New technologies are creating novel design engineeringopportunities for syntacticfoams, which are composite materials whose matrix is embedded withpreformed particles such as glass or ceramic microspheres. They are calledsyntactic because the spheres provide an ordered structure.

"Sometimes we like to refer to it as reinforced air," saysThomas J. Murray, a senior materials engineer who co-founded CMT Materials of Attleboro, MAto explore new applications opportunities for the technology.

The reference to "reinforced air" is half humorous but itgives syntactic foam unique characteristics compared to foam that is blown withchemicals or gases.

The hollow particles constitute half or more the volume ofthe composite, resulting in lower density, higher strength, resistance tocompressive stress and a lower coefficient of thermal expansion.

Recent U.S.patent approvals show some of the engineering potential for syntactic foam:

One of the big benefits of syntactic foam composites is theirability to be tailored for specific applications. The matrix material can be manydifferent types of metals, polymers or ceramics. The microballoons can be made fromglass, carbon and polymers. Some of the most commonly used products are glassbubbles from 3M,which are also widely used in plastic compounds for specific characteristicssuch as weight, sandability and sealing surfaces.

The compressive properties of syntactic foams derive fromthe properties of microballoons, while the tensile properties come from thematrix material. Properties can be adjusted by changing the volume of themicroballoons or by adjusting their wall thickness.

Thermoforming angle

CMT Materials developed thermoplastic matrix materials inthe 1990s for use as plug-assists in thermoforming dies.

"The thermoplastic materials allow much thinner sectionsbecause the epoxy composites can be quite brittle," says Murray. As a result, thermoformed packagesused for electronics or medical devices can be made with thinner sections.High-end thermoplastics are used to withstand temperatures above 300F.

Glass bubbles were developed in early 1960s for use asbuoyancy aid materials for marine applications.

One recent application for CMT Materials is buoyancy supportfor unmanned undersea vehicles developed by the Woods Hole OceanographicInstitute and now produced by Hydroid Inc. of Pocasset, MA.The purpose of the vehicles is to conduct environmental surveillance and searchfor mines and other underwater weapons. "There are 13 syntactic pieces on the Remus 6000," says Murray.

Another interesting new use is for a miniature computerweighing less than 5 oz that is attached to the backs of beaked whales withsuction cups.

The computer gives researchers insights into the behaviorand sounds of one of the more reclusive and deep-diving members of the species.One of the few known facts about these toothed whales is that they have beeninvolved in a number of mass strandings in recent years. One suspected cause isnaval sonar.

The digital tag wasdeveloped by biologists and engineers at the Woods Hole OceanographicInstitution (WHOI) to record sounds — those made by the whale, as well as otherwhales, boats, sonars and other sources. One way to think of the tag is as an MP3player, PDA and home medical monitor all rolled into one.

Suction cups are made from medical-grade silicon, and therole of the foam is to shoot the recorder to the surface for later retrieval.

So far, the device has recorded dives as deep as 6,000 ft,lasting 90 min.

New Syntactic Foams Create Engineering Opportunities A