Polymerisation of an industrial resin monitored by infrared fiber evanescent wave spectroscopy

In order to monitor the polymerisation process of an industrial resin, a study by infrared fiber evanescent wave spectroscopy has been carried out. To design the optical fiber, a special glass composition has been optimized in the Se–Sb–Ge–Ga vitreous quaternary system. This glass exhibits a large transparency in the mid-infrared range, and shows a temperature–viscosity behaviour that enables the preparation of optical fibers. Most important is the glass transition temperature, which is high enough to allow fiber measurement at about 200 °C. Infrared spectra have been recorded every 5 min during the curing process of the resin. A direct measurement of the integrated intensities of the relevant absorption band enabled to determine the times when the curing process started and when it finished. This direct analysis has been confirmed by principle component analysis, which is an unsupervised and very efficient method for applications at the industrial scale. Moreover, it has been shown that an autocatalytic kinetic model should be used to give a good account for the absorbency evolution versus time. From a practical point of view, the optical fiber can be integrated directly into the mould and be considered as a consumable. The use of chalcogenide glass fiber to record remotely infrared signals, coupled with modern spectral analysis techniques constitute a very powerful route for monitoring the curing of thermo-hardening resins.