Gas generant compositions with alkali oxide scavengers

Abstract

 A gas generant composition adapted for use with airbag restraint systems containing aluminum components includes a fuel which is 5-nitrobarbituric acid (or salts thereof), 5-nitroorotic acid (or salts thereof); an oxidizer which is an ammonium, alkali metal and/or alkaline earth metal salt of a chlorate, perchlorate or nitrate and/or is a transition metal oxide, such as cupric oxide; alumina and/or silica; and a binder.

Description

[0001] The present invention is directed to gas generant compositions suitable for automotive air bag restraint systems, particularly restraint systems in which the gas generant is encased in aluminum housing and/or generates gases which come into contact with aluminum components, such as filters.

[0002] 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.

[0003] US-A-5,015,309 to Wardle et al., teaches gas generant compositions in which fuels are salts of 5-nitrobarbituric acid, salts of 5-nitroorotic acid and nitrouracil (which doesn't form a salt), oxidizers include alkali (except sodium) and alkaline earth metal salts of nitrates, nitrites and hexanitrocobaltate, and binders, when needed for pelletization.

[0004] This patent teaches that minimal amounts of non-gas generating components should be added, except up to 1 wt% fibers, such as graphite or iron fibers, which increase the burning rate and transfer heat during combustion.

[0005] US-A-5, 015,309 teaches that the compositions are useful for inflators as taught in US-A-4,547,342 to Adams et al. The inflators taught in US-A-4,547,342 have aluminum housings and other aluminum components. A disadvantage with compositions such as taught in US-A-5,015,309 is that combustion products include K₂O which is highly corrosive to aluminum. Improved formulations are needed to reduce degradation of aluminum components during combustion of the gas generant composition. Also, it is highly desirable that the combustion products include a slag which is filterable. The compositions taught in US-A-5,015,309 are not filterable.

[0006] According to the present invention a gas generant composition using as the fuel component a compound selected from 5-nitrobarbituric acid, 5-nitroorotic acid, alkali and alkaline earth metal salts thereof; an oxidizer for the fuel selected from ammonium, alkali metal and alkaline earth metal salts of chlorates, perchlorates and nitrates and transition metal oxides; and a binder, also contains alumina (Al₂O₃) and/or silica (SiO₂) as a scavenger of alkali metal oxides.

[0007] The fuel comprises between 5 and 55 wt%, preferably at least 20 wt.%, of the gas generant composition. Preferred fuels are the potassium salts of 5nitrobarbituric acid (also called dilituric acid) and 5nitroorotic acid.

[0008] The oxidizer, which is used at a level of between 30 and 75 wt%, preferably at least 50 wt.%, is selected from ammonium, alkali metal and alkaline earth metal chlorates, perchlorates, nitrates, transition metal oxides, and mixture thereof.

[0009] Optionally, a portion of the oxidizer may be a transition metal oxide, such as cupric oxide. Such a transition metal oxide preferably comprises between about 5 to about 35 wt% of the composition. In addition to their oxidizing function, these oxides provide hard particles, facilitating compaction of the composition into pellets or other consolidated solid shapes.

[0010] As is taught in US-A-5,139,588, the teachings of which are incorporated herein by reference, the cations of the fuel salts and oxidizers are preferably mixture of alkali metal cations, i.e., lithium, sodium and potassium, and alkaline earth metal cations, i.e., magnesium, strontium, barium and cerium. Upon combustion, the alkali cations form liquid combustion products and the alkaline earth metal cations form solid combustion products, the mixture of liquid and solid combustion products forming clinkers which can be readily removed from the gas stream by filtration. The ratio of solid to liquid combustion products may be adjusted by the ratio of alkaline earth metal cations to alkali metal cations. Of alkali metal cations, sodium is preferred over potassium as sodium oxide is more readily scavenged by alumina or silica than potassium oxide.

[0011] In accordance with the present invention, alumina, silica or mixtures thereof are added to scavenge corrosive alkali metal oxides, such as sodium oxide and potassium oxide. Accordingly, the composition of the present invention contains alumina and/or silica at a level of between about 0.5 and about 10 wt%, preferably between about 1 and about 8 wt%. The alumina and/or silica may be in the form of particulates or as fibers, such as fibers of various silica/alumina content. Alumina is generally preferred over silica, being a more efficient scavenger.

[0012] A binder is added at a level of between about 1 and about 5 wt%. Suitable binder materials include but are not limited to molybdenum disulfide, graphite, polytetrafluroethylene, Viton® (a copolymer of vinylidene fluoride and hexafluoropropylene), nitrocellulose, polysaccharides, polyvinylpyrrolidones, polycarbonates, sodium silicate, calcium stearate, magnesium stearate and mixtures thereof. Preferred binder materials are molybdenum disulfide and polycarbonates.

[0013] Alkali metal and alkaline earth metal carbonates and/or oxalates may optionally be added up to 40 wt%. These act as coolants, lowering the combustion temperature. Lower combustion temperatures minimize production of toxic gases, such as CO and NO_x. Generally, if used, these coolants are used at a level of at least 1 wt%.

[0014] As noted above, the alumina and/or silica may be in the form of fibers. Fibers help to mechanically reinforce the consolidated unburned material and subsequently consolidate slag material formed by burning the composition. Graphite fibers, e.g., at between 1 and 10 wt%, may be also be used, either as the sole fibrous material or in conjunction with alumina-containing fibers to perform this reinforcing function.

[0015] The invention will now be described in greater detail by way of specific example.

Examples 1-8

[0016] Gas generant formulations according to the invention and their combustion properties are shown in the table below. Component levels are in weight percent.

Claims

1. A gas generant composition comprising
   between 5 and 55 wt% of a fuel which is selected from 5-nitrobarbituric acid, 5-nitroorotic acid, alkali and/or alkaline earth metal salts of the 5-nitrobarbituric acid or 5-nitroorotic acid, and mixtures thereof,
   between 30 and 75 wt% of an oxidizer selected from ammonium, alkali metal and alkaline earth metal chlorates, perchlorates, nitrates, transition metal ocides, and mixtures thereof,
   between 0.5 and 10 wt% alumina and/or silica, and
   between 1 and 5 wt% of a binder.

2. A generant composition according to claim 1 wherein said binder is selected from molybdenum disulfide, graphite, polytetrafluoroethylene, vinyl fluoride/hexafluoropropylene copolymer, nitrocellulose, polysaccharides, polyvinylpyrrolidones, polycarbonates, sodium silicate, calcium stearate, magnesium stearate and mixtures thereof.

3. A gas generant according to claim 1 wherein said binder is selected from molybdenum disulfide and polycarbonates.

4. A gas generant composition according to any preceding claim comprising between 5 and 35 wt% of a transition metal oxide.

5. A gas generant composition according to claim 4 wherein said transition metal oxide is cupric oxide.

6. A gas generant composition according to any preceding claim further containing between 1 and 40 wt% of a coolant selected from alkali metal and alkaline earth metal carbonates, oxalates and mixtures thereof.

7. A gas generant composition according to any preceding claim further containing between 1 and 10 wt% of graphite fibers.