Ambiguity in Assignment of Near Infrared Vibrational Bands for Polymerisation Monitoring of Epoxy System

The curing process of epoxy affects the chemical structure of the final network so mechanical and physical properties of the polymeric matrix for a composite may be modified according to the polymerisation conditions. This paper describes the ambiguity in assignment of reference bands to follow the cure of poly‐epoxy reactive systems using a laboratory‐made system which allows the coupling of dielectric analysis and Fourier Transform Near Infrared Spectroscopy (FTNIR). The dielectric measurements were obtained using interdigitated electrode. In situmonitoring of extent of reaction was carried out from room temperature up to 160 °C using fibre‐optic FTNIR spectroscopy. For the DGEBA/MCDEA system the epoxy band at 6060 cm−1was chosen in preference to the band at 4530 cm−1as representative of the epoxy function evolution during polymerisation because a small unknown peak probably due to the hardener appears in the 4530 cm−1region. The bands at 4620 and 4680 cm−1assigned to aromatic combination bands and widely used as reference bands are not unique for this formulation hence the band at 5980 cm−1is used as reference. The Principal Components Analysis (PCA) shows clearly also that the bands at 4620 and 4680 cm−1vary during the polymerisation. Surprisingly, the band at 4530 cm−1is equivalent to the one at 6060 cm−1to calculate the conversion rate. It is probably due to the fact that the hardener band near 4530 cm−1follows the same behaviour as the epoxy band at 4530 cm−1.