Defense Tech Briefs - Low-Water-Permeability Encapsulants for Acoustic Sensors

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Low-Water-Permeability Encapsulants for Acoustic Sensors

Naval Research Laboratory, Washington, DC

Oct 01 2007

Page 1 of 2

Small nanoparticle fillings in polymer matrices can significantly reduce permeabilities of composites.

Afamily of improved encapsulating materials for protecting underwater acoustic sensors has been invented. These materials could also be used for encapsulation or protective coating of marine hardware in general. These materials are formulated to exhibit ultra-low permeability by water, to be acoustically transparent or nearly transparent, and to be amenable to curing in place on the objects to be protected. Previously, none of the available underwater- acoustic-sensor-encapsulating material had all of these desired properties in combination.

Clay Nanoparticles In a Polymer Matrix are arranged so as to impede diffusion of water through the thickness.

A material in this family is a nanocomposite: it comprises a polymeric matrix filled with a small amount of chemically modified, high-aspectratio (thin flakes or plates) clay nanoparticles. As depicted schematically in the figure, the nanoparticles are positioned approximately in overlapping layers (reminiscent of stacking of shingles on a roof) interspersed with layers of the encapsulant. The clay nanoparticles are somewhat permeable by water along their larger dimensions perpendicular to their thicknesses, but are essentially impermeable by water through their thicknesses. In order to diffuse through a protective layer of the nanocomposite material, water molecules cannot pass directly through the clay nanoparticle sheets and must spend considerable amounts of time moving along around, and along the long dimensions within, each nanoparticle to reach the next polymer sheet layer. Hence, if the proportion of nanoparticle filling is sufficiently large and the nanoparticles are properly arranged in overlapping layers, the overall rate of diffusion of water through the nanocomposite can be much less than that through the neat polymeric matrix material.

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