10.1 Introduction

Microcellular foams refer to thermoplastic foams with cells of the order of 10 ?m in size. Typically these foams are rigid, closed-cell structures, although recently there is much interest in creating open-cell, porous structures that have cells in this size range. The microcellular process that sparked the growth in this field over the past two decades was invented at Massachusetts Institute of Technology, USA, in the early eighties [1], in response to a challenge by food and film packaging companies to reduce the amount of polymer used in their industries. As most of these applications used solid, thin-walled plastics, reducing their densities by traditional foaming processes that produced bubbles larger than 0.25 mm was not feasible due to excessive loss of strength. Thus was born the idea to create microcellular foam, where we could have, for example, 100 bubbles across 1 mm thickness, and expect to have a reasonable strength for the intended applications.

It would be reasonable to say that the potential of microcellular foams has yet to be realised. These materials have not yet appeared in mass produced plastic items, and the promised savings in materials and associated costs have yet to materialise. This is largely due to manufacturing difficulties encountered in scaling up for large scale production. However, enthusiasm for these materials remains high, and today researchers and commercial enterprises on every continent are in a global race to harness the potential benefits.

Much has been learned about...