| Low-Water-Permeability Encapsulants for Acoustic Sensors |
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| Naval Research Laboratory, Washington, DC | |
| Oct 01 2007 | |
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Advertisement: 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. Login first to download.
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