In-situ Observation of Curing and Adhesion Process of Epoxy Resin on a Carbon Fiber and Measurement of Resin Cure Shrinkage Using Microbond Method

Abstract

The present study proposed a method to evaluate the cure shrinkage process using the microbond specimens of an epoxy resin droplet adhered to a single carbon fiber, and discussed the interacted process of fiber-resin adhesion and resin curing in the residual stress development which occurs in the carbon-fiber-reinforced thermoset-epoxy composites. In our experiments, microbond specimens were first prepared and observed with an optical microscope under an isothermal heating condition. Moreover, we utilized the image processing techniques based on Carroll’s theory for the geometrical shape of resin droplets to evaluate the time variation in the characteristic shape parameters during the resin curing process, including droplet volume, diameter, contact angle on the fiber, and fiber embedded length. We compared the results among three different heating conditions for the resin curing and those between non-vacuum and vacuum conditions when the microbond specimen was prepared as well, and discussed the validity of the proposed method by investigating the characteristics of resin curing process. From the results, we confirmed that the volume change during the curing could be appropriately evaluated when the microbond specimen was placed in a vacuum for the degassing before the curing. Moreover, the results revealed that the fiber-resin interaction significantly reduced the cure shrinkage at the fiber/resin interface. It implies that the suppression of the cure shrinkage in the vicinity of the interface may lead to the build-up of the residual stress.