Method of controlling the amount of oxides of nitrogen in generated gas for airbags

Abstract: A gas generant composition devoid of azides which yields solid combustion products which are easily filtered rendering the gases useful for inflating automobile occupant restraint bags and further providing a reduction in the amount of toxic oxides of nitrogen in the gases produced.

Description: Gas generating compositions for inflating occupant restraint devices of over-the-road vehicles have been under development worldwide for many years and numerous patents have been granted thereon. Because of strict requirements relating to toxicity of the inflating gases, most gas generants now in use are based on inorganic azides, and especially sodium azide. One advantage of such known sodium azide gas generants is that the solid combustion products thereof generally produce a slag or "clinkers" which are easily filtered, resulting in a relatively clean gas. The ability of a gas generant to form a slag is a great advantage when the gases are used for inflation purposes, especially when the gases must be filtered as in the inflation of an automobile occupant restraint bag. However, the use of the sodium azide, or other azides as a practical matter, results in extra expense and risk in gas generant manufacture due to the extreme toxicity of unfired azides. In addition, the potential hazard and disposal problem of unfired inflation devices must be considered. Thus, a nonazide gas generant exhibits a significant advantage over an azide-based gas generant because of such toxicity related concerns. A fundamental problem that must be solved when using nonazide based gas generants is that it is easier to formulate slagging gas generants based on sodium azide than nonazide types because the combustion temperature is relatively low with azide-based gas generants. For example, the combustion temperature of a sodium azide/iron oxide slagging type generant is 969°C (1776°F) whereas, nonazide slagging type generants heretofore known have exhibited a combustion temperature of 1818°C (3304°F). Moreover, many common solid combustion products which might be expected from nonazide gas generants are liquids at the combustion temperature exhibited and are therefore difficult to filter out of the gas stream. For example, potassium carbonate melts at 891°C and sodium silicate melts at approximately 1100°C. The formation of solid combustion products which coalesce at high combustion temperatures, and at high gas flow rates, requires a special combination of materials. Early attempts at formulating nonazide gas generants resulted in semi-solid combustion products that were difficult to filter. It has been found that combustion products which are liquid at the combustion temperature must be cooled until solidified before filtering is successful because liquid products penetrate and clog the filter. It has also been found that cooling of the liquid combustion products results in cooling of the gas, which requires the use of more gas generant. A cooled gas is relatively less efficient for inflation purposes, especially with an aspirator system. The additional gas generant, in turn, requires more cooling and an additional filter as well as a larger combustion chamber. Most azide-free, gas generant compositions provide a higher yield of gas (moles of gas per gram of gas generant) than conventional occupant restraint gas generants. Although azide-free gas generating compositions offer numerous advantages over azide-based gas generants, it has been found difficult to produce gases which have sufficiently low levels of toxic substances. The toxic gases which are the most difficult to control are the oxides of nitrogen (NOx) and carbon monoxide (CO). Most azide-free gas generants consist of carbon and nitrogen containing ingredients which, upon combustion, produce small, but undesirable levels of NOx and CO in addition to the desired products, nitrogen and carbon dioxide. In combustion processes involving compounds containing both nitrogen and carbon it is possible to reduce or eliminate the CO by increasing the ratio of oxidizer to fuel. In this case, the extra oxygen oxidizes the CO to carbon dioxide. Unfortunately, however, this approach results in increased amounts of NOx. The ratio of oxidizer to fuel may also be lowered to eliminate excess oxygen and provide a fuel rich condition which reduces the amount of NOx produced. This approach, however, results in increased amounts of CO. Even though it is possible, by means of chemical equilibrium calculations, to find conditions of temperatures, pressure and gas generant composition which could reduce both NOx and CO to nontoxic levels it has been very difficult to accomplish this result in actual practice. The aforesaid problems are solved by the present invention, which discloses several types of nonazide gas generants that yield solid combustion products which form a slag or clinkers at the relatively high combustion temperatures encountered with nonazide gas generants. The gas generants disclosed herein allow the use of simple, relatively inexpensive filters which cool the gas less and result in better pumping in an aspirated system. Taken together, these factors result in a simpler, less expensive and smaller airbag inflation system. A problem solved by a preferred embodiment of this invention is that the NOx is controlled by means which are effective even though a limited amount of excess oxygen is present. This allows reduction of the CO level by, the excess oxygen while, at the same time, lowering the NOx concentration to acceptable values. DESCRIPTION OF THE PRIOR ART An example of prior art teachings relating to the subject matter of the instant invention is found in European Patent No. 0,055,547 entitled, "Solid Compositions for Generating Nitrogen, The Generation of Nitrogen Therefrom and Inflation of Gas Bags Therewith". This -patent describes use of alkali or alkaline earth metal salts of a hydrogen-free tetrazole compound and oxidizers of sodium nitrate, sodium nitrite and potassium nitrate or alkaline earth nitrates. A filter design is disclosed which utilizes fiberglass fabric that forms a tacky surface for particle entrapment. The filter has regions which cool and condense combustion solids. It is obvious from the disclosure and from the nature of the gas generating compositions that the solids produced do not form a slag and are difficult to filter. European Patent No. 0,055,904 entitled, "Azide Free Compositions for Generating Nitrogen, The Generation of Nitrogen Therefrom and Inflation of Gas Bags Therewith" describes a filter used for particle entrapment. Oxidizers which contain no oxygen are used, and no mention of slag formation is made. German Patent 2,004,620 teaches compositions of organic salts (aminoguanidine) of ditetrazole and azotetrazole that are oxidized using oxidizers such as barium nitrate or potassium nitrate. However, no compositions are mentioned which would lead to slag formation. U.S. Patent 3,947,300 entitled, "Fuel for Generation of Nontoxic Propellant Gases" discloses the use of alkali or alkaline earth metal azides that can be oxidized by practically any stable anhydrous oxidizing agent. The ratio of ingredients is selected to assure the formation of glass-like silicates with "as low a melting or softening point as possible" (column 2, lines 62-63 and column 4, lines 67-68). These silicates would be very difficult to filter in a high temperature system. U.S. Patent 4,376,002 entitled, "Multi-Ingredient Gas Generators" teaches the use of sodium azide and metal oxide (Fe 2 O 3 ). The metal oxide functions as an oxidizer converting sodium azide to sodium oxide and nitrogen as shown in the following equations: 6 NaN 3 + Fe 2 O 3 → 3 Na 2 O + 2 Fe + 9 N 2 or 4 NaN 3 + Fe 2 O 3 → 2 Na 2 O + Fe + Feo + 6 N 2 The sodium oxide then reacts with the Feo forming sodium ferrite or with silicon dioxide (if present) to form sodium silicate or with aluminum oxide to form sodium aluminate, as shown below: Na 2 O + 2 Feo → 2 Na FeO 2 (MP = 1347°C) Na 2 O + SiO 2 → Na 2 SiO 3 (MP = 1088°C) or 2 Na 2 O + SiO 2 → Na 4 SiO 4 (MP = 1018°C) Na 2 O + Al 2 O 3 → 2 Na A1O 2 (MP = 1650°C) However, the above reaction products melt at temperatures well below the combustion temperature of compositions described in this invention and would, therefore, be difficult to filter. U.S. Patent No. 4,931,112 entitled, "Gas Generating Compositions Containing Nitrotriazalone" discloses the use of nitrotriazolone (NTO) in combination with nitrates and nitrites of alkali metals (except sodium) and the alkaline earth metals calcium, strontium or barium. However, the compositions taught in the patent are not capable of forming useful solid clinkers. For example, the two compositions given in Example 2 consist of different ratios of NTO and strontium nitrate which, upon combustion, would produce strontium oxide and strontium carbonate as fine dust since there is no low-temperature slag former present. Compositions claimed, utilizing mixtures of NTO and potassium nitrate, likewise will not form a useful solid clinker since potassium carbonate would be produced which would be a liquid at the combustion temperature and no high temperature slag former is present. The hydroxides mentioned are very unlikely to be formed because the excess carbon dioxide would convert the metal oxides to carbonates in preference to hydroxides. Even if some hydroxides were formed they would be the wrong type of slag former to promote clinker formation. U.S. Patent 4,909,549 entitled, "Composition and Process for Inflating a Safety Crash Bag" discloses the use of alkali metal salts, alkaline earth metal salts or ammonium salt of a hydrogen containing tetrazole in the range of about 20 to about 65 Wt.%. The effectiveness of alkali metal compounds, at these or lower concentrations, was not known. The present invention provides a pyrotechnic, gas generating composition useful under combustion for inflating an automobile or aircraft safety crash bag, said pyrotechnic composition comprising at least one material from each of the following groups: a. a fuel selected from the group of azole compounds consisting of triazole, aminotetrazole, tetrazole, bitetrazole and metal salts of these compounds except alkali metal and alkaline earth metal salts; b. an oxygen-containing oxidizer compound selected from the group consisting of alkaline earth or alkali metal nitrates and perchlorates; c. a chemical additive which is an alkali metal carbonate, or an alkali metal salt of triazole, tetrazole, 5-aminotetrazole, bitetrazole, or 3-nitro-1,2,4-triazole-5-one, said chemical additive being present in said mixture in an amount sufficient to reduce the amount of toxic oxides of nitrogen from the combustion products produced by the mixture on combustion; and d. a low-temperature slag forming material selected from the group consisting of naturally occurring clays and talcs or silica; with the proviso that the composition does not simultaneously comprise the potassium salt of 5-aminotetrazole, 5-aminotetrazole, clay and strontium nitrate. The primary advantage of a new nonazide gas generant composition in accordance with the instant invention is that solid combustion products are easily filtered from the gas produced. The nonazide gas generant uses azole compounds or metal salts of azole compounds as the fuel and nitrogen source. The unique feature of this invention is the novel use of oxidizers and additives resulting in solid combustion products which coalesce into easily filtered slag or clinkers. Also, the gas generant compositions comprising this invention provide a relatively high yield of gas (moles of gas per gram of gas generant) compared to conventional occupant restraint gas generants. Another primary advantage of a preferred embodiment of this invention is that the NOx is controlled by means which are effective even though a limited amount of excess oxygen is present. This allows reduction of the CO level by the excess oxygen while, at the same time, lowering the NOx concentration to acceptable values. Since the ability to rapidly produce inflation gas which is relatively free of solid particulate matter is a requirement for automobile occupant restraint systems, even relatively nontoxic solids must be reduced to low levels. Almost any gas-solid mixture can be filtered to produce clean gas if a large expensive filter can be used. However, for automobile occupant restraint systems both filter size and cost must be minimized. The best way to accomplish this end is to produce solid combustion products which coalesce into large, easily filtered "clinkers" or slag. Many combinations of ingredients can be used to improve the filtering characteristics of the combustion products. For most practical applications, however, compromises are necessary to provide the desired combination of slag forming ability, burn rate, gas production, gas quality, pellet forming characteristics, and other processing factors. In accordance with the instant invention, several combinations of materials have been found which, produce easily filtered solid products as well as gases useful for inflation purposes. Such materials may be categorized as fuels, oxidizers, chemical additives and low-temperature slag formers. It is important that at least one material identified with each category be included in the mixture although certain materials can serve more than one of the categories as described below. In formulating a fuel for the gas generant of an automobile occupant restraint system, it is desirable to maximize the nitrogen content of the fuel and regulate the carbon and hydrogen content thereof to moderate values. Although carbon and hydrogen may be oxidized to carbon dioxide and water, which are relatively nontoxic gases, large amounts of heat are generated in the process. Tetrazole compounds such as aminotetrazole, tetrazole, bitetrazole and metal salts of these compounds as well as triazole compounds such as 1,2,4-triazole-5-one or 3-nitro-1,2,4-triazole-5-one and metal salts of these compounds are especially useful fuels. It should be noted that certain metal salts (alkaline earth metals) of these compounds can function, at least in part, as high temperature slag formers. For example, the calcium salt of tetrazole or bitetrazole forms, upon combustion, calcium oxide which would function as a high-temperature slag former. Magnesium, strontium, barium and possibly cerium salts would act in similar manner. In combination with a low-temperature slag former, a filterable slag would be formed. The alkali metal salts (lithium, sodium, potassium) could be considered, at least in part, as low-temperature slag formers since they could yield lower melting silicates or carbonates upon combustion. Oxidizers generally supply all or most of the oxygen present in the system. In addition, however, they are the preferred method of including a high-temperature slag former into the reaction system. The alkaline earth and cerium nitrates are all oxidizers with high-temperature slag forming potential, although most of these salts are hygroscopic and are difficult to use effectively. Strontium and barium nitrates are easy to obtain in the anhydrous state and are excellent oxidizers. Alkali metal nitrates, chlorates and perchlorates are other useful oxidizers when combined with a high-temperature slag former. Materials which function as high-temperature slag formers have melting points at, or higher, than the combustion temperature or decompose into compounds which have melting points, at or higher, than the combustion temperature. The alkaline earth oxides, hydroxides and oxalates are useful high-temperature slag formers. Magnesium carbonate and magnesium hydroxide are very useful high-temperature slag formers because they decompose before melting to form magnesium oxide which has a very high meeting point (2800°C). As mentioned above, oxidizers such as strontium nitrate are especially beneficial since they serve both as high-temperature slag former and oxidizer, thereby increasing the amount of gas produced per unit weight. Metal salts as fuels, such as the calcium or strontium salt of 5-aminotetrazole, tetrazole, or ditetrazole are also useful high-temperature slag formers, although not as efficient as the oxidizers. Other metal oxides having high melting points such as the oxides of titanium, zirconium and cerium are also useful high-temperature slag formers. Materials which function as low-temperature slag formers have melting points at or below the combustion temperature or form compounds during combustion which have melting points at or below the combustion temperature. Compounds such as silicon dioxide (SiO 2 ), boric oxide (B 2 O 3 ), vanadium pentoxide (V 2 O 5 ), sodium silicate (Na 2 SiO 3 ), potassium silicate (K 2 SiO 3 ), sodium carbonate (Na 2 CO 3 ) and potassium carbonate (K 2 CO 3 ) are examples of low-temperature slag formers. It should be noted that either the oxidizer or the fuel can act as a low-temperature slag former if it contains a suitable substance which can be converted during combustion. For example, sodium nitrate or the sodium salt of tetrazole, during the combustion reactions, can convert to sodium carbonate or sodium silicate, if silicon dioxide is also present. It is desirable to combine the fuel or oxidizer (or both) and the high temperature slag former into one ingredient. For example, strontium nitrate may serve as both the oxidizer and high-temperature slag former. In this case, the strontium nitrate will yield, upon combustion, strontium oxide (SrO), which has a high melting point (2430°C) as well as oxygen and nitrogen gases. Silicon dioxide, used as a low-temperature slag former is available in many forms ranging from very fine submicron particles to coarse ground sand with melting points from about 1500° to 1700°C. The combination of strontium oxide and silicon dioxide forms strontium silicate (SrSiO 3 ) with a melting point of approximately 1580°C. SrO + SiO 2 → SrSiO 3 Strontium oxide can also react with carbon dioxide, forming strontium carbonate which melts at approximately 1500°C at high pressure. SrO + CO 2 → SrCO 3 The extent of each of these reactions depends upon various conditions such as combustion temperature, pressure, particle size of each component, and the contact time between the various materials. It is believed that the function of the low-temperature slag former is to melt and glue the high-temperature solid particles together. With only low-temperature residue, the material is liquid and is difficult to filter. With only high-temperature materials, finely divided particles are formed which are also difficult to filter. The objective is to produce just enough low-temperature material to induce a coherent mass or slag to form, but not enough to make a low viscosity liquid. One preferred composition is one wherein the fuel comprises 5-aminotetrazole in a concentration of about 28 to about 32% by weight, the oxygen containing oxidizer compound comprises strontium nitrate in a concentration of about 50 to about 55% by weight, the chemical additive comprises potassium carbonate in a concentration of about 2 to about 10% by weight, and the low-temperature slag former comprises clay in a concentration of about 2 to about 10% by weight. Another preferred composition is one wherein the fuel comprises 5-aminotetrazole in a concentration of about 26 to about 32% by weight, the oxygen containing oxidizer compound comprises strontium nitrate in a concentration of about 52 to about 58% by weight, the chemical additive comprises sodium tetrazole in a concentration of about 2 to about 10% by weight, and the low-temperature slag former comprises clay in a concentration of about 2 to about 10% by weight. Still another preferred composition is one wherein the fuel comprises 5-aminotetrazole in a concentration of about 26 to about 32% by weight, the oxygen containing oxidizer compound comprises strontium nitrate in a concentraton of about 52 to about 58% by weight, the chemical additive comprises the potassium salt of 5-aminotetrazole in a concentration of about 2 to about 12% by weight, and the low-temperature slag former comprises talc in a concentration of about 2 to about 16% by weight. The invention importantly provides means of reducing the amount of the toxic gases NOx and CO in gas generant combustion products. This is accomplished by using an alkali metal salt mixed into the propellant. The primary effect of the salt is to reduce the amount of NOx but this allows formulation of the gas generant to provide an excess of oxygen, in the combustion products, which reduces the amount of carbon monoxides as well as the NOx. The invention contemplates application of these means to any gas generant which produces NOx and carbon monoxide. The type of alkali metal compound used is important. While all alkali metals are likely to be effective in controlling NOx, potassium is the most preferred alkali metal because of its availability, low cost and effectiveness. The alkali metal preferably should be incorporated into the propellant as part of an organic compound rather than an inorganic compound. Potassium carbonate also is effective. The preferred method of incorporating alkali metals into gas generants is as salts of organic acids. For gas generants used in automobile airbags it is advantageous to use compounds which have a high nitrogen content such as alkali metal salts of tetrazoles or triazoles. These materials serve multiple functions when incorporated into a gas generant. In addition to reducing the amount of NOx produced, these compounds serve as fuels which produce useful gases and as low temperature slag formers as described elsewhere herein. The range of alkali metal compounds which can be effectively used in a gas generant is quite broad. As little as 2% K5AT has been found to be effective as an additive and, in cases where the K5AT served as the primary fuel and gas producer, up to about 45% has been used. The preferred range, however, is about 2 to about 20% and the most preferred range is from about 2 to about 12% by weight. Regarding the chemical additive, as indicated, the organic acid salts and carbonates are effective. The salts of organic acids are most effective and are therefore preferred. The alkali metal salts of 5-aminotetrazole, tetrazole, bitetrazole and 3-nitro-1,2,4-triazole-5-one (NTO) are preferred because of their high nitrogen content. Lithium, sodium and potassium are preferred alkali metals; the invention also contemplates the use of rubidium and cesium. The most preferred alkali metal is potassium and the most preferred salt is the potassium salt of 5-aminotetrazole. The invention is illustrated by the following representative examples. REFERENCE EXAMPLE 1 A mixture of 5-aminotetrazole (5AT), strontium nitrate (SrN) and bentonite clay was prepared having the following composition in percent by weight: 33.1% 5AT, 58.9% SrN and 8.0% clay. These powders were dry blended and pellets were formed by compression molding. The pellets were burned in a Parr combustion bomb which was pressurized to 25 atmospheres pressure with nitrogen after flushing with nitrogen to remove any oxygen from the bomb. The pellets were ignited by means of a hot wire. A gas sample was removed from the bomb within 10 seconds after combustion of the gas generant in order to minimize interaction of NOx with the solid combustion products. Analysis of the gas sample showed the presence of a relatively high concentration of NOx: 2180 parts per million (ppm) of NOx. EXAMPLE 1 A mixture of 5AT, SrN, bentonite clay and the potassium salt of 5AT (K5AT) was prepared having the following composition in percent by weight: 28.6% 5AT, 57.4% SrN, 8.0% clay and 6.0% K5AT. This mixture was calculated by a chemical equilibrium computer program to have a small excess of oxygen in the resulting gas mixture. The above powders were prepared and tested as described in Reference Example 1. Two tests were performed resulting in measured NOx concentrations of 32 and 40 ppm. Example 1, by contrast with Reference Example 1, illustrates the large reduction in NOx concentration produced by the addition of K5AT. EXAMPLE 2 A mixture of 5AT, SrN, bentonite clay and potassium carbonate was prepared having the following composition in percent by weight: 31.1% 5AT, 55.4% SrN, 7.5% clay and 6.0% potassium carbonate. This mixture was prepared and tested as described in Reference Example 1. Two tests were performed resulting in measured NOx concentrations of 128 and 80 ppm. EXAMPLE 3 A mixture of 5AT, SrN, clay and the sodium salt of tetrazole (NaT) was prepared having the following composition in percent by weight: 30.4% 5AT, 54.2% SrN, 7.4% clay and 8.0% NaT. This mixture was prepared and tested as described in Reference Example 1. Two tests were performed resulting in measured NOx concentrations of 40 and 32 ppm. EXAMPLE 4 A mixture of 5AT, potassium nitrate (KN), Talc and K5AT was prepared having the following composition in percent by weight: 25.2% 5AT, 52.8KN, 16.0% Talc and 6.0% K5AT. This composition results in 2.5% by volume excess oxygen as calculated by a chemical equilibrium computer program. Small pellets of this mixture were prepared on an automatic tableting press. These pellets were tested as described in Reference Example 1. Two tests were performed resulting in 112 ppm NOx and 100 ppm carbon monoxide in the first test and 144 ppm NOx and 140 ppm carbon monoxide in the second test. This example illustrates that low concentrations of both NOx and carbon monoxide can be obtained by using K5AT in combination with excess oxygen. While the preferred embodiment of the invention has been disclosed, it should be appreciated that the invention is susceptible of modification without departing from the scope of the following claims.

Claims: A pyrotechnic, gas generating composition useful under combustion for inflating an automobile or aircraft safety crash bag, said pyrotechnic composition comprising at least one material from each of the following groups: a. a fuel selected from the group of azole compounds consisting of triazole, aminotetrazole, tetrazole, bitetrazole and metal salts of these compounds except alkali metal and alkaline earth metal salts; b. an oxygen-containing oxidizer compound selected from the group consisting of alkaline earth or alkali metal nitrates and perchlorates; c. a chemical additive which is an alkali metal carbonate, or an alkali metal salt of triazole, tetrazole, 5-aminotetrazole, bitetrazole, or 3-nitro-1,2,4-triazole-5-one, said chemical additive being present in said mixture in an amount sufficient to reduce the amount of toxic oxides of nitrogen from the combustion products produced by the mixture on combustion; and d. a low-temperature slag forming material selected from the group consisting of naturally occurring clays and talcs or silica; with the proviso that the composition does not simultaneously comprise the potassium salt of 5-aminotetrazole, 5-aminotetrazole, clay and strontium nitrate. A composition according to claim 1 wherein the chemical additive is an alkali metal salt of 5-aminotetrazole, tetrazole, bitetrazole or 3-nitro-1,2,4-triazole-5-one. A composition according to claim 2 wherein the alkali metal salt is a potassium, sodium or lithium salt. A composition according to any one of claims 1 to 3 wherein the chemical additive is an alkali metal carbonate. A composition according to claim 4 wherein the chemical additive is potassium carbonate. A composition according to any one of the preceding claims wherein the chemical additive is present in a concentration of from 2% to 45% by weight. A composition according to claim 1 wherein the fuel comprises 5-aminotetrazole in a concentration of from 28 to 32% by weight, said oxygen containing oxidizer compound comprises strontium nitrate in a concentration of from 50 to 55% by weight, said chemical additive comprises potassium carbonate in a concentration of from 2 to 10% by weight, and said low-temperature slag former comprises clay in a concentration of from 2 to 10% by weight. A composition according to claim 1 wherein the fuel comprises 5-aminotetrazole in a concentration of from 26 to 32% by weight, said oxygen containing oxidizer compound comprises strontium nitrate in a concentration of from 52 to 58% by weight, said chemical additive comprises sodium tetrazole in a concentration of from 2 to 10% by weight, and said low-temperature slag former comprises clay in a concentration of from 2 to 10% by weight. A composition according to claim 1 wherein the fuel comprises 5-aminotetrazole in a concentration of from 26 to 32% by weight, said oxygen containing oxidizer compound comprises strontium nitrate in a concentration of from 52 to 58% by weight, said chemical additive comprises the potassium salt of 5-aminotetrazole in a concentration of from 2 to 12% by weight, and said low-temperature slag former comprises talc in a concentration of from 2 to 16% by weight. A method of inflating an automobile or aircraft safety crash bag which comprises combusting a pyrotechnic gas generating composition as claimed in any one of claims 1 to 9 whereby gas is generated and inflates said crash bag.

Inventors: POOLE DONALD R, POOLE, DONALD R.
Applicants: AUTOMOTIVE SYSTEMS LAB, AUTOMOTIVE SYSTEMS LABORATORY INC.
Application year: 1992
Main classification: C06D5
IPC codes: C06D5