MadSci Network: Chemistry
Query:

Re: What is the reaction that fills an automobile airbag?

Date: Mon Aug 9 17:08:50 1999
Posted By: Adrian Popa, Directors Office, Hughes Research Laboratories
Area of science: Chemistry
ID: 934026840.Ch
Message:

Greetings:

A number of manufacturing trade secrets are involved in making solid 
propellant gas generators that produce a maximum amount of gas (usually 
smokeless) and that have a minimum amount of left over hazardous reside. I 
have combined the material found in the literature into chemical reactions 
that approximate the rapid explosive process used in gas generators.

You can find excellent 3D drawings of a typical automotive airbag system 
along with a description of their operation at the following URL (Note: the 
chemical reaction on these pages has been over simplified):
 http://
www.lemurzone.com/airbag/inflate.htm

The driver's airbag unit consists of an
airbag, and a gas generator. The unit is mounted in the steering wheel hub, 
concealed beneath a fabric covering. The gas generator is a metal canister 
filled with mixture containing mostly sodium azide (formerly a solid rocket 
propellant). When a car is involved in a crash severe enough to activate 
the sensing unit(s), an electrical charge is routed to the gas generator, 
which ignites the fuel. The fuel then creates a tremendous amount of inert 
nitrogen gas filling the airbag. The total elapsed time from contact to 
full deployment of the airbag will vary, but it should deploy within 1/25 
second. To reduce the airbag’s impact on people, several multistage gas 
generator concepts are now being developed enable the deployment to match 
the severity of the crash.

A typical (old design) gas generator is filled with sodium azide (NaN3) and 
mixtures of potassium nitrate (KNO3) and silicon dioxide (SiO2) which are a 
spin-off of military and rocket propellant industries. To initiate the 
inflation process a 12 volt output signal from the airbag control computer 
heats a resistive wire element which starts the chemical reaction. This 
exothermic chemical reaction decomposes the gas generating pellets in 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

As you can see, the reactions in steps 1 and 2 release a great deal of 
nitrogen gas. It is this hot nitrogen gas that fills the airbag. The 
potentially harmful sodium created in step 1 combines with potassium 
nitrate in step 2 to produce more nitrogen, potassium oxide, and sodium 
oxide. The final result is nitrogen gas and alkaline silicate powder. The 
sodium produced in step 1 may also react with moisture, temporarily forming 
sodium hydroxide. Because these reactions occur so rapidly, the multiple 
steps in the reaction are in reality occurring simultaneously. 

There has been much concern about the impact of airbag gas generators on 
the environment, particularly in automotive wrecking yards were many old 
unused airbag systems may end up. These old devices are considered a 
hazardous material and they must be disposed of in a suitable manner.
For example, California Assembly Bill 847 was enacted in response to 
concerns over the handling of hazardous materials contained in discarded 
major appliances and other items sent to scrap yards. (California Health & 
Safety Code § 25212(a)). These materials are called "materials that require 
special handling" and include: "sodium azide canisters from unspent 
airbags". 

Several new non-hazardous gas generators have been developed and are now 
being used in automotive systems. One of these systems, developed by 
Atlantic Research Corporation, is called ARCAIR ™. This system is described 
at the following URL:
  http://
www.atlantic-research.com/docs/adh.html

Best regards, Your Mad Scientist
Adrian Popa



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