Title of Invention: Autoignition main gas generant
Abstract: 
A gas generating composition of the present invention contains at least one fuel selected from amides, imides, and metal amine-based fuels, ammonium nitrate or phase stabilized ammonium nitrate, and at least one metal oxide. A gas generating system 200 containing a-gas generant in accordance with the present invention is also contemplated. 

Detailed Description: 

CROSS-REFERENCE TO RELATED APPLICATIONS 
 The present application claims the benefit of U.S. Provisional Application No. 60/761,017 having a filing date of Jan. 19, 2006.


TECHNICAL FIELD 
 The present invention relates generally to gas generating systems, and to gas generant compositions employed in gas generator devices for automotive restraint systems, for example. 
BACKGROUND OF THE INVENTION 
 The present invention relates to nontoxic gas generating compositions that upon combustion rapidly generate gases that are useful for inflating occupant safety restraints in motor vehicles and specifically, the invention relates to thermally stable nonazide gas generants having not only acceptable burn rates, but that also, upon combustion, exhibit a relatively high gas volume to solid particulate ratio at acceptable flame temperatures. 
 The evolution from azide-based gas generants to nonazide gas generants is well-documented in the prior art. The advantages of nonazide gas generant compositions in comparison with azide gas generants have been extensively described in the patent literature, for example, U.S. Pat. Nos. 4,370,181; 4,909,549; 4,948,439; 5,084,118; 5,139,588 and 5,035,757, the discussions of which are hereby incorporated by reference. 
 In addition to a fuel constituent, pyrotechnic nonazide gas generants contain ingredients such as oxidizers to provide the required oxygen for rapid combustion and reduce the quantity of toxic gases generated, a catalyst to promote the conversion of toxic oxides of carbon and nitrogen to innocuous gases, and a slag forming constituent to cause the solid and liquid products formed during and immediately after combustion to agglomerate into filterable clinker-like particulates. Other optional additives, such as burning rate enhancers or ballistic modifiers and ignition aids, are used to control the ignitability and combustion properties of the gas generant. 
 One of the disadvantages of known nonazide gas generant compositions is the amount and physical nature of the solid residues formed during combustion. When employed in a vehicle occupant protection system, the solids produced as a result of combustion must be filtered and otherwise kept away from contact with the occupants of the vehicle. It is therefore highly desirable to develop compositions that produce a minimum of solid particulates while still providing adequate quantities of a nontoxic gas to inflate the safety device at a high rate. 
 The use of phase stabilized ammonium nitrate as an oxidizer, for example, is desirable because it generates abundant nontoxic gases and minimal solids upon combustion. To be useful, however, gas generants for automotive applications must be thermally stable when aged for 400 hours or more at 107 degree C. The compositions must also retain structural integrity when cycled between −40 degree C. and 107 degree C. Further, gas generant compositions incorporating phase stabilized or pure ammonium nitrate sometimes exhibit poor thermal stability, and produce unacceptably high levels of toxic gases, CO and NO.sub.x for example, depending on the composition of the associated additives such as plasticizers and binders. 
 Yet another problem that must be addressed is that the U.S. 
 Department of Transportation (DOT) regulations require “cap testing” for gas generants. Because of the sensitivity to detonation of fuels often used in conjunction with ammonium nitrate, many propellants incorporating ammonium nitrate do not pass the cap test unless shaped into large disks, which in turn reduces design flexibility of the inflator. 
 Yet another concern includes slower cold start ignitions of typical smokeless gas generant compositions, that is gas generant compositions that when combusted result in at least 80 weight % of gaseous combustion products as compared to the overall weight of the combustion products. 
 Many compositions containing phase stabilized ammonium nitrate contain an azole-based fuel such as a tetrazole. Although proven to be satisfactory in many applications, one concern is that azole-based fuels sometimes have a relatively shorter burnout time thereby complicating the inflation profile requirements. Furthermore, it is also an ongoing effort to economize the design of an inflator by increasing the functionality of a given composition, as an autoignition (less than 160 Celsius, perhaps) and primary gas generant for example. 
 Accordingly, ongoing efforts in the design of automotive gas generating systems, for example, include other initiatives that desirably produce more gas and less solids without the drawbacks mentioned above.


BRIEF DESCRIPTION OF THE DRAWINGS 
 FIG. 1 is an exemplary inflator incorporating a composition of the present invention. 
 FIG. 2 is an exemplary gas generating system, in this case a vehicle occupant protection system, incorporating the inflator of FIG. 1. 


SUMMARY OF THE INVENTION 
 The above-referenced concerns are resolved by gas generating systems including a gas generant composition containing phase stabilized ammonium nitrate, stabilized in a known manner, metal oxides including transitional metal oxides such as copper oxide, and a non-azole fuel, that is a fuel not containing tetrazole, triazoles, furazans, or azoles. Accordingly, typical fuels include amides and imides such as azodicarbonamide for example, or metal amine-based fuels such as copper diamine dinitrate. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
 The above-referenced concerns are resolved by gas generating systems including a gas generant composition containing phase stabilized ammonium nitrate, stabilized in a known manner, metal oxides including transitional metal oxides such as copper oxide, and a fuel having non-azole character, that is a fuel not containing, or a fuel absent of any tetrazoles, triazoles, furazans, or azoles. Stated another way, the gas generant composition may be described as having a non-azole character because it does not contain an azole-based fuel as described herein. Accordingly, typical fuels include at least one of amides and imides such as dihydrazides, hydrazides, succinic dihydrazide, hydrazodicarbonamide, dicyandiamide, urea, carbohydrazide, oxamide, oxamic hydrazide, Bi-(carbonamide)amine, azodicarbonamide, derivatives thereof, d- or l-tartaric acid amide derivatives, and mixtures thereof, for example; metal amine-based fuels such as copper diamine di-nitrate; and mixtures thereof. Exemplary methods of stabilizing the phase stabilized ammonium nitrate include co-crystallization of the ammonium nitrate with potassium salts (e.g. KNO3 at about 10-15% by weight of the total weight of the PSAN), or by the solid-state melting of ammonium nitrate with transition metal oxides. 
 Ammonium nitrate or phase stabilized ammonium nitrate (PSAN) is provided at about 60-80%, and more preferably at about 65-75% by weight of the total composition. A metal oxide is provided at about 2-10%, and more preferably at about 3-7%, by weight of the total composition. The fuel is provided at about 18-38% by weight of the total composition. It will be appreciated that the various percentages may be varied based on design requirements such as autoignition temperature and burn rate. 
 One embodiment includes 68.27% PSAN, 3.25% copper oxide, and 27.50% azodicarbonamide. The resulting gas generation is 94.9% of the total combustion products. It will further be appreciated that a typical dry blend ratio of PSAN to the metal oxide is about 10 to 1 respectively, but may be modified as per the weight percents described above. Differential Scanning Calorimeter (DSC) laboratory results indicate a composition containing ammonium nitrate melt phase stabilized with copper oxide, combined with azodicarbonamide, exhibits an autoignition onset temperature of 150.20C, with a peak autoignition temperature of 155.48C. In contrast, nitrocellulose (smokeless powder) indicates an onset of 189.35C with a peak temperature of 214.19C. Accordingly, auto-ignition occurs relatively lower with compositions of the present invention. 
 In yet another aspect of the invention, the present compositions may be employed within a gas generating system. For example, a vehicle occupant protection system made in a known way contains crash sensors in electrical or operable communication with an airbag inflator in a steering wheel or otherwise within the vehicle, and also within a seatbelt assembly. The gas generating compositions of the present invention may be employed in both subassemblies within the broader vehicle occupant protection system or gas generating system. More specifically, each gas generator employed in the automotive gas generating system may contain a gas generating composition as described herein. 
 It should be noted that all percents given herein are weight percents based on the total weight of the gas generant composition. The chemicals described herein may be supplied by companies such as Aldrich Chemical Company and Polysciences, Inc. for example. 
 As shown in FIG. 1, an exemplary inflator incorporates a dual chamber design to tailor the force of deployment an associated airbag. In general, an inflator, containing a primary autoigniting gas generating composition 12 formed as described herein, may be manufactured as known in the art. U.S. Pat. Nos. 6,422,601, 6,805,377, 6,659,500, 6,749,219, and 6,752,421 exemplify typical airbag inflator designs and are each incorporated herein by reference in their entirety. It should also be appreciated that with regard to thermal stability and USCAR requirements, it has been found that the use of desiccant, such as zeolite, provided at about 1:1 weight ratios with regard to the gas generant 12, improves the thermal stability of the present compositions. Co-owned and co-pending U.S. application Ser. No. 11/604,628 filed on Nov. 27, 2006, incorporated herein by reference, further explains how the use of a desiccant may provide thermal stability advantage. 
 Referring now to FIG. 2, the exemplary inflator 10 described above may also be incorporated into a gas generating system such as an airbag or vehicle occupant protection system 200. Airbag system 200 includes at least one airbag 202 and an inflator 10 containing a gas generant composition 12 in accordance with the present invention, coupled to airbag 202 so as to enable fluid communication with an interior of the airbag. Airbag system 200 may also include (or be in communication with) a crash event sensor 210. Crash event sensor 210 includes a known crash sensor algorithm that signals actuation of airbag system 200 via, for example, activation of airbag inflator 10 in the event of a collision. 
 Referring again to FIG. 2, airbag system 200 may also be incorporated into a broader, more comprehensive vehicle occupant restraint system 180 including additional elements such as a safety belt assembly 150. 
 FIG. 2 shows a schematic diagram of one exemplary embodiment of such a restraint system. Safety belt assembly 150 includes a safety belt housing 152 and a safety belt 100 extending from housing 152. A safety belt retractor mechanism 154 (for example, a spring-loaded mechanism) may be coupled to an end portion of the belt. In addition, a safety belt pretensioner 156 containing propellant 12 and autoignition 14 may be coupled to belt retractor mechanism 154 to actuate the retractor mechanism in the event of a collision. Typical seat belt retractor mechanisms which may be used in conjunction with the safety belt embodiments of the present invention are described in U.S. Pat. Nos. 5,743,480, 5,553,803, 5,667,161, 5,451,008, 4,558,832 and 4,597,546, incorporated herein by reference. Illustrative examples of typical pretensioners with which the safety belt embodiments of the present invention may be combined are described in U.S. Pat. Nos. 6,505,790 and 6,419,177, incorporated herein by reference. 
 Safety belt assembly 150 may also include (or be in operable communication with) a crash event sensor 158 (for example, an inertia sensor or an accelerometer) including a known crash sensor algorithm that signals actuation of belt pretensioner 156 via, for example, activation of a pyrotechnic igniter (not shown) incorporated into the pretensioner. U.S. Pat. Nos. 6,505,790 and 6,419,177, previously incorporated herein by reference, provide illustrative examples of pretensioners actuated in such a manner. 
 It should be appreciated that safety belt assembly. 150, airbag system 200, and more broadly, vehicle occupant protection system 180 exemplify but do not limit gas generating systems contemplated in accordance with the present invention. 
 The compositions may be dry or wet mixed using methods known in the art. The various constituents are generally provided in particulate form and mixed to form a uniform mixture with the other gas generant constituents. The mixture is then pelletized or formed into other useful shapes in a safe manner known in the art. 
 In one aspect of the invention, it has been found that forming a complex between ammonium nitrate and the metal oxide, copper oxide for example, may best be accomplished by melting the two compounds and then homogeneously mixing the melt. A heating/mixing vessel may be employed wherein ammonium nitrate, or phase stabilized ammonium nitrate, is heated to its melting point. It has been found that heating ammonium nitrate or phase stabilized ammonium nitrate (co-precipitated with 10-15 wt % potassium nitrate for example) at about 150-175C provides a sufficient melt. Copper oxide, or any other metal oxide such as a transitional metal oxide, is then mixed in and melted as well. The contents of the vessel may be stirred and heated to complex the copper or copper oxide with the ammonium nitrate or phase stabilized ammonium nitrate. After stirring to provide a substantially homogeneous mixture, about 15-20 minutes for example, the heat is removed and the melt is preferably slowly cooled to room temperature. After the melt solidifies into the copper complex, the solid may be ground by mortar and pestle, or other known grinding techniques. Powdered fuel and powdered complex may then be homogeneously mixed in a planetary mixer for example, and then compacted and pelletized in a known manner. The melt constituents are of course provided in the weight percents characterized herein. 
 It should be noted that all percents given herein are weight percents based on the total weight of the gas generant composition. The chemicals described herein may be supplied by companies such as Aldrich Chemical Company and Polysciences, Inc., or Toyo Kasie Kogyo Co. of Takasago City, Japan, for example. Or, the various constituents may be made as known in the art. For example, d- or l-tartaric acid amide derivatives may be formed as described in U.S. Pat. No. 5,306,844, herein incorporated by reference in its entirety. 
 In sum, the present invention provides simplification of the inflator design by only requiring a gas generating composition (auto-igniting below 200C), rather than an auto-ignition composition and a separate gas generating composition. Furthermore, in many gas generators or inflators, a booster composition must also be employed to provide the energy needed to combust the primary gas generant in the event of a fire. By eliminating the need for an auto-igniting composition, the need for a booster composition may also be eliminated if desired. Furthermore, decomposition products typically associated with the decomposition of the auto-ignition composition in typical inflators is avoided. As such, the integrity of the propellant and the performance reliability of the inflator are favorably enhanced. 
 The present description is for illustrative purposes only, and should not be construed to limit the breadth of the present invention in any way. Thus, those skilled in the art will appreciate that various modifications could be made to the presently disclosed embodiments without departing from the scope of the present invention as defined in the appended claims. 



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