Abstract:
Gas generative compositions especially useful in inflators for protective passive restraint devices (e.g., motor vehicle air bags, escape slide chutes, lift rafts, and the like) include a nitrogen-containing fuel and an oxidizer selected from copper (II) oxide (CuO), cupric nitrate, basic copper nitrate (Cu(NO 3 ) 2 .3Cu(OH 2 ), strontium nitrate (Sr(NO 3 ) 2 ) and mixtures thereof. Most preferably the nitrogen-containing fuel is azodicarbonamidine dinitrate (AZODN) and/or 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20). The compositions of the present invention provide high burning rates with acceptable burning rate pressure exponents which allow their operation at lower pressures, thereby resulting in the use of less costly, lower weight, and lower strength materials for design and manufacture of the inflator pressure vessel.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to inflators for devices such as protective passive restraints or air bags used in motor vehicles, escape slide chutes, life rafts, and the like. More particularly, the present invention relates to gas generative compositions which exhibit low insoluble combustion products. 
     BACKGROUND AND SUMMARY OF THE INVENTION 
     Many devices, such as protective passive restraints or air bags used in motor vehicles, escape slide chutes, life rafts, and the like, are normally stored in a deflated state and are inflated with gas substantially instantaneously at the time of need. Such devices are generally stored and used in close proximity to humans and, therefore, must be designed with a high safety factor which is effective under all conceivable ambient conditions. 
     Inflation is sometimes accomplished solely by means of a gas generative composition. At other times, inflation is accomplished by means of a gas, such as air, nitrogen, carbon dioxide, helium, and the like, which is stored under pressure and further pressurized and supplemented at the time of use by the addition of high temperature combustion gas products produced by the burning of a gas-generative composition. 
     It is, of course, critical that the gas-generative composition be capable of safe and reliable storage without decomposition or ignition at temperatures which are likely to be encountered in a motor vehicle or other storage environment. For example, temperatures as high as about 107° C. (225° F.) may reasonably be experienced. It is also important that substantially all the combustion products generated during use be non-toxic, non-corrosive, non-flammable, particularly where the inflator device is used in a closed environment, such as a passenger compartment of a motor vehicle. 
     Broadly, the present invention is directed toward gas generative compositions which exhibit low concentrations of insoluble combustion products. In this regard, the gas generative compositions of the present invention are embodied in a solid mixture of a nitrogen-containing fuel and an oxidizer selected from oxides of copper, nitrates of copper and strontium, and mixtures thereof. Most preferably, the nitrogen-containing fuel is azodicarbonamidine dinitrate (AZODN) and/or 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane, colloquially known in the art as &#34;CL-20&#34;. The oxidizer is most preferably copper (II) oxide (CuO) and/or basic copper nitrate (also known as copper trihydroxynitrate (Cu(NO 3 )hd 2.3Cu(OH 2 ) and/or strontium nitrate (Sr(NO 3 ) 2 ). 
     It has been discovered that the compositions of the present invention provide high burning rates with acceptable burning rate pressure exponents which allow their operation at lower pressures, thereby resulting in the use of less costly, lower weight, and lower strength materials for design and manufacture of the inflator pressure vessel. In accordance with this invention, the use of nitrate and perchlorate salts of azodicarbonamidine, and in particular azodicarbonamidine dinitrate, or hexanitrohexaazaisowurtzitane (CL-20) and mixtures thereof, in combination with oxidizers such as copper oxide, basic copper nitrate, strontium nitrate or mixtures thereof, and optionally a binder for providing structural integrity, results in heterogeneous propellant compositions which provide greater total gas output and a lower concentration of insoluble solid combustion products, than when such oxidizers are used with prior art fuels such as guanidine nitrate, aminoguanidine nitrate, nitroguanidine, ethylenediamine dinitrate, cyclotrimethylenetrinitramine (RDX), cyclotetramethylenetetranitramine (HMX), and various tetrazole derivatives, such as 5-aminotetrazole, diammonium bitetrazole, and potassium 5-aminotetrazole. 
     As discussed above and in accordance with the present invention, a distinct advantage results from the use of azodicarbonamidine dinitrate (AZODN) and/or hexanitrohexaazaisowurtzitane (CL-20) as the major fuel component because the nitric acid salt of azodicarbonamidine, and CL-20, have a significantly better oxygen balance as compared to conventional fuels. This improved oxygen balance thereby allows the use of a significantly lower concentration of oxidizer to maintain the proper stoichiometry for burning to substantially innocuous gaseous combustion products consisting of carbon dioxide, nitrogen, and water vapor. In addition, because a lower concentration of solid oxidizer is required, an associated lower concentration of substantially insoluble and clinkerable solid combustion products are formed when compared with prior art formulations. Because a lower total concentration of solid combustion products is formed, acceptable filtration of the solid products which do not clink and are not trapped in the combustion chamber is readily achieved by using fewer mechanical screens or other filtering media within the inflator, and still retain a lowered susceptibility for initiating an asthmatic reaction from the occupant of the vehicle. 
     The gas generant compositions of this invention are particularly useful when employed in inflatable passive vehicle occupant restraint systems (e.g., air bag systems). Thus, the compositions of this invention may be employed as a monopropellant, fuel or partial fuel ingredient for use in hybrid inflation systems, airbag propellants, multiple airbag propellant combinations, ignition mixtures, and auto ignition pill (AIP) compositions. 
     These and other aspects and advantages of the present invention will become more clear after careful consideration is given to the following detailed description of the preferred exemplary embodiments thereof. 
    
    
     BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING 
     FIG. 1 depicts the ballistic pressure-time traces for a solid pyrotechnic gas generant composition identified as C1 in Table 2 below. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The compositions of the present invention will necessarily include a nitrogen containing fuel. Most preferably, the fuel is AZODN, CL-20 or a mixture thereof. The fuel will be present in the compositions of this invention in an amount between about 5 wt. % to about 95 wt. %, and more preferably between about 50 wt. % to about 90 wt. %. 
     The compositions of the present invention will also contain an oxidizer selected from copper (II) oxide (CuO), cupric nitrate, basic copper nitrate (Cu(NO 3 ) 2 .3)Cu(OH) 2 ), strontium nitrate (Sr(NO 3 ) 2 ) and mixtures thereof. The oxidizer will be present in the compositions of this invention in an amount between about 10 wt. % to about 60 wt. %, and more preferably between about 20 wt. % to about 60 wt. %. 
     Virtually any additive conventionally employed in gas generant compositions, such as binders, processing aids, ballistic additives, burn rate catalysts, colorants, slag formers, auxiliary oxidizers, multiple fuels and the like may be employed in the compositions of this invention. 
     Examples of auxiliary oxidizers include non-metallic, alkali metal, alkaline earth metal, lanthanide, rare earth, transition metal, and transition metal complex nitrates, nitrites, perchlorates, chlorates, chlorites, oxides, peroxides, superoxides, carbonates, hydroxides, sulfates, persulfates, permanganates, chromates, dichromates, and mixtures thereof. One specific example of a transition metal complex oxidizer includes hexammine cobalt (III) nitrate. 
     Examples of auxiliary fuels which may be utilized include derivatives and salts of guanidine, amino guanidine, diaminoguanidine triaminoguandine, triazines, triazoles, tetrazoles, bitetrazoles, azotetrazoles, amines, polyamines, linear and cylic nitramines, amides, polyamides such as azodicarbonamide, hydrazides, tetrazines such as 3,6-dihydrazino-s-tetrazines and mixtures thereof. An example of a triazine include trihydrazinotriazine. 
     The compositions of this invention may be uncatalyzed (i.e., the composition is void of a combustion catalyst), or may be catalyzed. That is, the composition may include a combustion catalyzing effective amount of a combustion catalyst. One preferred combustion catalyst that may be employed in the compositions of this invention is copper phthalocyanine (CuP). If used, the catalyst will preferably be present in a range between about 0.1 wt. % to about 5.0 wt. %. 
     The compositions may be used in the form of powders, granules, grains or compression-molded pellets. The compositions are most preferably used in the form of a solid compression-molded mixture of the above-noted components. In this regard, the compositions will therefore most preferably include a polymeric binder in an amount sufficient to bind the components into a solid form (e.g., pellet). The binder will therefore typically be present in an amount, based on the total composition weight, of between about 1.0 to about 6.0 wt. %, and preferably between about 2.0 to about 4.0 wt. %. Examples of binders include polyvinyl acetate (PVAC), cellulose acetate butyrate (CAB), and poly(alkylene carbonates). The preferred binders are those poly(alkylene carbonates) commercially available from Pac Polymers, Inc. As Q-PAC® 40, a poly(propylene carbonate) copolymer, and AQ-PAC® 25, a poly(ethylene carbonate) copolymer, or mixtures thereof. 
     The present invention will be further understood from the following non-limiting Examples. 
     EXAMPLES 
     Example 1 
     Basic copper nitrate (copper trihydroxy nitrate) was combined with each of the fuel components noted below in Table 1 to obtain binary compositions. Theoretical calculations were conducted for the binary compositions of Table 1 at a combustion pressure of 5000 psia and an oxidation ratio of 0.95. The results are summarized in Table 1. 
     In Table 1 below, the following abbreviations were employed: 
     PVAC=polyvinyl acetate 
     CAB=cellulose acetate butyrate 
     QPAC=poly(alkylene carbonate) 
     5-AT=5-aminotetrazole 
     GN=guanidine nitrate 
     AZODN=azodicarbonamidine dinitrate 
     
                       TABLE 1______________________________________  Binder  and/or   Amt.            Gas,Sample Fuel     (wt. %)  T.sub.C, ° K.                           m/cwt  Ash, wt. %______________________________________A      PVAC     15.152   1542   2.0653 44.9B      CAB      17.669   1544   2.0200 43.6C      QPAC     20.704   1488   2.0606 42.0D      5-AT     32.773   2023   3.4795 35.6E      GN       57.067   1868   3.1519 22.7F      AZODN    74.091   2575   3.2897 13.7______________________________________ 
    
     All of the ash in the compositions noted in Table 1 above is elemental copper because it is not a strong enough reducing agent to reduce either CO 2  or H 2  O. As can be seen, Composition F is the best in terms of high gas output, and low ash. 
     Example 2 
     Table 2 below provides examples of propellant compositions in accordance with the present invention and some of their respective physical properties. 
     
                       TABLE 2______________________________________COMPOSITION AND PROPERTIES OF HIGH BURNING RATEAZODN GAS GENERATOR PROPELLANTS     C1       C2         C3______________________________________Composition, Wt %Azodicarbonamidine       68.66      60.15      67.00DinitrateStrontium Nitrate       28.34      --          --Copper (II) Oxide       --         39.85      --Basic Copper Nitrate       --         --         31.00Polyalkylene       3.00       --         2.00Carbonate BinderCombustion Products:Gaseous Reaction       83.70      72.30      83.00ProductsMoles of Gas per       3.5        2.7        3.1100 gms:Solid Reaction       16.30      27.70      17.00Products:Ballistic Properties:Burning Rate,       0.50       0.80       0.641000 psi, ipsPressure Exponent, n:       0.48       0.63       0.51Pellet CrushStrength Studies:Baseline, stress, psi:       3948       --         3827Age at 107° C.,       3107       3703       --400 Hrs, psiTemp. Cycling,       5258       4082       --200 cycles, -40/107° C., psiHazards Properties:Threshold   Green Line Green Line Green LineImpact:     &gt;45 kgcm   &gt;50 kgcm   &gt;45 kgcmFriction, ABL:       Neg &gt; 100 psi                  Neg &gt; 1800 psi                             Neg &gt; 100 psi       @90°                  @90°                             @90°Electrostatic       Neg &gt; 1.4  Neg &gt; 6    Neg &gt; 1.4Discharge;  Joules     Joules     Joules______________________________________ 
    
     FIG. 1 shows the ballistic pressure-time results for the solid pyrotechnic gas generant composition consisting of azodicarbonamidine dinitrate, strontium nitrate, and polyalkylene carbonate binder identified as composition C1 in Table 2 above when evaluated in an &#34;all pyro&#34; PD-67 inflator unit (Atlantic Research Corporation, Gainesville, Va.) at ambient (-21° C.) and low temperature (-40° C.) conditions. It will be observed that the propellant provides the desired results with regard to the time of ignition, action time, inflator pressure, tank pressure, rate of gas production, and total gas production. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.