Blowing agent

A blowing agent is a substance which is capable of producing a cellular structure via a foaming process in a variety of materials that undergo hardening or phase transition, such as polymers, plastics, and metals. They are typically applied when the blown material is in a liquid stage. The cellular structure in a matrix reduces density, increasing thermal and acoustic insulation, while increasing relative stiffness of the original polymer.

Blowing agents (also known as 'pneumatogens') or related mechanisms to create holes in a matrix producing cellular materials, have been classified as follows:

Physical blowing agents e.g. CFCs (however, these are ozone depletants, banned by of 1989), HCFCs (replaced CFCs, but are still ozone depletants, therefore being phased out), hydrocarbons (e.g. pentane, isopentane, cyclopentane), liquid CO₂. The bubble/foam-making process is irreversible and endothermic, i.e. it needs heat (e.g. from a melt process or the chemical exotherm due to cross-linking), to volatile a liquid blowing agent. However, on cooling the blowing agent will condense, i.e. a reversible process.

Chemical blowing agents e.g. isocyanate and water (for PUs), azo-, hydrazine and other nitrogen-based materials (for thermoplastic and elastomeric foams), sodium bicarbonate (aka baking soda, used in thermoplastic foams). Here gaseous products and other by-products are formed by a chemical reaction(s), promoted by process or a reacting polymer's exothermic heat. Since the blowing reaction occurs forming low molecular weight compounds acting as the blowing gas, additional exothermic heat is also released. Powdered titanium hydride is used as a foaming agent in the production of metal foams, as it decomposes to form hydrogen gas and titanium at elevated temperatures.Zirconium(II) hydride is used for the same purpose. Once formed the low molecular weight compounds will never revert to the original blowing agent(s), i.e. the reaction is irreversible.

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