Optical fiber evanescent-wave cure monitoring of epoxy resins

Two types of optical fiber sensors (OFS) were investigated for use in monitoring the cure of an epoxy-amine resin system: (1) an evanescent wave sensor and (2) a refractive index sensor. The evanescent wave sensor was used to detect changes in concentration of the active chemical species involved in the cure reaction via evanescent wave near-infrared spectroscopy. By using the optical fiber as an attenuated total reflection waveguide, spectra were collected over the range 1490 - 1570 nm at regular time intervals during the cure. This technique enabled the depletion of amine to be monitored. Results obtained via this method were fitted to kinetic models which allowed prediction of the reaction rate at different cure temperatures and conversions. The optical fiber evanescent wave sensor results were compared with data obtained using an established cure monitoring technique (FT-IR spectroscopy). A theoretical model of the evanescent sensor has been used which describes the relationship between evanescent absorption as a function of absorber concentration and refractive index. Predictions of sensor response were undertaken using absorption data from FT-IR spectroscopy and refractive index results as a function of cure time. The predicted sensor response was then compared with experimentally obtained sensor data. An optical fiber sensor which monitored the cure process via refractive index change was also investigated. Sensors were set up to allow simultaneous collection of data during cure from the OFS, together with data from transmission near-infrared spectroscopy and Abbe refractometry. In this way the response of the sensor to changes in the cure state of the resin, refractive index and temperature was compared.