Dielectric cure monitoring and optimization of film adhesives

Abstract

Abstract Dielectric loss factor measurements were studied as a means for cure cycle monitoring and optimization of structural film adhesives. Three film adhesives were included in the study: a high-temperature-curing epoxy (175°C cure), a low-temperature-curing epoxy (120°C cure) and a 175°C curing polyimide. Generally, three regions can be distinguished in the dielectric loss factor with respect to heating time. First, the softening or flow region, where the loss factor exhibits a plateau and in which the resin viscosity attains low values. Second, the polymerization peak where the loss factor attains a maximum, and finally the region where the loss factor approaches an asymptote, indicating completion of curing. In the present study the three regions were first characterized as functions of heating time for each adhesive using a laboratory oven. Next, the heating conditions were optimized in an autoclave where lap shear specimens were prepared for mechanical properties evaluation, in addition to in situ monitoring of the dielectric loss factor. In the final stage of the study, dielectric cure monitoring was shown to be adequate for characterizing and establishing optimized time-temperature-pressure processing conditions for aged epoxy film adhesives.