Cure Terms Glossary
Physical Blowing Agent
Physical blowing agents achieve foaming with any chemical change (unlike a chemical blowing agent). They are soluble additives which can achieve blowing by decompression or the action of heat. Examples include low-boiling hydrocarbons or halogenated species of various types (e.g. CFC, HCFC, HCC or HFC), such as those listed below.
Volatility and molecular weight are important parameters. The higher the volatility, then the more efficient the blowing (i.e. higher expansion) provided that a cell nucleation and growth is controllable. In principle, the direct injection of carbon dioxide (Bp -78°C) offers scope for very high expansions, but the challenge of controlled decompression limits the ultimate potential. Molecular weight impacts on the thermal conductivity of the resultant cells, with higher molecular weights offering scope for better thermal insulation. Such considerations made CFCs a popular choice until environmental concerns came to light.
In the case of hydrocarbons, volatility impacts on flash point - e.g., isobutane -82°C (Bp -12°C), isopentane -51°C (Bp 28°C), cyclopentane -37°C (Bp 49°C) - and all C4 and C5 variants are classified highly flammable. Meeting safety and environmental concerns can present major challenges, although physical blowing agents remain a preferred choice for insulating foams.
The action of vaporisation
consumes latent heat, and so physical blowing is an endothermic process.
In some cases this may limit the levels of blowing that can be achieved
without suppressing the cure, although the energy uptake may provide an
additional level of control to highly-exothermic formulations such as
acid-catalysed resole cures.