Real-time study of frequency dependence of attenuation and velocity of ultrasonic waves during the curing reaction of epoxy resin

Abstract

The frequency dependence of the phase velocity and attenuation of ultrasonic waves were measured as a function of time during the polymerization (curing) reaction of epoxy resins. The phase velocity and attenuation were evaluated from the amplitude and phase spectra of ultrasonic signals transmitted through a layer of curing epoxy resin. The measurements were made in the frequency range of 2–20 MHz. From the experimental data follows an important conclusion: The attenuation coefficient increases linearly with frequency at all stages of the curing reaction from the viscous liquid to the solid state. The slope of the attenuation coefficient as a function of frequency is strongly dependent on the time of cure (degree of cure). The linear behavior of attenuation versus frequency suggests that the attenuation effect cannot be explained by classical viscothermal absorption or relaxation theory. This type of behavior (so-called hysteresis behavior) is poorly understood on the molecular level and was found previously for some highly viscous liquids, for solid polymers, and for biological tissue. The phase velocity data were evaluated from the phase spectrum of the transmitted signal. The ultrasonic velocity changes with time according to an S-shaped curve. It is also moderately dependent on the frequency.