Cure Monitoring of Epoxy Matrices for Composites by Ultrasonic Wave Propagation

Although ultrasonic wave propagation is a well-known technique for non-destructive analysis, it can be also applied for dynamic mechanical characterization (DMA) of polymers and composites. Most of DMA characterizations at ultrasonic frequencies are performed on thermoplastics and only a few articles are available on the characterization of the reactive properties of thermosetting resins. Therefore, in this work a complete characterization of the cure of a model epoxy system is presented, by comparing isothermal and nonisothermal data. The propagation of ultrasonic waves acting as a dynamic mechanical deformation at high frequencies can be used for the calculation of complex longitudinal bulk moduli during the cure of the epoxy resin. The evolution of attenuation and velocity during reaction is related to the strong physical changes occurring during the cure process. Furthermore, a comparison between the degree of reaction measured by Differential Scanning Calorimetry and ultrasonic data is proposed. The ultrasonic velocity (or the bulk longitudinal modulus) can be considered the most interesting parameter for cure monitoring because it follows the growth and evolution of the mechanical stiffness of the resin during cure. In particular, the obtained results suggest that the measurement of longitudinal velocity or L? could be exploited for on-line measurements of post-gel properties. Finally, an immediate correlation is also proposed between the gel time and the end of cure and the ultrasonic data. © 1999 John Wiley & Sons, Inc.