5 - Fundamentals of Polymeric Foams and Classification of Foam Types

Abstract

Foam is commonly defined as a dispersion of gas bubbles in a liquid. In the case of “solid foam,” the liquid is now into a gel or a solid phase after the gas dispersion and expansion. The volume fraction ϕg of the gas in foams can range from as low as 5% for very high-density foams to as high as 98% for very low-density foams. If ϕg is greater than 75%, the bubbles push against each other and deform losing their spherical shape to form polyhedral cellular morphologies. For the majority of conventional foams, the average cell size is greater than 300µm, and the cell density is less than 106 (cells/cm3). There are foams with fine cellular structures where the average cell size falls in the range 10–300µm, with cell densities in the range 106–109 (cells/cm3). Yet even finer celled foams sometimes referred to as micro-cellular foams are currently being developed with average cell sizes below 10µm and cell density greater than 109 (cells/cm3). Special classes of micro-cellular foams often termed nano-foams are now being developed. These foams have extremely small cells with cell size in the nanometer range, typically less than 100 nanomicrons. These foams find special applications in areas requiring very high insulation due to their very low thermal conductivities. This arises from special heat transport mechanisms resulting from their very fine cellular structures