Chemists in France have synthesised new compounds from old, for an alternative for air bag inflation.

Cobalt complexes could fulfil the automotive industry's safety criteria for air bag use

Air bags inflate when energetic materials decompose to release gases. These gases will eventually enter the atmosphere, so with widespread aims to reduce environmental pollution the gases must contain only nitrogen, oxygen or water vapour, avoiding carbon mono- and dioxides.

Sylviane Sabo-Etienne from the Laboratory of Coordination Chemistry, Toulouse, and colleagues have created new energetic materials for potential use in air bags. Their aim was to combine different metal salts to avoid using carbon-containing ligands, so that no carbon mono- or dioxide could form.

The French chemists began by considering cobalt salts since cobalt hexamine trinitrate was patented ten years ago for use as a catalyst for gas generation in air bags. The group combined cobalt hexamine with a metal nitrate counterion. The high oxygen content in the final complexes promotes their full combustion to the metal oxides, nitrogen, oxygen and water vapour.

Sabo-Etienne has shown that the new compounds have the potential to fulfil the automotive industry's safety criteria. Karl Rink, who works in the area of airbag initiators as part of his research at the National Institute for Advanced Transportation Technology at the University of Idaho, Moscow, US, describes the work as important, although he adds that further tests will need to be done to ensure their suitability for use in vehicles. 'Specifications governing gas generant use in air bag applications are very demanding and ultimately very difficult to meet,' he says.

'The industry is under incessant pressure to reduce both production and material costs,' Rink warns. 'The material cost is already a concern for cobalt hexamine trinitrate generants.' With cost in mind, Sabo-Etienne and colleagues worked to find an alternative aqueous pathway to the complexes, both to make the synthesis cheaper, and because it is 'greener' for industry to work in water. They found that both the manganese and zinc analogues could be made in water under ambient conditions.

Sabo-Etienne is now working towards using these compounds for an automatic syringe for microfluidic devices. She explains: 'The compound is placed into a chamber behind a membrane. When the compound decomposes, it inflates the membrane - in a similar way to the car air bag - and the liquid will then be pushed wherever you want it to go.' The compounds could have many potential applications, says Sabo-Etienne.

Rachel Davies