The sodium azide is a solid pellet system (and shouldn't exfiltrate from the bag), which under ignition reacts with solid potasium nitrate to produce the nitrogen gas that inflates the bag: It is destroyed in the ignition process
http://auto.howstuffworks.com/airbag.htm
The reaction is "a three step process which proceeds as follows:
This initial reaction forms sodium and hot nitrogen gas which inflates the airbag.
2 NaN3 —> 2 Na + 3 N2
The sodium byproduct of the first reaction and the potassium nitrate generate additional nitrogen in the secondary reaction.
10 Na + 2 KNO3 —> K2O + 5 Na2O + N2
And finally the previous two reactions leave potassium oxide and sodium oxide to react with the third component of the mixture, silicon dioxide, forming alkaline silicate "glass".
K2O + Na2O + SiO2 —> alkaline silicate"
http://www.madsci.org/posts/archives/aug99/934263277.Ch.r.html
I would still assume the plastic cover has a materials problem, as the physical structure of the bag doesn't support a release of sodium azide, which tends to be reactive more with metals rather than plastics (we used plastic or glass beakers and plastic spatulas when we worked with it in the labs).
Apparently the 15 year life is more related to break down of the semi-porous bag material, and the igniter is somewhat more reliable, (but still limited to 15 years). This implies that bag ignition on an older bag may work properly, may only partially inflate a degrading bag, or may not inflate a torn bag at all.
BTW, I went ahead and replaced the 16+ year old seatbelt in my car, as I suspected ozone, UV, and dust abbrasion damage to the belt, as well as fatigue to the reel and lock up mechanism. I also knew that the fatiqued spring had allowed the belt to be slammed in the door at least 4-5 times as well. But I had overlooked that the tensioner is also an explosive device, possibly with a finite shelf life!? The tensioner, reel, belt, etc. are sold as one unit. It looks rather like a tapered barrel single shot pistol with a small handle (explosive charge capsule) and a fishing reel tacked on the side. The tech secured the old device in a vise and applied a voltage to the tensioner from a remote location (full range safety rules
). The propellent was still good, as it sounded like a 410 shotgun going off! This made the belt system safe for disposal.
The new belt reels in much more easily, looks and feels clean and is more supple than the old belt, and was a 20-30 minute installation. About $250 parts and labor covered everything. So apparently, for both the belts and bags, the fabric is more of a weak link than the explosive charge, from what I have read and observed in this (admittedly) small experiment.