Time temperature indicator film for monitoring composite repair adhesive cure using thermochromic fluorescent molecules

Carbon fiber composite part in-service repair can be challenging due to the need to carefully control the repair epoxy adhesive's cure conditions. Overheating the repair can cause damage to the undamaged matrix epoxy; underheating can cause a weak bond. The use of thermocouples to monitor and guide repair may not always be feasible. We have developed a polymer film that uses thermally activated fluorescent probe molecules in an effort to more easily determine the thermal exposure over the entire repair area. The films show dramatic color and fluorescence change related to the time and temperature of a thermal exposure following a first-order kinetic reaction model. In simulated repair scenarios the films identified areas of proper and improper exposure in both simple imaging and spectroscopic analysis, and a good/bad criterion for a repair thermal exposure was established based on the spectroscopic data.