Solid smokeless propellants and pyrotechnic compositions for rocket and gas generation systems

Abstract

Solid gas generant compositions produce low particulates of the slightly-soluble or insoluble type when combusted. More specifically the solid gas generants and/or ignition compositions are provided which include an effective combination of phase stabilized ammonium nitrate (PSAN) as a oxidizer component and azodiformamidine dinitrate (AZODN) as the fuel component. Most preferably, the AN is phase stabilized with a phase stabilizing effective amount of a phase stabilizer selected from potassium perchlorate (KP) and/or potassium nitrate (KN).

Description

FIELD OF THE INVENTION

[0001] The present invention relates generally to gas generant compositions. In preferred forms, the present invention relates to solid propellants used in gas-generators and rocket systems and especially gas generant compositions employed in the inflation of vehicle occupant passive restraint systems.

BACKGROUND AND SUMMARY OF THE INVENTION

[0002] Various inflators for inflating vehicle occupant passive restraint systems (known colloquially in the art as “air bags”) are known. One of the various types of known inflators utilizes a quantity of stored compressed gas which is selectively released to inflate the air bag. A related type of inflator generates a gas source from a combustible gas-generating material which, upon ignition, provides a quantity of gas sufficient to inflate the air bag. In still another type (known as a hybrid inflator), the air bag inflating gas is provided by the combination of a stored compressed gas and the combustion products of the gas generating material.

[0003] Inflators which depend entirely or partially on the generation of gases by virtue of combustion of combustible materials have several disadvantages. For example, the burning of the propellant and the initiator materials in such inflators results in the production of undesired particulate matter. Thus, using inflators that are particulate -containing or which generate particulates upon combustion as part of a passive restraint system in a vehicle might result in undesirable particulates being released into the occupant zone of the vehicles and thereby inhaled by the occupants. In particular, asthmatic reactions may be caused by inhalation of particulate matter, creating a health risk for the occupants. For this reason, automobile manufacturers limit the quantity and type of particulates released by the inflator system. Insoluble particulates are preferred over soluble particulates, as the latter are believed to cause greater reaction.

[0004] Particulates may arise from certain components of solid rocket propellants or gas generants and ignition systems, as well as through secondary combustion of inert components used in rocket and inflation systems. Reduction in the contribution of particulates from one or more of these components will result in a beneficial reduction in visible (“smoke-like”) particulates for the whole assembly.

[0005] One prior inflator in the area of automotive passive restraint systems is disclosed in commonly owned U.S. Pat. No. 5,589,141 to Sides et al.1 In the Sides et al '141 patent, the inflation system utilizes a propellant comprised of an ammonium nitrate oxidizer and a suitable fuel, e.g., aminoguanidine nitrate or a nitramine, such as cyclotrimethylenenitramine (RDX) and/or cyclotetramethylenetetranitramine (HMX), which is burned in the presence of argon and a molecular oxygen-containing gas. The ratio of the oxygen-containing gas to argon is variably selected so as to provide only non-toxic reaction products in the exhaust gas. 1 The entire content of each prior-issued U.S. Patent cited herein is expressly incorporated by reference hereinto.

[0006] U.S. Pat No. 6,136,113 to Wheatley et al teaches the use of copper compounds, such as copper phthalocyanine, as additives for improving combustion efficiency of gas generants containing a eutectic mixture or solid solution of ammonium nitrate, guanidine and/or aminoguanidine nitrate, potassium or cesium nitrate or perchlorate, and a polyvinyl alcohol binder. The use of azodiformamidine dinitrate (AZODN) is not specifically disclosed or claimed therein.

[0007] Burns et al., in U.S. Pat. No. 6,074,502, describes thermally stable gas generant compositions incorporating nonazide primary and secondary high nitrogen fuels. The primary fuels are selected from tetrazoles, bitetrazoles, triazoles, and salts thereof. The secondary fuels are selected from azodicarbonamide and hydrazodicarbonamide. The compositions of the above invention also contain phase stabilized ammonium nitrate (PSAN). The compositions are said to provide when combusted, a greater yield of gaseous products, lower combustion temperatures, acceptable burn rates, and thermal stability. However, the use of derivatives or salts of azodiformamidine (azodicarbonamidine), a distinctly different molecular structure than azodicarbonamide, is not disclosed.

[0008] Lundstrom et al., in U.S. Pat. No. 6,156,137, disclose slag forming gas generative compositions which form a low concentration of water insoluble combustion products. The compositions of Lundstrom et al '137 contain azodicarbonamidine dinitrate (AZODN), a nitrogen-containing organic fuel, an oxidizer selected from the group consisting of strontium nitrate, basic copper nitrate, copper oxide, cupric nitrate, or mixtures thereof, and a polycarbonate binder. The compositions provide high burning rates with acceptable pressure exponents which allow their use at low operating pressures. However, the specific use of phase stabilized ammonium nitrate (PSAN) as an oxidizer with azodicarbonamidine dinitrate, and the objective of formation of low particulate, substantially all gaseous, essentially smokeless and odorless non-slag forming combustion products, is not discussed.

[0009] Lundstrom et al, in U.S. Pat. No. 6,017,404; teaches the use of azodicarbonamidine dinitrate (azodiformamidine dinitrate), AZODN, as a non-solids forming low pressure combustion enhancing additive with or without copper phthalocyanine in a nonazide ammonium nitrate based gas generant composition with fuels such as high bulk density nitroguanidine, tetrazoles such as diammonium bitetrazole or triazoles; and an oxidizer comprising phase stabilized ammonium nitrate (PSAN). However, the use of azodicarbonamidine dinitrate as a singular primary fuel component in a composition containing PSAN, which results in lower demand for the oxidizer component which results in a significant decrease in the amount of phase stabilizer required in the total composition, and an associated decrease in particulate formation when combusted, is not disclosed.

[0010] Scheffee et al., in U.S. Pat. No. 5,726,382, describes a gas generant based on a eutectic solution of ammonium nitrate and either aminoguanidine nitrate or guanidine nitrate with a minor amount of potassium nitrate. The composition in the form of a pellet is used to generate a particulate-free, non-toxic, odorless, and colorless gas. However, no disclosure is made for any use of azodiformamidine dinitrate (AZODN) in gas generants, or for that matter, eutectic solutions of ammonium nitrate and azodiformamidine dinitrate (AZODN).

[0011] U.S. Pat. No. 5,545,272 to Poole, discloses the use of gas generant compositions consisting of phase stabilized ammonium nitrate (PSAN) with nitroguanidine. Poole '272 notes, however, that the use of PSAN or pure ammonium nitrate in many compositions results in prohibitively low melting points and poor thermal stability. Again however, no discussion is presented therein for the use of AZODN with PSAN.

[0012] U.S. Pat. No. 5,531,941 to Poole, teaches the use of PSAN with two or more specified fuels provided in specified groups. In order to achieve acceptable burning rates, triaminoguanidine nitrate (TAGN) is used as a major constituent. However, TAGN is a sensitive explosive which is expressly banned by the U.S. Department of Transportation, and which complicates raw material purchase and delivery schedules. Again, however, no discussion is present therein for the use of AZODN with PSAN.

[0013] Ramnarace, in U.S. Pat. No. 4,111,728, teaches gas generants based on mixtures of ammonium nitrate, oxamide, guanidine nitrate, and a polyester binder. Again, no discussion is made for the use of AZODN in gas generants.

[0014] Theory predicts that a successful ignition material for the main charge gas generant of an inflator, when ignited, will give off hot particles or gases that will subsequently condense onto the cooler surface of the main charge being ignited and thereby transfer heat and produce “hot spots” which, in turn, lead to ignition. However, as noted previously, high levels of condensed species in the exhaust products of air bag inflators is undesirable due to potentially undesirable respiratory effects on the occupants when an airbag is inflated during an automobile accident. Furthermore, condensed species are preferred to be water insoluble as these are considered to have less impact on occupants with respiratory ailments, such as asthma. Similarly, such high levels of condensed species is undesirable for shoulder-launched rocket systems as they will result in a visible exhaust plume when fired thereby potentially revealing a soldier's position to hostile forces and possible return fire. Therefore, a balance is required between the ignition material and the main charge gas generant material with regard to particulate formation during combustion in an inflator or a shoulder-fired rocket system

[0015] Broadly, the present invention is embodied in solid rocket propellant and gas generant compositions which produce a very low concentration of solid particulates and minimal smoke when combusted. More specifically the present invention is embodied in combustible compositions including, propellants, solid gas generants, ignition enhancing and/or ignition compositions, which are comprised of an effective combination of phase stabilized ammonium nitrate (PSAN) as a oxidizer component and azodiformamidine dinitrate (AZODN) as the fuel component. Most preferably, the AN is phase stabilized with a phase stabilizing effective amount of a phase stabilizer selected from potassium perchlorate (KP) and/or potassium nitrate (KN). In addition, the PSAN and AZODN components in the compositions of the present invention may be present as a mixture or as a solid solution of such components.

[0016] The gas generation compositions of the present invention are usefully employed in inflatable passive vehicle occupant restraint systems (e.g., air bag systems), gas generators for aerospace operations (e.g., missile and space vehicle control systems), and various types of rocket propulsion systems. As used herein and in the accompanying claims, therefore, the term “gas generant” is meant to be a term generic to other terms ascribed to combustible gas generation compositions such as, propellants, gas generators, pyrotechnics, boosters, igniters and the like. In contrast to many other solid igniter-booster formulations, the combustible gas generation compositions of the present invention sustain combustion and burn readily at low (ambient) temperature and atmospheric pressure with minimal ash output.

[0017] With the above in mind, one objective of this invention is to provide a gas generating composition containing ammonium nitrate which is less susceptible to hygroscopic degradation than prior art ammonium nitrate based compositions.

[0018] Another objective of this invention is to provide a gas generating composition as either a blend, solid solution or eutectic mixture containing phase stabilized ammonium nitrate (PSAN), which when formulated to burn to substantially innocuous gaseous combustion products, consisting of carbon dioxide, nitrogen, and water vapor, requires a significantly lower stabilizer concentration based on the total composition than those of the prior art.

[0019] A further objective of this invention is to provide a gas generating composition containing a yellow colored oxygenated fuel component, which when blended to a uniform color with the other major constituent of the composition, PSAN, assures a visual verification of a thorough blending and coating operation of the white colored hygroscopic oxidizer component, which in turn assures an improvement over the prior art with regard to improved ignitability and uniformity with respect to combustion and mechanical properties of the final product.

[0020] An additional objective of this invention is to provide a gas generating composition, the major ingredients of which include an oxidizer component consisting of PSAN, an oxygenated fuel component consisting of a nitric acid salt of azobisformamidine, and an oxygenated binder and coating component such as a polyalkylcarbonate; which due to the self-deflagrating nature of the fuel in conjunction with the favorable oxygen balance of the combined fuel and binder/coating constituents, requires a lower oxygen demand from the hygroscopic ammonium nitrate based PSAN oxidizer component. This represents an advance over the prior art because it results in (1) a much lower demand of the hygroscopic ammonium nitrate based oxidizer component needed for providing the proper reaction stoichiometry for formation of the desired innocuous combustion products, (2) less stringent storage and transportation requirements, and (3) improvement in reliability with regard to ignitability and ballistic performance.

[0021] Another objective of this invention, considered an improvement over the prior art, is to provide a substantially smokeless gas generating composition, in the form of a blend, solid solution or eutectic mixture containing a decreased level of PSAN, for use in military and civilian applications, which results in the formation of a lower concentration of solid smoke producing combustion particulates, due to an associated lower demand of phase stabilizer required in the overall formulation.

[0022] Another objective of the invention, and an improvement over the prior art, is to provide a gas generating composition based on the use of PSAN as the oxidizer component, which provides up to 97.5% substantially innocuous and smokeless gaseous combustion products when the formulation is burned, essentially eliminating the requirement of a filtration system in the inflation device of an automobile passive restraint safety system (airbag).

[0023] An additional objective of the invention is to provide a gas generating composition based on the use of PSAN as the oxidizer component which provides rapid combustion at lower pressures, than previously observed for the prior art, and provides the necessary gas output at a lower pressure in a lighter weight and less costly inflator housing within the constraints of the operational envelope of an automotive airbag safety device.

[0024] Another objective of the invention is to provide a gas generating composition which uses PSAN as the oxidizer component, said composition having an acceptable burning rate of 0.25 to 0.39 inches per second at 1000 psi, greater overall density for use in volume limited systems, than compositions of the prior art such as those using PSAN as the oxidizer component with guanidine nitrate and/or nitroguanidine as the fuel component.

[0025] A further objective of this invention is to improve upon the prior art and provide a method of rapid formation of substantially innocuous gaseous combustion products in order to provide a fluidic medium for use in inflation, pressurization, and rocket or other propulsion devices.

[0026] An additional objective of this invention is to provide a system which includes a gas generating composition in conjunction with a device such as an inflator housing or pressure chamber for use in automotive airbag safety systems or rocket propulsion systems.

[0027] Another objective of this invention, which is an improvement over the prior art, is to provide a gas generating composition which is capable of, when combusted, providing a substantially innocuous gas output of greater than 96%; following ignition, capable of maintaining self-sustained combustion at a sea level atmospheric pressure of 14.7 lbs/sq. in.; and acceptable burning rates ranging from 0.30 to 0.39 inches per second at 1000 psi for use in automotive inflators for airbags or gas generation and rocket propulsion devices.

[0028] Another objective of the present invention, and an improvement over the prior art, is to provide a thermally stable gas generating composition containing ammonium nitrate, which achieves a burning rate of less than 0.40 inches per second at 1000 psi, ignites reliably and does not result in “no-fires” in inflator, gas generator, or rocket propulsion devices wherein none or only a portion of the composition is combusted, and provides predictable performance resulting in a gas production response time which is rapid enough for use in automotive airbag or rocket propulsion, or other gas inflation or pressurization devices.

[0029] An additional objective is to provide an improvement in the processibility of a gas generating composition containing the difficultly processible hygroscopic oxidizer component PSAN, by incorporating significantly lower concentrations of PSAN in the composition than previously disclosed in prior art formulations, and still retaining the capability of forming when combusted, substantially innocuous gaseous combustion products consisting of carbon dioxide, nitrogen, and water vapor for use in automotive airbag inflation and shoulder launched rocket systems.

[0030] Another objective of the invention, and an advance over the prior art, is to provide a gas generating composition which provides a substantially innocuous gas output greater than 96%, burns to completion when ignited at atmospheric pressure, and tested with differential scanning calorimetry (DSC) has a first exotherm peaking at about 132-133*C, a second exotherm peaking at about 168-169*C, and a major exotherm peaking at about 316-319*C, allowing the use of a conventional low temperature 125-175*C range autoignition material (AIM) with said composition.

[0031] The aforementioned objectives are achieved by a gas generant, gas generating composition, or propellant that may be used in a inflation device, fire suppression device, gas pressurization device, or rocket propulsion device. The terms gas generant, gas generating composition, and propellant will be used interchangeably throughout this document and shall be considered one and the same. Said propellant comprises an oxidizer component consisting of phase stabilized ammonium nitrate (PSAN), a highly oxygenated fuel component consisting of a nitric acid salt of azodiformamidine, and a highly oxygenated binder/coating component consisting of a polyalkylene carbonate.

[0032] These objectives, as well as other aspects and advantages will become more clear from the following detailed description of the preferred exemplary embodiments thereof.

DETAILED DESCRIPTION OF THE PREFERRED EXEMPLARY EMBODIMENTS

[0033] The gas generation compositions of this invention necessarily include a ammonium nitrate (AN) as an oxidizer component, and azodiformamidine dinitrate (AZODN) as a fuel component in the form of a blend, solid solution or eutectic mixture. In this regard, the AZODN fuel will be present in an amount between about 30 wt. % to about 80 wt. %, preferably between about 40 wt. % to about 70 wt. %. (Unless indicated to the contrary, all weight percentages expressed herein are based on the total composition weight.)

[0034] A particularly preferred oxidizer is phase-stabilized ammonium nitrate (PSAN). In this regard, PSAN is AN which includes a phase stabilizing effective amount of one or more phase stabilizers such as potassium perchlorate (KP) or potassium nitrate (KN). However, other phase stabilizers may be used, such as nickel oxide or copper oxide. Typically, PSAN is prepared as an aqueous solution of a mixture of AN/KN (92.3 wt. %/7.7 wt. %) or AN/KP (85.9 wt. %/14.1 wt. %). These solutions are then vacuum dried by conventional techniques to form a dry powder thereof. The phase stabilizer is typically incorporated into the AN in relatively low concentrations and thus will be present in the composition in amounts typically ranging between about 2 wt. % to about 20 wt. %, and preferably between about 3 wt. % to about 15 wt. %, based on the total composition weight. The PSAN will most preferably be present in amounts effective to provide AN in the composition of between about 20 to about 80 wt. %, preferably between about 30 wt. % to about 60 wt. %, based on total composition weight.

[0035] The compositions of this invention may be used in the form of powders, granules, or a variety shaped solid structures (for example, pellets, one-piece monolithic grains and the like). Thus, the compositions may be in the form of compression-molded solid structures, extruded configurations, various cast and cured configurations, or in the form of a monolithic grain for high burning rate compositions. The compositions are most preferably used in the form of a solid compression-molded mixture or solid solution of the components. It is important that the form of the invention have sufficient strength to withstand forces due to initiator shock or long term thermal aging and/or thermal cycling without loss of physical integrity. In this regard, the compositions will therefore most preferably include a polymeric binder in an amount sufficient to bind the components into a more durable, stronger solid form (e.g., pellet).

[0036] A binder will typically be present in an amount based upon the total composition weight, of between 1 and 15 wt. %, and preferably between about 2 and 5 wt. %. Various binders may be used. However, the preferred binders are highly oxygenated and include polymethylmethacrylate (PMMA with about 32 wt. % oxygen), polyvinyl alcohols (PVA with about 36 wt. % oxygen), and/or polyalkylene carbonates. Examples of polyalkylene carbonates that may be employed in this invention are those that are commercially available from PAC Polymers, Inc. as poly(propylene carbonate) copolymer (QPAC-40 with about 47 wt. % oxygen) and poly(ethylene carbonate) copolymer (QPAC-25 with about 54 wt. % oxygen).

[0037] The especially novel compositions of the present invention employ AZODN as a highly oxygenated fuel component in combination with PSAN. In this regard, a stoichiometric combination of the disclosed ingredients herein, results in a significantly lower concentration of PSAN (and therefore an associated decrease in the total amount of phase stabilizer being required) as compared to conventional PSAN-containing formulations. Because AZODN is a more highly oxygenated fuel component in this invention (in contrast to prior art oxygenated fuels such as guanidine nitrate (GN) or nitroguanidine (NQ), or non-oxygenated fuels such as tetrazole derivatives), significantly less AN is required for burning to the relatively innocuous gaseous carbon dioxide, nitrogen and water vapor combustion products. Because less AN is used with an associated decrease in stabilizer content, a lower concentration of solid particulates and smokey combustion products is formed. In other words, the greater the concentration of AN required in a composition for burning to carbon dioxide, nitrogen and water vapor, the higher the concentration of phase stabilizer is needed which, in turn results, in a higher concentration of solid products when the formulation is burned.

[0038] The present invention will be further understood from the following non-limiting Examples.

EXAMPLES

Example 1

Comparison of AN Concentration Required with AZODN vs. AN Required for Various Prior Art Fuels to Burn to CO2, N2 and H2O Vapor

[0039] Reactions of several AN/fuel mixtures are shown below in the proper stoichiometric proportions and balanced to burn to relatively innocuous gaseous reaction products consisting of carbon dioxide, nitrogen and water vapor. The reactions of example 1 are shown without the use of phase stabilizers in the AN so that a direct comparison of the required concentration of AN for use with the selected fuel can be made. It is apparent from reviewing Example 1 that the higher the oxygen balance of the selected fuel, the lower the concentration of the AN oxidizer required and the lower the associated concentration of stabilizer, which results in solid particulates, when PSAN is used in a formulation.

[0040] 1. Reaction of AZODN and AN: 1

[0041] 2. Reaction of guanidine nitrate (GN) and AN: 2

[0042] 3. Reaction of nitroguanidine (NQ) and AN: 3

[0043] 4. Reaction of diammonium 5,5′-bitetrazole (DABT) and AN: 4

Example 2

Formulations C1 through C5 According to Invention and Comparative (Prior Art) Formulation CE1

[0044] Compositions were prepared in accordance with Table 1 below. Each composition was analyzed in terms of combustion products, ballistic properties and thermal stability. In comparison, a conventional formulation containing guanidine nitrate (GN) as a fuel component in combination with non-phase stabilized AN was also analyzed. All ingredients of the compositions listed below are expressed in weight percent (wt. %) of the total composition weight. 1 TABLE 1 Composition: C1 C2 C3 C4 C5 CE1 Ammonium Nitrate (AN) 43.30 38.05 38.45 38.85 39.25 60.00 Azodiformamidine dinitrate (AZODN) 48.09 50.72 51.25 51.78 52.32 — Guanidine Nitrate (GN) — — — — — 24.00 Potassium Perchlorate (KP) — 6.23 6.30 6.37 6.43 9.00 Potassium Nitrate (KN) 3.61 — — — — — Copper Phthalocyanine — — — — — 2.00 Polyalkylene Carbonate Binder (QPAC- 5.00 5.00 4.00 3.00 2.00 — 40) Polyvinyl Alcohol Binder (PVA) — — — — — 5.00 Theoretical Combustion Products: Gas Output, moles/100 gms 4.00 3.93 3.89 3.85 3.81 4.09 Gas Output, wt. % 97.54 96.65 96.62 96.59 96.57 94.54 Solids, wt. % 2.46 3.35 3.38 3.41 3.43 5.06 Theoretical Flame Temperature, Deg. K 2470 2556 2562 2565 2568 2259 Ballistics: Burning Rate: 1000 psi, ips 0.31 0.35 0.37 0.38 0.39 0.35 2000 psi, ips 0.54 0.65 0.73 0.72 0.70 0.58 4000 psi, ips 0.98 1.16 — 1.39 — 0.87 Exponent (1-2 kpsi) 0.80 0.84 0.91 0.90 0.86 0.73 Exponent (1-4 kpsi) 0.80 0.84 — 0.90 — 0.66 Self-Sustained Combustion @ yes yes yes yes yes no Atmospheric Pressure Thermal Stability: Baseline Stress, psi 7960 6606 6633 6123 5058 7403 Diameter, in. .522 .522 .522 .522 .522 .522 400 hrs (17 days) @ 107° C. Stress, psi 6945 7166 7706 5963 7288 4847 Diameter, in. .526 .525 .525 .528 .525 .531 1000 hrs (42 days) @ 90° C. Stress, psi 8432 7981 8105 6809 6734 — Diameter, in. .522 .522 .522 .522 .522 — 100 cycles (14 days), −40 to +107° C. Stress, psi 5829 5547 6078 4913 4166 — Diameter, in. .533 .531 .529 .530 .531 — 100 cycles (14 days), −40 to +90° C. Stress, psi 6748 7423 6259 4935 5140 — Diameter, in. .527 .526 .526 .526 .526 — 200 cycles (28 days), −40 to +107° C. Stress, psi 3486 5060 4952 4163 3730 5552 Diameter, in. .545 .533 .532 .532 .533 .531 200 cycles (28 days), −40 to +90° C. Stress, psi 5997 6637 5285 5832 5232 — Diameter, in. .528 .527 .528 .527 .527 —

[0045] As can be seen from the data, the formulations of the present invention which contain phase stabilized ammonium nitrate (PSAN) result in the use of a much lower concentration of stabilizer 3-6% vs. 9% with an associated greater gas output and lower concentration of solid particulates when combusted, as compared to the conventional (prior art) formulation CE1. In formulations C2-C5, the stabilizer employed in the PSAN is potassium perchlorate (KP) and thus a direct comparison can be made with the CE1 composition. The formulations C1 through C5 in accordance with the present invention contain phase stabilized ammonium nitrate (PSAN) prepared as an aqueous solution of a mixture of AN/KN (92.3/7.7) or AN/KP (85.9/14.1) followed by drying to a powder, blending with the remaining ingredients and pressing into ½-inch diameter pellets. Another attractive feature of the compositions of the present invention is that a significantly lower concentration of the hygroscopic constituent (AN) results in easier processing and a less critical storage requirement.

[0046] As the data above demonstrate, the compositions of the present invention burn readily at ambient pressure with minimal ash output, in contrast to the conventional (prior art) formulation CE1.

[0047] It will also be observed from the combustion data as noted above that the percent gas output is greater, and the percent of solids is lower for the formulations of the present invention C1 through C5 as compared with the conventional (prior art) formulation CE1. Furthermore, as shown in the ballistics data, the formulations of this invention offer a variety of acceptable burning rates for use in gas generator and inflator applications which are comparable to that of the conventional formulation CE1 allowing the capability of controlling batch to batch variations of ballistic properties. In accordance with this invention, the tailoring of the burning rate is readily obtained by variations in the concentration of the AZODN. Also, in accordance with the present invention, in contrast to conventional formulations, the AZODN-containing compositions burn rapidly at atmospheric pressure. This ability of the compositions to burn at atmospheric pressure is important for use with dual level inflator applications where less than the entirety of the gas generant composition may be combusted during the inflation event, known in the art as a “soft” inflation event. The gas generating compositions of the present invention are therefore capable of being burned at atmospheric pressure immediately after the soft inflation event so that any residual gas generant present in the inflator at that time is eliminated entirely and is no longer a combustion threat.

[0048] With regard to the thermal stability data, it will be observed that all of the formulations of the present invention provide excellent mechanical properties and structural integrity when thermally cycled from −40 to +90° C. and −40 to +107° C. for 200 cycles. The compositions also exhibited excellent mechanical properties after being aged at 107° C. for 400 hours and 90° C. for 1000 hours.

[0049] 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 to 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.

Claims

1. A solid gas generant composition comprising azodiformamidine dinitrate (AZODN), and phase-stabilized ammonium nitrate (PSAN).

2. The gas generant composition as in claim 1, wherein the PSAN includes phase stabilizer selected from potassium nitrate (KN) or potassium perchlorate (KP).

3. The gas generant composition as in claim 1, wherein the phase stabilizer is present in amounts effective to stabilize the ammonium nitrate (AN).

4. The gas generant composition as in claim 3, wherein the phase stabilizer is present in an amount between about 2 wt. % to about 20 wt. %.

5. The gas generant composition as in claim 1, wherein the PSAN is present in an amount effective to achieve between about 20 to about 80 wt. % of AN in the composition.

6. The gas generant composition as in claim 2, wherein the PSAN is present in an amount effective to achieve between about 30 to about 60 wt. % AN in the composition.

7. The gas generant composition of claim 1 or 5, wherein the AZODN is present in an amount between about 30 to about 80 wt. %.

8. The gas generant composition as in claim 7, where the AZODN is present in an amount between about 40 to about 70 wt. %.

9. The gas generant composition as in claim 1, further comprising a binder.

10. The gas generant composition as in claim 9, wherein the binder is at least one selected from polyvinyl alcohols, poly(alkylene) carbonates, and polymethyl methacrylates.

11. The gas generant composition of claim 10, wherein the binder is present in an amount between about 1 wt. % to about 15 wt. %.

12. The gas generant composition of claim 11, wherein the binder is present in an amount between about 3 to about 5 wt. %.

13. A solid gas generant composition which comprises, based on total composition weight, of between about 30 to about 80 wt. % of azodiformamidine dinitrate (AZODN), and phase stabilized ammonium nitrate (PSAN) in an amount effective to provide between about 20 wt. % to about 80 wt. % of ammonium nitrate (AN) in the composition, and between about 2 wt. % to about 20 wt. % of a phase stabilizer selected from potassium perchlorate (KP) or potassium nitrate (KN), and between about 1 to about 15 wt. % of a binder selected from the group consisting of polyvinyl alcohols, poly(alkylene) carbonates and polymethyl methacrylates.

14. A shaped solid structure formed of the gas generant composition of claim 1 or 13.

15. The shaped solid structure of claim 14, in the form of a monolithic grain.

16. The shaped solid structure of claim 14, which is compression-molded, cast or extruded.

17. The gas generant composition of claim 1 or 13 in the form of powders or granules.

18. A method of making a solid gas generant composition comprising forming a mixture of azodiformamidine dinitrate (AZODN), and phase-stabilized ammonium nitrate (PSAN).

19. The method as in claim 18, wherein the PSAN includes phase stabilizer selected from potassium nitrate (KN) or potassium perchlorate (KP).

20. The method as in claim 18, wherein the phase stabilizer is present in amounts effective to stabilize the ammonium nitrate (AN).

21. The method as in claim 18, which is practiced to form a solid eutectic solution of said mixture.

22. A method of inflating a gas-inflatable device containing an amount of the gas generant composition as in any one of claims 1-13 in excess of that needed to sufficiently inflate the device, which method comprises combusting under pressure during an inflation event a first portion of the composition so as to generate sufficient amount of gas to inflate the device, and thereafter combusting under atmospheric pressure a remaining portion of the gas generant composition.