NUWC XP-1 polyurethane-urea: a new, "acoustically transparent" encapsulant for underwater transducers and hydrophones

Almost all acoustic projectors and hydrophones are encapsulated in polymeric materials. These coatings protect the underlying sensors and electronics from physical damage and water ingression, while allowing the passage of acoustical energy without significant reflection, loss, or distortion. Critical material parameters for superior acoustical performance include: (1) density and sound speeds close to those of seawater; (2) low mechanical loss tangent; (3) medium to high shear loss tangent; (4) low shear storage modulus; (5) Poisson\´s ratio near 0.50; and (6) no major polymeric transitions in/near the frequency and temperature ranges of interest. Recently, a new polyurethane-urea elastomer was synthesized at Naval Undersea Warfare Center (NUWC), Division Newport for use as an encapsulant for transducers and hydrophones. This material, "NUWC XP-1", utilizes a toluene diisocyanate/polyether polyol base (molecular weight about 1500) and dimethylthiotoluenediamine as the cross-linking/curing agent. NUWC XP-1 cures overnight at room temperature to 75 Shore A hardness. Its sound speed is very close to that of seawater at room temperature, and at 25/spl deg/C, its acoustic impedance is 1.71/spl times/10/sup 6/ metric Rayls. NUWC XP-1 exhibits a sub-ambient glass transition temperature (-31/spl deg/C) and its dynamic mechanical properties are almost ideal for underwater transducer/hydrophone encapsulants. Side-by-side comparisons with several commonly used, commercially-manufactured polyurethane encapsulants (PRC-DeSoto\´s PR-1547 and PR-1592; Cytec\´s Conathane EN-7) determined that properties of NUWC XP-1 are equal to, or better in almost every category required for effectively encapsulating acoustic devices and sensors. A distinct benefit to the industry is that the chemical formula of NUWC XP-1 is controlled by the U.S. Navy. The material is made from basic raw materials, thus its future availability is more assured than many other commercial products.