Cure Terms Glossary
In the polymer industry, blowing is the process of pressurisation by a gas (or vapour) for expanding a polymer melt. When applied on (as distinct from in) the polymer, blowing streches the profile to reduce wall thickness (film blowing) or assist moulding (blow moulding). When the pressurization is effected within the material, then bulk volume is expanded (i.e. density reduced) through the creation of a foamed or cellular structure. Foaming can be used to reduce modulus or component weight, or to enhance thermal or acoustic insulation.
Foaming can be achieved in a number of ways. For example, nitrogen gas or liquid carbon dioxide can be injected into the polymer. Alternatively, if the viscosity is suitably low (e.g. with a rubber latex) air can be mechanically beaten into the polymer. However, commonly, it is achieved by means of additives. Such additives are classified according to the mechanism by which they operate: a physical blowing agent simply volatilises to generate the foam whilst a chemical blowing agent produces a foaming gas by chemical reaction.
In a foaming liquid,
cell growth is driven by hydrostatic pressure and resisted by surface
tension effects. In a liquid undergoing cure, cell growth is resisted
by the elasticity of the matrix once gelation
has set in. Thus, unless the network is subject to tension set, volume
expansion in foaming effectively stops at the gel