Primers, or detonators as they are alternatively known, are the means used to detonate an explosive charge. Those devices may be detonated by electrical or purely mechanical means. They are comprised of a primary explosive which may be initiated by stab, friction or impact, an intermediate explosive composition that is set off by the primer composition, and a base charge of a secondary explosive such as RDX or HMX to provide the desired explosive output. These primers ordinarily coupled with booster charges and a propellant to form an explodable ignition chain.
Primer/detonator compositions are employed in a variety of applications, including weapon systems, razing buildings, and in creating excavations. Primer/detonators are also used to initiate the explosive chain in automobile air bag systems. Explosive charges are uniquely suited for these systems because the explosive reaction instantaneously generates a large volume of gas that fills the air bag. It is essential that the air bag be filled within that instant between collision and the occupant's impact with the dashboard.
As the primer/detonator functions to initiate the explosive reaction, the primer must possess some means for igniting the chain. This is the function of the sensitizer, which may be in either electrical or mechanical form.
Airbag systems are ordinarily housed in a metallic structure. As it is desirable from an engineering and design standpoint to choose the lightest materials available, aluminum is the material of choice for housing.