CURE MONITORING OF AEROSPACE EPOXY-RESINS AND PREPREGS BY FOURIER-TRANSFORM INFRARED-EMISSION SPECTROSCOPY

Spectral analysis of the infrared radiation emitted from thin films of resin transferred from the surface of high performance aerospace carb on fibre-epoxy composite prepregs and heated to the cure temperature a llows the cure chemistry and kinetics to be monitored in real time. Qu antitative spectra with excellent signal-to-noise ratio are obtained b y heating a thin resin film on a platinum hotplate fitted to the exter nal optics of a Fourier transform infrared (FTIR) spectrometer and ref erencing the resulting emission (with the platinum emission subtracted ) to a graphite black body at the same temperature. The resulting spec tra are identical to absorption spectra and the quantitative features of the analysis are demonstrated by the appearance of isosbestic point s during the curing reactions, so indicating that concentration profil es of the reacting species may be obtained. From the initial rate of a mine and epoxy consumption, activation energies of 75 kJ mol(-1) were obtained for both functional groups in the uncatalysed resin 4,4'-tetr aglycidyl diamino diphenyl methane (TGDDM) with 27% 4,4'-diaminodiphen ylsulfone (DDS) while values of 74 and 89 kJ mol(-1) were obtained for amine and epoxy consumption from the TGDDM/DDS prepreg catalysed with boron trifluoride monoethylamine (Hercules 3501-6), consistent with h omopolymerization occurring in the prepreg as well. as amine-epoxy add ition. Analysis of the FTIR emission at 177 degrees C of resin from pr epreg aged up to 90 h at 23 degrees C and 55% relative humidity shows a lowering of epoxy and amine concentration and a higher rate of cure, consistent with the formation of catalytic species. This technique ma y be used to monitor changes in surface properties such as tack and re sin transfer, in addition to changes in the cure profile of the aged e poxy prepreg.