New technologies are creating novel design engineering opportunities for syntactic foams, which are composite materials whose matrix is embedded with preformed particles such as glass or ceramic microspheres. They are called syntactic because the spheres provide an ordered structure.
"Sometimes we like to refer to it as reinforced air," says Thomas J. Murray, a senior materials engineer who co-founded CMT Materials of Attleboro, MA to explore new applications opportunities for the technology.
The reference to "reinforced air" is half humorous but it gives syntactic foam unique characteristics compared to foam that is blown with chemicals or gases.
The hollow particles constitute half or more the volume of the composite, resulting in lower density, higher strength, resistance to compressive stress and a lower coefficient of thermal expansion.
Recent U.S. patent approvals show some of the engineering potential for syntactic foam:
- An implant device material can replace living bone tissue. In this application from McDonnell Douglas Corp. of St. Louis, MO, polyetherimide thermoplastic is used to wet and bond microballoons in an array. The molded or machined syntactic foam material is highly biocompatible and stable with no apparent adverse effects on a recipient.
- A lightweight, watertight acoustic sensor module for use in towed array retrieval systems is produced by casting transducers and supporting telemetry into syntactic foam composite. The U.S. Navy patented the device to reduce maintenance costs of current O-ring-based systems used on submarines.
- A single-walled container insulated by syntactic foam was developed by Insulation Dimension Corp. of Leland, NC, for storing hot or cold foods or liquids, such as coffee sold at a fast-food restaurant.
One of the big benefits of syntactic foam composites is their ability to be tailored for specific applications. The matrix material can be many different types of metals, polymers or ceramics. The microballoons can be made from glass, carbon and polymers. Some of the most commonly used products are glass bubbles from 3M, which are also widely used in plastic compounds for specific characteristics such as weight, sandability and sealing surfaces.
The compressive properties of syntactic foams derive from the properties of microballoons, while the tensile properties come from the matrix material. Properties can be adjusted by changing the volume of the microballoons or by adjusting their wall thickness.
CMT Materials developed thermoplastic matrix materials in the 1990s for use as plug-assists in thermoforming dies.
"The thermoplastic materials allow much thinner sections because the epoxy composites can be quite brittle," says Murray. As a result, thermoformed packages used 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 as buoyancy aid materials for marine applications.
One recent application for CMT Materials is buoyancy support for unmanned undersea vehicles developed by the Woods Hole Oceanographic Institute and now produced by Hydroid Inc. of Pocasset, MA. The purpose of the vehicles is to conduct environmental surveillance and search for mines and other underwater weapons. "There are 13 syntactic pieces on the Remus 6000," says Murray.
Another interesting new use is for a miniature computer weighing less than 5 oz that is attached to the backs of beaked whales with suction cups.
The computer gives researchers insights into the behavior and 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 been involved in a number of mass strandings in recent years. One suspected cause is naval sonar.
The digital tag was developed by biologists and engineers at the Woods Hole Oceanographic Institution (WHOI) to record sounds â those made by the whale, as well as other whales, boats, sonars and other sources. One way to think of the tag is as an MP3 player, PDA and home medical monitor all rolled into one.
Suction cups are made from medical-grade silicon, and the role 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.