Cure Terms Glossary

Chemical Blowing Agent

Chemical blowing agents generate a foaming gas by a chemical reaction - i.e. they do not foam directly themselves (unlike a physical blowing agent). An example is the use of water in a polyurethane formulation where carbon dioxide is produced by isocyanate hydrolysis.

—NCO + H2O → —NH2 + CO2

Water is always used for low density polyurethane foams. Water alone can reach densities below 25 kg/m3 - the limit of expansion often determined by the exotherm of reaction and the risk of scorch or even fire. Large-scale production of the lowest density slabstock foams uses some physical blowing agent in conjunction with water.

Carbon dioxide can also be generated by the (endothermic) thermal decomposition of sodium bicarbonate, sometimes activated with citric acid. The temperatures required make these suitable blowing agents for rubber undergoing vulcanisation.

Organic species with the capability to release nitrogen on heating provide an exothermic alternative to foaming at vulcanisation temperatures. Examples include: azodicarbonamide (ADC), p-toluenesulphonyl semicarbazide (TSH) and p,p'-oxy-bis(benzenesulphonyl hydrazide (OBSH).