Determination of gel point and completion of curing in a single fiber/polymer composite

Abstract By embedding both a single fiber Bragg grating (FBG) sensor and a thermocouple during the cure cycle of a room temperature cure epoxy, a novel in-situ approach was developed to identify certain properties of the polymer. Residual strains during curing and cooling of the epoxy were determined as a function of time for three different ambient curing temperatures. Comparing the thermocouple based temperatures to those of the ambient environment allowed for the calculation of the degree of cure and the full cure time of the epoxy. Analyzing the wavelengths of the FBG sensor and compensating for the temperature measured by the internal thermocouple, the gel point was precisely identified and validated using Flory-Stockmayer theory. As expected the residual strains increased with the curing temperature while the gel time and the completion of curing time decreased with temperature. The integrated approach of using both FBG sensors and embedded thermocouples presented in this study could be used for other polymer systems and polymer matrix composites during their manufacturing.