8.1 Introduction

Although polyolefin foams are relatively recent additions to the range of polymeric foam materials, having been first marketed in the early sixties, they have found a use in almost every industry. Areas of application include packaging, sports and leisure, toys, insulation, automotive, military, aircraft, buoyancy, cushioning and others. This wide range results from the scope to vary properties from hard and tough through to soft and resilient. Hard (though not brittle) foams are obtained using polypropylene or high density polyethylene as the basic polymer, whilst softer materials are obtained using ethylene or propylene co-polymers such as ethylene vinyl acetate (EVA). This ability to vary foam properties by changes in the polymer is similar to that seen in polyurethane foams, although the technologies are very different since almost all polyurethane foams result from liquid technology with in situ polymerisation and blowing whilst polyolefin foams are all produced starting with the basic thermoplastic polymer.

No single foaming method dominates polyolefin foam manufacture and both continuous and batch processes are operated. The manufacturing process is a factor determining both the form of the foam (sheet, block, bead) and foam properties since the process determines the foam structure, degree of crosslinking (if any) and level of residual by-products, e.g., from the blowing agent.

Polyolefin foams are normally closed cell, though open cell products can be made by a post manufacture processing operation. The foams may be crosslinked or non-crosslinked, different processes being used for each type. Crosslinked foams retain...