P3K-3 Investigation of Low Glass Transition Temperature Epoxy Resin Blends for Lossy, yet Machineable, Transducer Substrates

In the context of our on-going investigation of low-cost two-dimensional (2D) arrays, we studied the temperature-dependent acoustic properties of epoxy blends that could serve as a component in a lossy backing for a compact 2D transducer array. The acoustic impedance and attenuation of five epoxy blends - ranging from "soft" (low T_g) to "hard" (high T_g) - were analyzed across a 35degC temperature range. Fiberglass- and tungsten-filled samples were also fabricated and tested. We established that the hardest epoxy has a constant impedance (versus temperature) of 2.2 MRayl and constant attenuation of 1 dB/mm, while the softer epoxies are more temperature dependent. One soft epoxy has an impedance that declines from 2.7 to 2.0 MRayl and attenuation that increases from 5 to 10 dB/mm. In our application, unfilled epoxies could provide a 24 dB attenuation of backing block echoes, while filled epoxies may provide up to 40 dB attenuation. These materials may be machined (when chilled) or molded to form compact Z-axis conductive backing blocks with improved attenuation of echoes using existing methods.