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 singlecomponent 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 rubbertoughened 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.

This Brief includes a Technical Support Package (TSP).
Castable, Relatively Stiff Acoustic-Damping Materials

(reference NRL-0011) 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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