Cure monitoring of epoxy matrices for composites by ultrasonic wave propagation

Although ultrasonic wave propagation is a well-known technique for nondestr uctive analysis, it can be also applied for dynamic mechanical characteriza tion (DMA) of polymers and composites. Most of DMA characterizations at ult rasonic frequencies are performed on thermoplastics and only a few articles are available on the characterization of the reactive properties of thermo setting resins. Therefore, in this work a complete characterization of the cure of a model epoxy system is presented, by comparing isothermal and noni sothermal data. The propagation of ultrasonic waves acting as a dynamic mec hanical deformation at high frequencies can be used for the calculation of complex longitudinal bulk moduli during the cure of the epoxy resin. The ev olution of attenuation and velocity during reaction is related to the stron g physical changes occurring during the cure process. Furthermore, a compar ison between the degree of reaction measured by Differential Scanning Calor imetry and ultrasonic data is proposed. The ultrasonic velocity (or the bul k longitudinal modulus) can be considered the most interesting parameter fo r cure monitoring because it follows the growth and evolution of the mechan ical stiffness of the resin during cure. In particular, the obtained result s suggest that the measurement of longitudinal velocity or L' could be expl oited for on-line measurements of post-gel properties. Finally, an immediat e correlation is also proposed between the gel time and the end of cure and the ultrasonic data. (C) 1999 John Wiley & Sons, Inc.