Explosive and thermic compositions or charges – Containing hydrazine or hydrazine derivative
Reexamination Certificate
1998-07-13
2001-05-22
Miller, Edward A. (Department: 3641)
Explosive and thermic compositions or charges
Containing hydrazine or hydrazine derivative
C149S019700, C149S045000, C149S046000, C149S061000, C149S076000, C149S077000
Reexamination Certificate
active
06235132
ABSTRACT:
FIELD OF THE INVENTION
The invention generally relates to pyrotechnic materials, comprising at least one fuel and an oxidizer comprising ceric ammonium nitrate. The pyrotechnic materials of the invention are useful in most pyrotechnic applications requiring a mixture of a fuel and oxidizer, but are particularly useful in gas generating compositions or gas generants, such as those used in “air bag” passive restraint systems, that have a low solids output on combustion.
BACKGROUND OF THE INVENTION
Gas generators, i.e., devices for producing gas, have become more common place in the field of pyrotechnics over the last 20 years, mainly due to the increased use of automotive air bags. Typically, an automotive air bag gas generator, which is referred to as an inflator, contains a gas generant, i.e., a pyrotechnic material that generates a gas during combustion. The gas generant in the inflator in a vehicle air bag passive restraint system is typically a pyrotechnic material comprising a fuel and an oxidizer or, in the case of mono-propellants, such as nitrocellulose, a fuel having an integral oxidizer. The gas generant must provide the gas required to deploy and fill the air bag in a matter of milliseconds when an actuation signal is received by the system, and the air bag inflator must perform properly during an accident at any point in the useful life of the vehicle. The fact that an inflator may be required to rapidly fill an air bag after 10 or more years of storage places a number of constraints on inflator design, which are dictated by the required performance of the restraint system, i.e., the time required for the full deployment of the air bag, reliability (including environmental exposure and storage life), the safety and health of vehicle occupants, air bag volume, and the interface between the restraint system and the vehicle. The inflator specification that results from these constraints defines the form, fit, and function criteria for the inflator.
An example of a filterless inflator is provided in parent U.S. Pat. No. 5,551,725, which is incorporated herein by reference. The inflator described in the above identified application comprises a contained volume, a source of gas for producing an inflation gas, an initiating system for initiating the conversion of the source of gas to the inflation gas, and an exhaust orifice that provides an exhaust path and controls the flow of the inflation gas. The source of gas is typically a mixture of a fuel and oxidizer that is stable, and will not ignite until the initiating system ignites the mixture to produce the inflation gas.
A typical inflator functions by converting an electrical or mechanical initiating signal into the generation of a precisely controlled quantity of gas at precisely controlled rates. Generally, this is accomplished by an inflator pyrotechnic train, which comprises an ‘initiation’ device called an initiator, an enhancer charge, and a main gas generant charge, all of which are contained in the body of the inflator. In response to the initiating signal, the initiator ignites and produces a hot gas, particulates, and/or flame. The flame output of the initiator is typically small, and often requires enhancement to ignite the main gas generant charge. The initiator flame ignites the enhancer charge, which is a hot burning propellant, and augments the initiator output sufficiently to ignite the main gas generant charge. Once ignited, the gas generant burns to produce the hot gas required at a rate sufficient to fill the air bag module in the required time.
The restraint system performance is dictated, in part, by the need to fill and deploy the air bag in a matter of milliseconds. Under representative conditions, only about 60 milliseconds elapse between the primary impact of a vehicle in an accident and the secondary impact of the driver or passenger (herein after “an occupant”) with a portion of the vehicle interior. Therefore, a very rapid generation or release of gas is required to fill the bag, and prevent the secondary impact. The amount and rate of gas generation or release is determined by the volume of the air bag required for the vehicle and the time between primary and secondary impacts.
In addition, to meet environmental and occupant safety and health requirements, the inflation gas produced by the inflator should be non-toxic and non-noxious when the inflator is functioned in an air bag module in a typical vehicle. The gas generated or released must also have a temperature that is sufficiently low to avoid burning the occupant and the air bag, and it must be chemically inert, so that the mechanical strength and integrity of the bag are not degraded by the gas.
The stability and reliability of an inflator gas generant over the life of the vehicle are extremely important. The gas generant must be stable over a wide range of temperature and humidity conditions, and should be resistant to shock, so that the propellant pellets, grains, granules, etc. maintain mechanical strength and integrity during the life of the vehicle.
Vehicle manufacturers have developed a number of quantitative tests to determine whether an air bag restraint system will operate reliably when needed during any part of a vehicle's useful life. Although these tests and the performance requirements that an inflator should meet in these tests vary somewhat from manufacturer to manufacturer, the design criteria of all the vehicle manufacturers are essentially the same.
In a typical prior art passive restraint system the inflation gas is nitrogen, which is produced by the decomposition reaction of a gas generant containing a metal azide, typically sodium azide (NaN
3
). The metal azide is the fuel and the principal gas generating compound in the gas generant used in the inflator. A typical metal azide gas generant is disclosed in U.S. Pat. No. Re. 32,584.
The gas produced in sodium azide based inflators is relatively pure nitrogen. Because there is no carbon in the fuel, oxides of nitrogen, NO
x
, can be controlled easily by running the propellant under slightly fuel rich conditions. In contrast, the combustion of gas generants containing carbon, nitrogen, and oxygen, when formulated to be fuel rich, results in the production of carbon monoxide (CO), a toxic gas. If excess oxygen is present in such a composition to assure the complete oxidation of CO to carbon dioxide, the excess oxygen will react with nitrogen at the propellant combustion temperature to form oxides of nitrogen, which can also be toxic. Therefore, the mixture of oxidizer and fuel must approach a stoichiometric balance in gas generants of this type to avoid the production of toxic gases.
Inflator designs based on sodium azide have been shown to meet the requirements of vehicle manufacturers, and are used today in most passive restraint systems. However, there are disadvantages to this technology, including the production of large quantities of hot, solid particulates during combustion, such as sodium oxide, a highly caustic material, which results in added complexity and cost in the inflator design. The relatively high toxicity of the raw sodium azide (oral rat LD
50
of about 45 mg/kg), which must be handled during the inflator manufacturing process, can also create a disposal problem at the end of the useful life of the vehicle.
Because typical gas generants used in inflators produce solid particulates, filters must be incorporated into the inflator to separate the hot particulates from the gas prior to exhausting the gas from the inflator into the air bag. Filters are required in virtually all driver and passenger side air bag inflators that incorporate purely pyrotechnic gas generants, including sodium azide based air bag inflators because of the significant amounts of solids produced during the decomposition of the oxidizer and the combustion of the fuel. The solids produced during the combustion of the gas generant are separated from the gas stream to prevent exposure of vehicle occupants to excessive or toxic levels of airborne particulates during and after ai
Knowlton Gregory D.
Ludwig Christopher P.
Miller Edward A.
Talley Defense Systems, Inc.
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