Monitoring Cross-Linking, the Evolution of Refractive Index and the Glass Transition Temperature of an Epoxy Resin Using an Optical Fiber Sensor

Hyphenated analytical techniques enable the simultaneous measurement of relevant processing and materials parameters under identical environmental conditions. In the current study, a power-compensated differential scanning calorimeter (DSC) was custom-modified to enable the integration of an optical fibre sensor to monitor in situ the progression of the cross-linking reactions by inferring the evolution of the refractive index. A cleaved optical fibre was used and it served as a Fresnel reflection sensor (FRS). The DSC was calibrated with and without the integrated FRS and it was demonstrated that it did not influence the performance of the DSC. The FRS was calibrated using reference refractive index oils within the DSC. An epoxy/amine resin system was cross-linked at 70 ^oC and the enthalpy of cross-linking and the evolution of the refractive index were monitored simultaneously using the DSC and FRS respectively. After the cross-linking was completed, the DSC was programmed to perform a ramped heating schedule from ambient temperature to 150 ^oC. The FRS was capable of detecting glass transition temperature (Tg) of the cross-linked resin. An excellent correlation was observed for the Tg obtained by the FRS and DSC. The contribution of factors affecting the resolution of the data from the FRS are discussed.