Abstract:
A gas generating composition is disclosed wherein the combustible reactants are an alkali metal azide and a metal oxide which upon thermal initiation react to produce alkali metal oxide, metal and pure nitrogen gas which can be used to inflate an impact protection air cushion of an automotive restraint system.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to gas generating compositions and, in particular, to such compositions which are suitable for inflating an air cushion in vehicle restraint systems of the type which utilize such a cushion to protect vehicle occupants upon sudden stopping or deceleration of the vehicle in which they are riding. The air cushion, which is commonly formed of an inflatable bag, acts to prevent the secondary collision between the occupants and interior structural components of the vehicle resulting from the primary collision between the vehicle and another object. 
     2. Description of the Prior Art 
     One method commonly proposed for inflating the air cushion of vehicle restraint systems involves the use of a sealed cylinder of compressed gas which communicates with the collapsed bag mounted in the interior of the vehicle. The compressed gas is released by impact responsive actuators or sensors which sense a rapid change in velocity of the vehicle as, for example, when the vehicle collides with another object. 
     Another method proposed for inflating the air cushion involves the use of an ignitable propellant system, where the inflating gas is generated by the exothermic reaction of the reactants forming the propellant composition. 
     The bags used in restraint systems of the type described must in order to accomplish their purpose be inflated to a sufficient degree within a very short time span, generally on the order of a few milliseconds. For example, under certain representative conditions only about 60 milliseconds elapse between occurrence of the primary and secondary collisions. In addition, the gas itself must meet several rather stringent requirements. The gas must be nontoxic and non-noxious, and its temperature as it is generated must be low enough not to burn the bag or of itself undermine the mechanical strength of the bag. Furthermore, the temperature of the gas must be sufficiently cool so as not to burn the passengers in the vehicle in the event the bag ruptures. 
     In systems which utilize an ignitable propellant, the stability and reliability of the propellant composition over the life of the vehicle are highly important. Generally, the propellant composition must possess sufficient stability to temperature, humidity and shock so that it is virtually incapable of being set off except upon deliberate initiation by the actuating sensors employed. 
     One gas which possesses the required characteristics is nitrogen. In the past, compositions containing sodium azide as one of the reactants have been used to generate pure nitrogen. However, with heretofore known compositions of this nature, the decomposition of the azide leaves a residue of sodium metal which disadvantageously burns spontaneously upon contact with air presenting an extremely undesirable fire hazard to vehicle occupants. Also, the sodium residue disadvantageously exhibits a propensity to react with moisture to produce hydrogen. Hydrogen quite obviously is a highly undesirable by-product, also because of its high flammability characteristics. To eliminate the formation of hydrogen, various organic compounds have been used in previous azide-containing compositions to react with sodium metal residue. But with all such organic compounds, some carbon monoxide is generated as a side product of the reaction. This also is undesirable for the applications herein discussed because of the toxic properties of carbon monoxide. 
     The present invention provides a novel gas generating composition which, when ignited, generates nitrogen gas without any of the deleterious side-products associated with the reaction of known compositions. 
     SUMMARY OF THE INVENTION 
     In accordance with the teachings of the present invention, a solid gas generating composition is provided which, when thermally initiated, rapidly generates pure nitrogen which can be used for inflating an impact protection air cushion. The solid reactants of the composition include an alkali metal azide and a metal oxide. In the preferred embodiment, an excess stoichiometric mixture of sodium azide and copper oxide is used. The decomposition products of the reaction are pure nitrogen gas, liquid copper and sodium oxide. Importantly the gas generated is free of noxious and toxic impurities and can be produced at temperatures low enough to preclude damage to the air cushion or to vehicle occupants. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The propellant gas generated in accordance with this invention comprises nitrogen gas which is released by the exothermic reaction of an alkali metal azide with the oxide of a metal lower in the electromotive series than the alkali metal of the azide. The reaction is thermally initiated at temperatures between about 600° to 800° Farenheit. Because of the occurrence of spontaneous combustion when alkali metals are exposed to air, the metal of the oxide must not be an alkali metal. Although the exact mechanism is not known, this reaction is believed to proceed in two steps. The first step involves the decomposition of the alkali metal azide to form alkali metal and nitrogen. The second step of the reaction is thought to involve the reaction of the alkali metal formed in the first stage of decomposition with the metal oxide to form alkali metal oxide and metal. These reactions can be represented as follows: ##STR1## wherein M&#39; is an alkali metal, M&#34; is a metal (other than an alkali metal) lower in the electromotive series than M&#39;, and wherein x is the valence of M&#34;. Examples of alkali metal azides include lithium azide, sodium azide, and potassium azide. Examples of suitable metal oxides include copper oxide, iron oxide, stannic oxide, titanium dioxide, lead oxide, chromium oxide and zinc oxide. 
     It is to be noted that if there is any excess azide present in the mixture, complete reaction will not occur and alkali metal will be formed, which is highly undesirable for reasons hereinbefore discussed. Thus, the amount of metal oxide in the mixture must be sufficient to react with substantially all of the alkali metal of the azide. To insure complete reaction of the azide, it is preferred that the mixture contain a slight stoichiometric excess (a few percent) of the metal oxide. A mixture containing a stoichiometric excess of the metal oxide is one in which the amount of metal oxide exceeds the amount which represents the theoretical stoichiometric amount. It is also to be noted however that if an amount of metal oxide greater than a few percent above the stoichiometric amount is used, the efficiency of the reaction drops and, consequently, less gas per unit weight of the propellant composition is produced. Examples of suitable compositions preferred according to the invention are as follows: 
     EXAMPLE 1 
     A mixture comprising 70.9% by weight of sodium azide and 29.1% by weight of iron oxide. (Fe 2  O 3 ). 
     EXAMPLE 2 
     A mixture containing 63.3% by weight of sodium azide together with 36.7% by weight of stannic oxide. 
     EXAMPLE 3 
     A mixture comprising 76.5% of sodium azide together with 23.5% by weight of titanium dioxide. 
     EXAMPLE 4 
     A mixture comprising 62.0% of sodium azide together with 38.0% by weight of copper oxide. 
     EXAMPLE 5 
     A mixture comprising 55.2% of lithium azide together with 44.8% by weight of copper oxide. 
     EXAMPLE 6 
     A mixture comprising 67.1% of potassium azide together with 32.9% by weight of copper oxide. 
     EXAMPLE 7 
     A mixture comprising 64.8% of Lithium azide together with 35.2% by weight of iron oxide. (Fe 2  O 3 ). 
     EXAMPLE 8 
     A mixture comprising 75.3% of potassium azide together with 24.7% by weight of iron oxide. (Fe 2  O 3 ). 
     Particularly preferred is the mixture set forth in Example 4 above. This mixture was prepared by taking sodium azide in granular form and reducing the size of the granular particles by grinding in a hammer mill to a particle size of less than 50 microns. The ground azide and untreated copper oxide in commercially available powdered form were blended in a twin shell V-blender in the proper proportions until a heterogeneous mixture was obtained. 
     This mixture exhibits a life of about 30 years and high stability to shock, temperature and humidity. The composition is stable in these respects in that it can be stored for long periods of time without noticeable deterioration, it can be stored at temperatures up to about 240° F. for several days and it can withstand relative humidity up to about 80% for at least 30 days. Furthermore, the composition of the preferred embodiment will not detonate as a result of static electricity nor will it detonate upon initiation by a No. 8 blasting cap through a tetryl booster acting on 1 quart of the propellant composition. Thus the composition is extremely stable and can be initiated only at an ignition temperature above 600° F. 
     The most significant aspect of the reaction of this mixture is the rapid generation of pure, non-toxic nitrogen gas without any of the harmful side-products formed upon decomposition of heretofore known azide compositions and at a temperature not harmful to human beings. For example, 110 grams of the preferred mixture ignited at a temperature of 796° F. produced enough pure nitrogen to fully inflate an air cushion having a volume of 2.6 cubic feet in less than 25 milliseconds. And, 250 grams of this mixture ignited at the same temperature produced enough gas to fully inflate an air cushion having a volume of 6.0 cubic feet in less than 25 milliseconds. 
     Generally, any size air cushion can be inflated in the requisite time period by employing a sufficient amount of propellant, the ratio between amount of propellant and air cushion volume remaining roughly constant. 
     The above description has been made primarily with reference to the best mode contemplated for practicing the invention. It will be recognized that various modifications can be made to the invention without departing from the spirit and scope of the following claims.