TABLE OF CONTENTS David Eaves
Although polymer foams are based on a wide variety of materials and are manufactured in many different forms, there are some principles and concepts which apply to all foams. These have been reviewed in depth elsewhere [1, 2]. Since these concepts are useful in understanding the relationships between polymer properties and foam structure and can facilitate the prediction of the physical behaviour of foams in typical applications, this chapter provides a summary of some of these fundamentals.
Most foams are formed by a process involving nucleation and growth of gas bubbles in a polymer matrix (exceptions being syntactic foam where micro-beads of encapsulated gas are compounded into a polymer system, and latex foam). As the bubbles grow, the foam structure changes through stages which may be described as follows:
Initially, small dispersed spherical bubbles are generated in a liquid matrix, with a small reduction in density.
Whilst the bubbles grow but still remain spherical, the lowest foam density is achieved when bubbles reach close packed structure.
Further growth and lower foam density then involves distortion of cells to form polyhedral structures, sometimes idealised as pentagonal dodecahedrons.
Viscous and surface tension effects subsequently cause material to flow towards intersecting cell elements to form junctions of tricuspid cross-section.
A final stage may involve rupture of cell walls to result in an open cell foam.
Depending on the degree of expansion and the particular formulation, foams can have any or...
