An Original Method to Characterize the Expansion of Thermally Self-Expandable Adhesive Thermoset Formulations Using Optical Microscopy

The present work focuses on a characterization method using optical microscopy to follow the swelling of expanded adhesive formulations. The original aspect of the method consists of incorporating a heating plate into the optical microscope in order to apply a curing process to the adhesive and to directly observe its expansion between two glass plates with a defined gap. Different heating rates and curing temperatures were applied to observe the influence on the expansion process, characterized by the time needed to fill the gap between the glass plates, the homogeneity and quality of the contact on the upper plate and the stability of the adhesion. Bisphenol A diglycidyl ether/dicyandiamide adhesive formulations were particularly considered in this study. Adhesives were also analyzed by DSC to clearly define the curing process in function of the heating rate and curing temperature. The expansion process presented in this work implies the use of a physical expansion agent to validate the analytical method. Their expansion was also studied under an optical microscope to define a blowing profile matching the curing process of the epoxy-amine adhesive. Results showed that the synchronization between the expansion and the curing of the adhesive could be followed and analyzed qualitatively by the proposed method and could be applied to various expanded formulations.