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.
Thermoforming angle
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 — 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.
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