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 (1) mixing a liquid resin with air
by mechanical agitation, then hardening the polymer by means of heat or chemical
reaction; (2) 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; and (3) 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