Gas generant compositions containing stabilizer

To a gas generant composition comprising a fuel component which includes a triazole compound and/or tetrazole compound with an acidic hydrogen and an oxidizer component which includes a transition metal oxide, is added a chelating agent, such as ethylenediaminetetraacetic acid (EDTA) to provide long-term stability to the gas generant composition.

The present Invention is directed to gas generant compositions for 
inflating automotive air-bags and other devices in which rapid production 
of high volumes of gas is required. More particularly, the invention is 
directed to such compositions where tetrazoles and/or triazoles are the 
fuel component and metal oxides are employed as oxidizers and 
stabilization of such compositions. 
BACKGROUND OF THE INVENTION 
Most automotive air bag restraint systems, presently in use, use gas 
generant compositions in which sodium azide is the principal fuel. Because 
of disadvantages with sodium azide, particularly instability in the 
presence of metallic impurities and toxicity, which presents a disposal 
problem for unfired gas generators, there is a desire to develop non-azide 
gas generant systems, and a number of non-azide formulations have been 
proposed. However, to date, non-azide gas generants have not made 
significant commercial inroads. 
Alternatives to azides which have been proposed, e.g., in U.S. Pat. No. 
5,035,757, the teachings of which are incorporated herein by reference, 
include azole compounds, particularly tetrazole and triazole compounds. 
Tetrazole compounds include, for example, 5-amino tetrazole (5-AT), 
tetrazole, and bitetrazole. Triazole compounds include, for example, 
1,2,4-triazole-5-one, and 3-nitro 1,2,4-triazole-5-one. Although all of 
the above azole compounds are useful fuels in accordance with the present 
invention, 5-AT is the most commercially important of these. 
Gas generant systems include, in addition to the fuel component, an 
oxidizer component. Proposed oxidizers for use in conjunction with azole 
fuels include alkali and alkaline earth metal salts of nitrates, chlorates 
and perchlorates. Another type of oxidizer for tetrazoles and triazoles, 
as taught, for example, in U.S. Pat. No. 3,468,730, the teachings of which 
are incorporated herein by reference, are metal oxides, particularly 
transition metal oxides. Transition metal oxides suitable as oxidizers 
include, but are not limited to cupric oxide, ferric oxide, lead dioxide, 
manganese dioxide and mixtures thereof. Metal oxides are desired as 
oxidizers in that they tend to lower combustion temperatures, thereby 
lowering the generated levels of toxic oxides, such as CO and NO.sub.x. 
Several gas generant processing procedures utilize water. Water-processing 
reduces hazards of processing gas generant materials. It is therefore 
desirable that gas generant compositions be formulated so as to facilitate 
water processing. 
One Example of water processing, taught, e.g., in U.S. Pat. No. 5,015,309, 
the teachings of which are incorporated by reference, involves the steps 
of 
1. Forming a slurry of the generant ingredients with water. 
2. Spray drying the slurry to form spherical prills of diameter 100-300 
microns. 
3. Feeding the prills via gravity flow to a high speed rotary press. 
Another common production technique, (e.g. U.S. Pat. No. 5,084,218), the 
teachings of which are incorporated herein by reference, involves the 
following steps: 
1. Forming a slurry of the generant ingredients with water. 
2. Extruding the slurry to form spaghetti like strands. 
3. Chopping and spheronizing the strands into prills. 
4. Tableting of the prills as described previously. 
A problem has been found with gas generant compositions containing both a 
triazole and/or a tetrazole having an acidic hydrogen plus a metal oxide 
oxidizer, a problem particularly seen if the composition is 
aqueous-processed, is poor long-term stability (as demonstrated by 
accelerated heat-aging experiments). Over time, the amount of the fuel is 
found to decrease and the performance decreases. Thus, if such a gas 
generant were used in an automotive airbag inflator, the inflator, over 
time, might become insufficiently effective. While Applicants are not 
bound by theory, it is believed that the metal ion of the metal oxide 
replaces, over time, acidic hydrogens of tetrazoles and/or triazoles, 
producing metal salts or complexes. These metal salts or complexes are 
somewhat unstable and, over time, decompose. 
It is a primary object of the invention to stabilize gas generant 
compositions containing tetrazoles and/or triazoles having an acidic 
hydrogen plus a transition metal oxide oxidizer. 
SUMMARY OF THE INVENTION 
In a gas generant composition comprising a fuel component and an oxidizer 
component and in which at least part of the fuel component is a tetrazole 
compound having an acidic hydrogen and/or a triazole compound having an 
acidic hydrogen and in which at least part of the fuel component is a 
transition metal oxide, enhanced stability is provided by incorporating 
between about 0.05 and about 5 wt %, relative to total fuel component plus 
total oxidizer component (fuel component plus oxidizer component being 100 
wt %), of a chelating agent. The preferred chelating agents are 
aminocarboxylic acids and salts thereof, particularly 
ethylenediaminetetraacetic acid (EDTA) and salts thereof. 
DETAILED DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS 
By acidic hydrogen on a triazole or tetrazole compound is meant herein a 
hydrogen that is on a triazole ring nitrogen or tetrazole ring nitrogen. 
When a triazole or tetrazole compound is compounded with a metal oxide, 
long-term instability tends to result. The use of a chelating agent in 
accordance with the invention eliminates or minimizes this instability 
problem. 
The tetrazole and/or triazole compound of the fuel component may be 
selected from any of those listed above and mixtures thereof. From an 
availability and cost standpoint, 5-aminotetrazole (5-AT) is presently the 
azole compound of choice, although the instability problem addressed by 
the present invention is applicable to any tetrazole or triazole compound 
having an acidic hydrogen. The fuel may be entirely tetrazole, e.g., as 
per above-referenced U.S. Pat. No. 3,468,730, and/or triazole, but may be 
a mixture of fuels including a tetrazole and/or triazole and another fuel. 
Stability problems of significance in any such gas generant wherein the 
tetrazole and/or triazole comprises 10 wt % or more by weight of the total 
of the fuel component plus oxidant component. Likewise, the oxidizer may 
be entirely a metal oxide or mixture of metal oxides or a mixture of metal 
oxide(s) and non-metal oxide oxidizers. Stability problems of significance 
occur in any such gas generant wherein the metal oxide component comprises 
about 5 wt % or more of the total of the fuel component plus oxidizer 
component. The purpose of the fuel is to produce carbon dioxide, water and 
nitrogen gases when burned with an appropriate oxidizer or oxidizer 
combination. The gases so produced are used to inflate an automobile gas 
bag or other such device. By way of example, 5-AT is combusted to produce 
carbon dioxide, water and nitrogen according to the following equation: 
EQU 2CH.sub.3 N.sub.5 +7/2O.sub.2 .fwdarw.2CO.sub.2 +3H.sub.2 O+5N.sub.2. 
In accordance with the invention, long-term stability is provided by 
inclusion of a metal chelating agent at a level of between about 0.05 and 
about 5 wt %, preferably between 0.1 and 1 wt %, relative to the total of 
the fuel component plus the oxidizer component. Preferred chelating agents 
are aminocarboxylic acids and their salts. From a cost and availability 
standpoint, the preferred chelating agent is EDTA and its salts, such as 
disodium EDTA, tetrasodium EDTA, and potassium salts of EDTA. Example of 
other aminocarboxylic acids are hydroxyethylenediaminetriacetic acid, 
nitrilotriacetic acid, N-dihydroxyethylglycine, and 
ethylenebis(hydroxyphenylglycine). Suitable alternative types of chelating 
agents include polyphosphates, 1,3-diketones, hydroxycarboxylic acids, 
polyamines, aminoalcohols, aromatic heterocyclic base, phenols, 
aminophenols, oximes, Schiff bases, tetrapyrroles, sulfur compounds, 
synthetic macrocyclic compounds, and phosphoric acids. 
To facilitate processing in conjunction with water, a minor portion of the 
fuel, i.e., between about 15 and about 50 wt % of the fuel, is preferably 
water soluble. While water-soluble oxidizers, such as strontium nitrate 
also facilitate water-processing, over-reliance on such water-soluble 
oxidizers tend to produce undesirably high combustion temperatures. 
Specific desirable characteristics of water soluble fuels are: 
The compound should be readily soluble in water, i.e., at least about 30 
gm/100 ml. H.sub.2 O at 25.degree. C.; 
The compound should contain only elements selected from H, C, O and N; 
When formulated with an oxidizer to stoichiometrically yield carbon 
dioxide, nitrogen, and water, the gas yield should be greater than about 
1.8 moles of gas per 100 grams of formulation; and 
When formulated with an oxidizer to stoichiometrically yield carbon 
dioxide, water and nitrogen, the theoretical chamber temperature at 1000 
psi should be low, preferably, less than about 1800.degree. K. 
Compounds that most ideally fit the above criteria are nitrate salts of 
amines or substituted amines. Suitable compounds include, but are not 
limited to, the group consisting of guanidine nitrate, aminoguanidine 
nitrate, diaminoguanidine nitrate, semicarbazide nitrate, 
triaminoguanidine nitrate, ethylenediamine dinitrate, hexamethylene 
tetramine dinitrate, and mixtures of such compounds. Guanadine nitrate is 
the currently preferred water-soluble fuel. 
Generally any transition metal oxide may serve as an oxidizer. The 
preferred transition metal oxide is cupric oxide which, upon combustion of 
the gas generant, produces copper metal as a slag component. The purpose 
of the oxidizer is to provide the oxygen necessary to oxidize the fuel; 
for example, CuO oxidizes 5-AT according to the following equation: 
EQU 4CH.sub.3 N.sub.5 +14CuO.fwdarw.14Cu+4CO.sub.2 +6H.sub.2 O+10N.sub.2. 
The transition metal oxide may comprise the sole oxidizer or it may be used 
in conjunction with other oxidizers including alkali and alkaline earth 
metal nitrates, chlorates and perchlorates and mixtures of such oxidizers. 
Of these, nitrates (alkali and/or alkaline earth metal salts) are 
preferred. Nitrate oxidizers increase gas output slightly. Alkali metal 
nitrates are particularly useful as ignition promoting additives. 
It is frequently desirable to pelletize the gas generant composition. If 
so, up to about 5 wt %, typically 0.2-5 wt % of a pressing aid or binder 
may be employed. These may be selected from materials known to be useful 
for this purpose, including molybdenum disulfide, polycarbonate, graphite, 
Viton, nitrocellulose, polysaccharides, polyvinylpyrrolidone, sodium 
silicate, calcium stearate, magnesium stearate, zinc stearate, talc, mica 
minerals, bentonite, montmorillonite and others known to those skilled in 
the art. A preferred pressing aid/binder is molybdenum disulfide. If 
molybdenum disulfide is used, it is preferred that an alkali metal nitrate 
be included as a portion of the oxidizer. Alkali metal nitrate in the 
presence of molybdenum disulfide results in the formation of alkali metal 
sulfate, rather than toxic sulfur species. Accordingly, if molybdenum 
disulfide is used, alkali metal nitrate is used as a portion of the 
oxidizer in an amount sufficient to convert substantially all of the 
sulfur component of the molybdenum disulfide to alkali metal sulfate. This 
amount is at least the stoichiometric equivalent of the molybdenum 
disulfide, but is typically several time the stoichiometric equivalent. On 
a weight basis, an alkali metal nitrate is typically used at between about 
3 and about 5 times the weight of molybdenum disulfide used. 
The gas generant composition may optionally contain a catalyst up to about 
3 wt %, typically between about 1 and about 2 wt %. Boron hydrides and 
iron ferricyanide are such combustion catalysts. Certain transition metal 
oxides, such as copper chromate, chromium oxide and manganese oxide, in 
addition to the oxidizer function, further act to catalyze combustion. 
To further reduce reaction temperature, coolants may also optionally be 
included at up to about 10 wt %, typically between about 1 and about 5 wt 
%. Suitable coolants include graphite, alumina, silica, metal carbonate 
salts, and mixtures thereof. The coolants may be in particulate form, 
although if available, fiber form is preferred, e.g., graphite, alumina 
and alumina/silica fibers.

The invention will now be described in greater detail by way of specific 
examples. 
EXAMPLE 1 
A gas generant composition was prepared by mixing 15 wt % 5-aminotetrazole 
(5-AT) with 85 wt % cupric oxide. Two mixtures were prepared by combining 
the ingredients in an aqueous slurry, mixing well, and drying in a vacuum 
oven. A control sample contained only the CuO and the 5-AT. To an 
experimental sample was added 0.1% Na.sub.2 -EDTA. Accelerated aging was 
conducted by subjecting each of the Control and Experimental samples to 
107.degree. C. heat for 100 hours. Results are as follows: 
______________________________________ 
Burn rate 
Sample wt % 5-AT* in/sec Appearance 
______________________________________ 
Control/no aging 
15.08 .420 Navy blue 
Control/aged 
12.88 .421 Navy blue 
Exp./no aging 
14.21 .520 Grey/black 
Exp./aged 14.92 .660 Grey/black 
______________________________________ 
*analyzed 
The lower 5-AT content of the Experimental sample (no-aging) was due to a 
higher initial moisture content in the Experimental sample as well as a 
small amount of dilution by the added Na.sub.2 EDTA. Heat aging of the 
Experimental sample drove off the excess water, and the 5-AT content 
increased as a percentage of the mixture comparable to that of the control 
(no heat age) sample. However, in the Control sample, the 5-AT content 
decreased to 12.88% upon heat aging, indicating a loss of 5-AT. The lower 
burn rates obtained with the Control samples is believed to be due to the 
formation of the copper salt or complex of 5-AT and decomposition thereof 
during the manufacturing process. Also, the formation of the salt or 
complex is believed to be responsible for the blue color observed in the 
Control samples. It is believed that addition of EDTA to the mix prior to 
slurrying inhibits formation of this salt; thus, the higher burn rates and 
lack of blue color in the Experimental samples. The increase in burn rate 
observed in the heat aged Experimental sample relative to the non-heat 
aged Experimental sample is believed to be due to removal of excess 
moisture during heat aging.