Phases of Foam

Phases of Foam

By Ray Erikson

Syntactic foams may be considered two-phase, three-phase, or multiphase. A two-phase foam consists of a matrix material, such as a resin or metallic alloy, and microspheres. Resin-based two-phase foams with glass microspheres will typically have a density of 0.5 to 1.0 grams per cubic centimeter (30 to 60 pounds per cubic foot). Foams in this range are virtually waterproof.

For comparison: honeycombs range from 0.03 to 0.15 g/cc; water has a density of 1.0 g/cc; woods range from 0.3 to 1.1 g/cc (yes, limbs of the ironwood tree sink in water); advanced laminated composites run around 1.8 g/cc; and solid aluminum is about 2.7 g/cc.

A simple three-phase foam consists of matrix material, microspheres, and either macrospheres or voids between the microspheres not completely filled with matrix material. Practical, resin-based three-phase foams can reach densities as low as 0.15 g/cc.

Down to 0.3 g/cc, three-phase foams are still largely water-resistant, closed-cell materials. Below 0.3 g/cc, the resin matrix becomes sufficiently sparse so that the voids can connect to form channels. This makes the material susceptible to water intrusion, although experiments conducted for the F-22 fighter program showed that even fully saturated, ultralight foams suffered no loss in structural integrity, even at -65°C.

A multiphase foam combines additional components with a two- or three-phase basic foam. These may take the form of fibers for reinforcement, pigments for color, or special materials that modify the foam's electrical or thermal properties.