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Castable, Relatively Stiff Acoustic-Damping Materials

Two-component materials are synthesized to overcome limitations of single-component materials.

A recently invented family of acoustic-damping materials offers advantages over prior acoustic-damping materials:

  • Relative to soft rubbers and the like previously used for acoustic damping, these materials have high moduli of elasticity; that is, these materials are stiffer and, therefore, better suited for applications in which some stiffness is required.
  • One prior acoustic-dumping material contains lead and is produced by casting into blocks that must then be machined to desired sizes and shapes. The release of lead particles during machining poses a toxicity hazard. In contrast, the present materials have little toxicity and can be cast in molds to final sizes and shapes, without machining.

It is not been possible to obtain a desired combination of high modulus of elasticity (E) and high damping in a rubber or a similar single-component material for the following reasons:

  • A rubber or similar material typically undergoes a glass-to-rubber transition in a temperature range characterized by a middle temperature Tg (denoted the glass-transition temperature). It is well established that such a material dissipates vibrations more effectively at Tg than at higher or lower temperature but also tends to be relatively soft (to have low E) at Tg.
  • It is also well established that the rate at which acoustic energy enters the material is proportional to E1/2. Hence, if a material has low E, it may not absorb acoustic energy at a rate high enough to be considered an efficient damper, even at Tg.

Although it is not possible for a single component material to exhibit high damping and a high value of E at the same time, it is possible to obtain this desired combination of properties by synthesizing a two- component material, wherein a higher- damping, lower modulus component is dispersed within a lower- damping, higher- modulus component. This principle underlies the present invention, in which two-component materials are synthesized following a phase-segregation approach common to that followed in synthesis of rubber toughened epoxy materials.

In a material according to the invention, the higher-damping, lower-modulus component is a carboxy-terminated butadiene nitrile (CTBN) formulated to have a Tg at or near the intended use temperature, and the lower-damping, higher-modulus component is an epoxy. In the first step of the synthesis of the material, a CTBN or a suitable mixture of CTBNs is mixed into an epoxy resin (typically in a proportion of 1 to 3 parts of CTBN to 10 parts of epoxy by weight or volume) at a temperature of about 150°C. (A lower temperature can be used if more time is available.) Once the epoxy resin has become modified by reaction with the CTBN, it is cooled, then mixed with the epoxy-curing agent. The curing reaction involves both cross-linking and gelling of the resin molecules. During the curing reaction, the CTBN component becomes segregated into a separate phase comprising discrete, approximately spherical rubbery domains, between 1 and 10 μm in diameter, dispersed throughout the epoxy resin. Because most of the volume of the material is occupied by the relatively high-modulus epoxy and the Tg of the rubbery domains occupying part of the volume is at or near the intended use temperature, the material can have the desired combination of sufficient stiffness and sufficient damping.

This work was done by Thomas S. Ramotowski of the Naval Research Laboratory.

Topics:
Acoustics Materials Mechanical Components Physical Sciences
This Brief includes a Technical Support Package (TSP).
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Castable, Relatively Stiff Acoustic-Damping Materials

(reference NRL-0011) is currently available for download from the TSP library.

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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 for a castable and high-modulus acoustic dampening material, which is designed to absorb acoustic energy effectively. This invention is particularly relevant for governmental purposes, as it can be manufactured and utilized by the U.S. government without incurring any royalty fees.

    The background section outlines the field of the invention, emphasizing the importance of polymers in acoustic dampening. The ability of a polymer to absorb acoustic energy is linked to energy-absorbing transitions that occur within its operational temperature range. The glass transition temperature (Tg) of the polymer is identified as the most effective energy-absorbing transition. Below this temperature, the polymer exhibits stiffness and brittleness, while above it, the polymer becomes soft and more effective at dampening acoustic energy.

    The document also discusses prior art, indicating that various materials have been explored for their dampening properties. However, it notes that natural rubber, while effective, has certain drawbacks that limit its application. The invention aims to overcome these limitations by providing a material that maintains high performance in acoustic energy absorption.

    In the detailed description of the invention, the document specifies that the high-modulus acoustic dampening material incorporates carboxy-terminated butadiene or, preferably, carboxy-terminated butadiene nitrile (CTBN) as the primary dampening element. This choice of material is significant as it enhances the dampening properties of the polymer.

    Additionally, the document mentions the formulation of the dampening material, indicating that it can include a variety of epoxy resins and curing agents, which are well-known to those skilled in the art. The specific formulation details, such as the parts per hundred of resin (phr), are also provided, suggesting a precise approach to achieving the desired properties of the dampening material.

    Overall, this patent application presents a novel approach to acoustic dampening, highlighting the potential for improved performance in various applications, particularly in governmental contexts. The invention's focus on high-modulus materials and specific polymer formulations positions it as a significant advancement in the field of acoustic energy absorption.

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