Blowing agents

Abstract

The production of a foam normally but not inevitably involves using a blowing agent to generate gas within the polymer, while the latter is temporarily in a viscous liquid state. Foam formation has the advantage of greatly decreasing the weight of the finished product. It also reduces the modulus, while improving other properties such as energy absorption and thermal and acoustic insulation performance. The insulation characteristics of a foam can depend markedly on the gas contained in the cells, and therefore the choice of blowing agent is not simply a matter for consideration during the processing stage. It also affects the ultimate properties. These properties depend on the detailed microstructure of the foam and on the volume fraction of cavities. This in turn depends on the surface tension of the liquid, the rheology of the mix at the time of foam formation, the prevailing temperature, the number of gas bubbles per unit volume, the average wall thickness of the cells etc. The formation of a foam has to be synchronized with the formation of the finished part.