Introduction and synopsis

Foams have unique thermal, electrical and acoustic properties. Among these are: exceptionally low thermal conductivity, making them a prime choice for thermal insulation; very low dielectric loss, allowing transmission of microwaves without attenuation or scattering; and the ability to absorb sound, suiting them as materials for noise abatement.

In this chapter we survey the thermal, electrical and acoustic properties of foams. Where possible, the underlying physical understanding of the behaviour is emphasized, since it is this which allows a degree of predictive modelling of foam properties. Case studies are used to illustrate some of the results.

Thermal properties

More foam is used for thermal insulation than for any other purpose. Closed-cell foams have the lowest thermal conductivity of any conventional non-vacuum insulation. Several factors combine to limit heat flow in foams: the low volume fraction of the solid phase; the small cell size which virtually suppresses convection and reduces radiation through repeated absorption and reflection at the cell walls; and the poor conductivity of the enclosed gas. This low thermal conductivity is exploited, at one extreme of sophistication, in the insulation for liquid oxygen rocket tanks and, at the other, in disposable cups for hot drinks.