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An Objective Characterization of the Cell Size of Microcellular Foams

Microcellular foams are foams whose cell or pore size is much smaller than that obtained in conventional foams. For example, blown polystyrene foam will usually have a cell size of 50 to 100 micrometers (p,m), whereas a microcellular polystyrene foam will have a cell size of 1 to 10 ~.m and in some cases as small as 0.1 lim [1,2]. The name derives from observations made on conventional foams of very small cells (microcells) sometimes occurring in the walls or struts of the foam [3]. We are finding a wide variety of uses for microcellular foams. In all applications the small cell size is a necessary requirement for their intended use. Some applications occur in high energy physics experiments (such as inertial confinement fusion) in which very small cell size structural foams are needed. The small cell size is required so that the foam mimics a uniform media [4]. Many biomedical applications have been found for microcellular foams, such as for vascular grafts [5,6]. Microcellular foams are being investigated as catalyst supports because the small cell size occurs simultaneously with a very high surface area. They could also have applications in chromatography and ion exchange. A unique filtration application is in the collection of comet dust [7]. In this application, the small-celled foam is needed to collect dust at high velocities (15 km/sec) without melting the dust. Only foams have been successful for this application. Undoubtedly, there are many additional uses for microcellular foams.