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Low-Water-Permeability Encapsulants for Acoustic Sensors

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.

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.

Topics:
Acoustics Coating/Surface Modification Coatings, colorants and finishes Coatings, colorants, and finishes Composite materials Materials Nanotechnology Polymers Polymers Resins Sensors Sensors and actuators Thermoplastics Water
This Brief includes a Technical Support Package (TSP).
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Low-Water-Permeability Encapsulants for Acoustic Sensors

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    Defense Tech Briefs Magazine

    This article first appeared in the October, 2007 issue of Defense Tech Briefs Magazine (Vol. 1 No. 5).

    Read more articles from the archives here.

    Overview

    The document pertains to a patent application from the Naval Undersea Warfare Center, specifically focusing on an invention titled "Ultra-Low Permeability Polymeric Encapsulants for Acoustic Applications." The patent application, identified by Attorney Docket No. 95961, was filed on May 10, 2006, by inventor Thomas S. Ramotowski. It is available for licensing, and inquiries can be directed to the Patent Counsel at the center.

    The invention addresses the limitations of existing materials used to coat naval platforms and encapsulate acoustic sensors, which have been described as "barely adequate." These limitations pose challenges for the U.S. Navy's future hardware concepts, including miniaturized and distributed sensors, large area sensors, smart skins, and high-powered acoustic sources. The invention aims to improve performance while aligning with the Navy's goals of reducing total ownership costs and extending service lifetimes.

    The encapsulant is made from an acoustically clear polymer, such as polyurethane, and incorporates high aspect ratio clay nanoparticles. These nanoparticles are arranged in overlapping layers within the polymer substrate, which enhances the encapsulant's properties. The invention also outlines a method for creating this acoustically transparent, low permeability encapsulant. This method involves treating the clay nanoparticles to make them organophilic, mixing them with a polymer resin to form an intercalated mixture, and then adding a curing agent to set the mixture.

    The document highlights the advantages of using chemically modified clay nanoparticles, which significantly reduce water permeation through the polymer while maintaining acoustic clarity. The required amount of filler is minimal (approximately 2-8%), which helps to avoid issues related to density and sound speed changes.

    Overall, this invention represents a significant advancement in materials science for naval applications, providing a solution that enhances the performance of coatings and encapsulants used in acoustic technologies. The document concludes with a statement of government interest, indicating that the invention may be manufactured and used by the U.S. government without the payment of royalties. This underscores the potential impact of the invention on governmental projects and initiatives.

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