Common gases used in polymer foams are air, nitrogen, and carbon dioxide. The proportion of gas can range up to 90% or more. The gas is introduced into the polymer by several methods, called foaming processes. These include:
Mixing a liquid resin with air by mechanical agitation, then hardening the polymer by means of heat or chemical reaction
Mixing a physical blowing agent with the polymer — a gas such as nitrogen (N2) or pentane (C5H12), which can be dissolved in the polymer melt under pressure, so that the gas comes out of solution and expands when the pressure is subsequently reduced
Mixing the polymer with chemical compounds, called chemical blowing agents, that decompose at elevated temperatures to liberate gases such as CO2 or N2 within the melt.
The way the gas is distributed throughout the polymer matrix distinguishes two basic foam structures, illustrated in Fig1:
Closed cell, where the gas pores are roughly spherical and completely separated from each other by the polymer matrix; and
Open cell, in which the pores are interconnected to some extent, allowing passage of a fluid through the foam
A closed cell structure makes a satisfactory life jacket; an open cell structure would become waterlogged. Other attributes that characterize the structure include the relative proportions of polymer and gas and the cell density (number of cells per unit volume) which is inversely related to the size of the individual air cells in the foam.