cure time and temperature. The DDS spectra show a progressive red shift up to 24 nm when the primary amine is reacted with epoxide to become the secondary and the tertiary amines. The spectral shift of the DDS is also correlated with the extent of epoxide reaction determined by the Fourier transform infrared (FTIR) spectroscopy. Both data exhibit a linear relation, consistent with the behavior of the DDS peak shift, which increases linearly with the amine reaction. The excitation spectra also show a temperature dependency such that the amount of red shift increases with the measurement temperature in a manner that can be described by an exponential function. The temperature effects also depend on the state of cure in the sample. The temperature correction can be made by the application of an empirically developed equation. Thus, a direct comparison can be made among the on‐line data obtained under varying conditions of cure, by reducing the spectral data to any reference temperature. This intrinsic fluorescence technique is much simpler than the previously reported extrinsic fluorophore technique, which requires the addition of an extrinsic fluorophore and an internal dye, and can be applied to any commercial prepregs containing DDS, thus making it a very powerful and widely applicable monitoring tool for composite processing.

Thermal curing of glass‐epoxy prepregs by luminescence spectroscopy

“…Bands located in the red region were attributed to the secondary and tertiary amines that were formed through the curing reaction. An increase in the band relative intensity was observed at 450 nm 5, 9, 34, 35…”

Fiber optic strain monitoring of textile GFRP during RTM molding and fatigue tests by using embedded FBG sensors

“…Among these sensors, fiber optic sensors are promised due to their small size, light weight, high strength and high flexibility. Several kinds of fiber optic sensors, near-infrared spectroscopy (NIRS)-based sensors, fluorimetry-based sensors and refractive index-based sensors are used for only cure monitoring [2][3][4]. These sensors respond variation of chemical structure of resin.…”

Curing of linear and crosslinked epoxy systems: A fluorescence study with dansyl derivatives

“…The intrinsic fluorophore techniques monitor the emission of a fluorophore moiety, which is part of the reaction component. They monitor changes in its emission characteristics with the progress of the curing reaction 10–12…”