Acoustic and mechanical properties of polyurethanes based on polybutadiene soft segment

Abstract

Mechanical and acoustic properties of castable polyurethane elastomers comprising polybutadiene polyol, diphenyl methane diisocyanate, and 2-ethylhexane-1,3-diol chain extender are reported. Such elastomers are used to encapsulate sonar transducers. For this application the principal acoustic requirement is that a good impedance match to water be obtained by making both sound velocity and density similar to that of water. Perfect matching would give perfect transmission of the signal from the water into the material but due to the viscoelastic nature of polymers there is always some residual mismatch. Increasing chain extender content improves strength and increases hardness and can bring about formation of phase separated structures. The stronger materials have poorer acoustic performance due to increased shear modulus and damping. A compromise must be reached between acoustic properties and strength, which depends primarily on the layer thickness and the frequency of the sound energy in the particular application.