MadSci Network: Chemistry |
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
Try the links in the MadSci Library for more information on Chemistry.