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.

The choice of the matrix material is governed partly by the requirement that for acoustic transparency, the product of the mass density and the speed of sound must be as close as possible to that of seawater. Suitable polymeric matrix materials that satisfy the acoustic- transparency and other requirements notably include polyurethanes. The proportion of clay nanoparticles in the nanocomposite must be large enough to effect a sufficient reduction in the rate of diffusion of water but not so large as to effect either a significant change in the speed of sound or a significant change in mass density. It has been estimated that typically, the best compromise between preserving acoustic transparency and reducing permeability is obtained by setting the proportion of nanoparticles between 2 and 8 weight percent of the polymer matrix.

Clay particles ordinarily have hydrophilic silicate surfaces that make them unsuitable for mixing with organic polymer resins. Therefore, it is necessary to render them organophilic through chemical pretreatment. Suitable pretreatments include ion-exchange reactions with organic cations (typically, alkylammonium ions) or functionalization with silanes. In a typical application, pretreated clay nanoparticles are mixed into a polyurethane resin in a proportion suitable to obtain the desired arrangement of overlapping layers of nanoparticles. Then a diamine curing agent is added to the mixture, causing the resin to polymerize. Alternatively, depending on details of a specific application, pretreated nanoparticles could be mixed into a polymer solution from which the solvent is then allowed to evaporate, leaving a solid polymer/nanoparticle composite. In yet another alternative approach, pretreated nanoparticles could be mixed into a molten thermoplastic, which could then be poured into place and cooled into a solid composite.

This work was done by Thomas R. Ramotowski of the Naval Undersea Warfare Center for the Naval Research Laboratory.

This Brief includes a Technical Support Package (TSP).
Low-Water-Permeability Encapsulants for Acoustic Sensors

(reference NRL-0014) is currently available for download from the TSP library.

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This article first appeared in the October, 2007 issue of Defense Tech Briefs Magazine.

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