Fuel compositions for use in hybrid inflators containing stored oxidizing gas

Description

[0001] This invention relates to a hybrid gas generating system for an airbag inflator. More particularly, it relates to a novel system which minimizes particulates in the generated gas and which provides internal ballistic control of the inflator. Still more particularly, it relates to the use of a solid organic fuel in which some level of an oxidizer has been incorporated to generate heat and vaporize the fuel so that it can then be burned in a stored oxidizing gas.

[0002] Hybrid gas generators contain a stored gas which may be an oxidizing gas, an inert gas, or a mixture of the two. The oxidizing gas may be mixed with gaseous or liquid fuel and burned to produce heated gaseous products which may be used to inflate an airbag in an automobile. When the stored gas is totally inert a pyrotechnic heater cartridge must be used to heat the gas prior to inflation of the airbag. Exemplary of the many patents issued in this field are U. S. Patent Nos. 3,692,495 (Schneiter et al); 3,723,205 (Scheffee); 3,756,621 (Lewis et al); 3,785,149 (Timmerman); 3,897,285 (Hamilton et al); 3,901,747 (Garner); 3,912,562 (Garner); 3,950,009 (Hamilton); 3,964,255 (Catanzarite); 4,128,996 (Garner et al); and 4,981,534 (Scheffee).

[0003] In the case of a mixture of an oxidizing gas and an inert gas, the combustion of a gaseous or liquid fuel with the oxidizing gas heats the stored gas and augments the stored gas for inflation of the airbag.

[0004] Control over the airbag inflation rate is desirable to minimize the forces a person experiences during dynamic response to an automobile collision. Control over the rate of heating the stored gas provides some measure of control over the rate of gas efflux from the inflator and, hence, the rate of airbag inflation. The extrusion of pyrotechnic materials into various shapes and their formulation to achieve certain burning rates may be used to control the rate of inflation. A disadvantage of using a totally inert gas and a pyrotechnic heater, however, is the production of objectionably high level of particulates to which the occupants of the automobile are subjected. Several criteria must be met by a pyrotechnic gas generant to be satisfactory for inflatable restraint systems. It must produce non-toxic, non-flammable and smokeless gas over a wide range of temperatures and other environmental conditions. The pyrotechnic must be safe to handle and must be capable of generating a large amount of gas within a very short time frame, i.e., about 35 milliseconds.

[0005] Some mixed gas systems have been proposed in which either a metal, a gas, or a liquid is used as the fuel. The use of a metal (see U.S. 5,230,532) suffers from the same particulates disadvantage as the pyrotechnic heater cartridge. The reaction of a gaseous fuel such as hydrogen (see U.S. 5,263,740) or a liquid is generally very rapid and does not allow for an easily controlled release of gas from the inflator. The orifice size in the inflator is sometimes the only control over the efflux rate. On the other hand, if the reaction rate of the gas or liquid is too slow, possibly toxic, unreacted components are exhausted into the airbag and thence into the automobile. In one embodiment of U.S. 5,263,740, a mixture of a flammable gas, an inert gas, and the amount of an oxidizing gas required for complete combustion of the fuel is used. In another, a pyrotechnic material containing PVC and a stoichiometric amount of oxidizer is used.

[0006] It is an object of this invention, therefore, to provide a hybrid gas generating system whereby the rate at which stored gas is heated depends on the rate at which a solid fuel, the source of the heat, is vaporized.

[0007] It is another object of this invention to provide a hybrid system in which the vaporization rate is the controlling factor rather than the relatively very fast combustion rate of the vaporized fuel.

[0008] It is another object of this invention to provide a pyrotechnic composition which contains an amount of oxidizer sufficient to vaporize the fuel therein.

[0009] It is a related object of this invention to provide an air bag inflator system wherein the level of solids in the inflating gas is very low.

[0010] It is a related object of this invention to provide a hybrid gas generating system wherein the rate of heating the stored gas is a function of the surface area of a solid pyrotechnic material in the system.

[0011] It is a related object of this invention to provide a hybrid gas generating system wherein the rate of heating the stored gas is a function of the linear regression rate of a solid pyrotechnic material in the system.

[0012] It is yet another object of this invention to provide a method for inflating an air bag by vaporizing an under-oxidized solid fuel composition and introducing the vaporized fuel into an oxygen-containing gas within a pressure vessel, wherein it is fully oxidized to generate sufficient heat to raise the pressure within the vessel and force the pressurized gas into the air bag.

[0013] These and other objects of the invention which shall become apparent from the following description of the invention are achieved by a method for inflating an air bag in an automobile by igniting an under-oxidized solid fuel composition in response to a signal from a deceleration sensor in the automobile, vaporizing said fuel with heat thus generated, introducing the vaporized fuel into an oxygen-containing gas within a pressure vessel, fully oxidizing the fuel in said gas, thereby generating sufficient heat to raise the pressure within the vessel and force the pressurized gas into an air bag communicating with the vessel.

[0014] The invention is further described as a hybrid gas generating system comprising a pressure vessel containing oxygen under pressure, a container adapted for communication with the pressure vessel, a solid combustible fuel having a heat of combustion of at least about 3500 calories per gram and an oxidizer within said container, said oxidizer being present in said container in an amount sufficient only to support combustion of the fuel to generate heat to vaporize the fuel, said amount being less than 60% of the stoichiometric amount required for the oxidation of all available carbon in said fuel to carbon dioxide and all available hydrogen in said fuel to water, an igniter for a mixture of the solid fuel and oxidizer, thereby causing said mixture to burn and increasing the pressure of the gas within the pressure vessel.

[0015] The solid fuel suitable for use in the pyrotechnic material of this invention has the empirical formula C_xH_yO_zN_u wherein x and y are positive integers, and z and u are independently zero or positive integers. The stoichiometric amount of oxidizer required for the oxidation of all available carbon in said fuel to carbon dioxide and all available hydrogen in said fuel to water is 2x + y/2-z moles of oxygen per mole of fuel. The level of oxidizer present in the under-oxidized formulations is preferably such that less than one gram of solid particulate is exhausted from a standard-sized airbag module when the fuel composition is oxidized by the internal oxidizer and the oxidizing gas. An airbag module is defined herein as an inflator device plus an inflatable airbag cushion.

[0016] Examples of a solid fuel suitable for the purposes of this invention include polyolefins, waxes, asphalts, and internally partially oxidized compounds such as sugars, polyesters, polyethers, acrylic polymers, phenols, polysaccharides, e.g., cellulose or starch, cellulose ethers, or cellulose esters, and nitrate salts of amines, nitramines, nitro compounds, nitrate esters, and mixtures of two or more of said compounds.

[0017] The heat of combustion of a suitable fuel is at least about 3500, preferably about 4000 or more, calories per gram. With reference to the empirical formula of a internally partially oxidized fuel, the ratio of the number of moles of oxygen present in the fuel molecule per the quantity (2 x moles of C + 0.5 x moles of H), or O/F ratio, is preferably from 0 to about 0.4. For example, the O/F ratio for methyl cellulose (C₁₈H₃₁O₁₁) is 0.214 and its heat of combustion is-4960 calories per gram. The O/F for poly methylmethacrylate (C₅H₈O₂) is 0.143 and its heat of combustion is -6530 calories per gram. Suitable solid fuels are exemplified by lactose, ethyl cellulose, hydroxyethyl cellulose, cellulose acetate, cellulose propionate, stearic acid, polyacetal, polyethylene, polypropylene, polystyrene, naphthalene, resorcinol, and a saturated linear polyester such as that sold under the BOSTIC 4156 trademark and number. All of the foregoing fuels are readily available from purveyors of commodity chemicals.

[0018] Any stable, solid inorganic oxidizer is suitable for the purposes of this invention. Examples of such suitable oxidizers include the sodium, potassium, lithium, and other alkali metal chromates, dichromates, nitrates, chlorates, and perchlorates. The corresponding ammonium salts are also useful. The selection of the oxidizer depends upon the type of solid fuel.

[0019] From 0 to about 50 % of the total weight of the solid pyrotechnic material may be a plasticizer such as the alkyl and alkoxyalkyl adipates, sebacates, phthalates, azelates, and nitrates.

[0020] Catalysts and burn rate modifiers are optional and, when used, the maximum amount is about 5% of the weight of the pyrotechnic material. Examples of such include boron hydrides and transition metal oxides such as copper oxide, manganese oxide, and vanadium oxide.

[0021] The rate of producing heat for the stored gas is a function of the surface area and the linear regression rate of the solid pyrotechnic material. The surface area may be tailored easily by the physical shape of the pyrotechnic. A granular pyrotechnic material will have a high surface area while cylindrical particles will have a small surface area. The shaping of the particles may be by extrusion, solvent extrusion, molding, spray drying and pelletization, or extrusion and spheronization.

[0022] The amount of oxygen in the stored gas must be sufficient to complete the oxidation of all available carbon in said fuel to carbon dioxide and all available hydrogen in said fuel to water. The oxygen-containing gas may be 100 % oxygen or a mixture of oxygen and an inert gas such as nitrogen or argon. Air is suitable.

Example 1

[0023] Conventionally, the oxidizer portion of a pyrotechnic composition is by far the major portion, the ratio of oxidizer to fuel being as high as 4:1 or even higher. When the combustion of the fuel is completed in an oxygen atmosphere, however, the total amount of pyrotechnic material can be reduced greatly along with the proportion of oxidizer. For example, a stoichiometric ratio of 30.00 % of a polyester, 16.16 % of potassium perchlorate, and 52.85 % oxygen is utilized in the heating of x grams of an oxygen/argon mixture in an inflator device when only 11.73 grams of an under-oxidized pyrotechnic material containing 65.73 % of polyester and 34.22 % of the perchlorate is burned in said mixture. In comparison, 28 grams of a stoichiometric formulation of polyester and potassium perchlorate (1:4 by weight) is required to heat x grams of argon. About 1 gram of solid residue is produced from the under-oxidized pyrotechnic material. The amount of solids in the inflating gas may be even less with certain formulations. The heat of reaction generated by the combustion of the under-oxidized pyrotechnic in the oxygen/argon mixture is greater than when all of the oxidizer is present in the pyrotechnic material.

Example 2

[0024] The stoichiometric formulation of the ethyl cellulose and potassium perchlorate is 19.7 : 80.3 parts by weight. An under-oxidized pyrotechnic material consisting of 80.5 parts by weight of ethyl cellulose and 19.5 parts by weight of potassium perchlorate is burned in a hybrid inflator containing an oxygen/argon mixture in a pressurized cylinder. The proportion of fuel, perchlorate and oxygen taking part in the combustion is 33.2 : 8.0 : 58.8. The combustion releases 1840 calories per gram of pyrotechnic material in one hundred grams of the combustion mixture. Thirty thousand (30,000) calories of heat are required for the inflation of a passenger side air bag by the hybrid inflator; thus, the inflation requires the combustion of 16.3 grams of the pyrotechnic material. About 1.7 grams of potassium chloride are produced.

Claims

1. A combustible gas generant comprising at least one solid fuel having a heat of combustion of at least 3500 calories per gram and an oxidizer in an amount sufficient to support combustion which generates sufficient heat to volatilize the fuel, said amount being less than 60% of the stoichiometric amount required for the oxidation of all available carbon in said fuel to carbon dioxide and all available hydrogen in said fuel to water.

2. A composition according to claim 1 wherein the fuel has the empirical formula C_xH_yO_zN_u wherein x and y are positive integers, and z and u are independently zero or positive integers.

3. A composition according to claim 1 or claim 2 wherein the fuel is selected from polyolefins, waxes, asphalts, internally partially oxidized compounds, and mixtures thereof.

4. A composition according to claim 1 wherein the fuel is an internally partially oxidized compound selected from the group consisting of sugars, polyesters, polyethers, acrylic polymers, phenols, polysaccharides, amine nitrates, nitrate esters, nitramines, and mixtures thereof.

5. A composition according to claim 1 or claim 2 wherein the fuel is a cellulose ether.

6. A composition according to any preceding claim wherein the oxidizer is selected from nitrates, perchlorates, and chlorates of an ammonium ion, alkali metals, and alkaline earth metals, transition metal oxides, and mixtures thereof.

7. A hybrid gas generating system comprising a pressure vessel containing oxygen under pressure, a container adapted for communication with the pressure vessel, a solid combustible fuel having a heat of combustion of at least 3500 calories per gram and an oxidizer within said container, said oxidizer being present in an amount sufficient only to support combustion of the fuel to generate heat to vaporize the fuel, and an igniter for a mixture of the fuel and oxidizer.

8. A system according to claim 7 wherein said amount is less than 60% of the stoichiometric amount required for the oxidation of all available carbon in said fuel to carbon dioxide and all available hydrogen in said fuel to water.

9. A system according to claim 7 or claim 8 wherein the oxygen in the pressure vessel is sufficient to complete the oxidation of all carbon in the vaporized fuel to carbon dioxide and of all hydrogen in the vaporized fuel to water.

10. A system according to any one of claims 7 to 9 wherein the oxygen is mixed with an inert gas.

11. A system according to any one of claims 7 to 10 wherein the fuel has the empirical formula C_xH_yO_zN_u wherein x and y are positive integers, and z and u are independently zero or positive integers.

12. A system according to any one of claims 7 to 11 wherein the fuel is selected from polyolefins, waxes, asphalts, internally partially oxidized compounds, and mixtures of two or more such fuels.

13. A system according to any one of claims 7 to 11 wherein the fuel is an internally partially oxidized compound selected from sugars, polyesters, polyethers, acrylic polymers, phenols, polysaccharides, amine nitrates, nitrate esters, nitramines, and mixtures of two or more of said compounds.

14. A system according to any one of claims 7 to 11 wherein the fuel is a cellulose ether.

15. A system according to any one of claims 7 to 14 wherein the oxidizer is selected from nitrates, perchlorates, and chlorates of an ammonium ion, alkali metals, and alkaline earth metals, transition metal oxides, and mixtures thereof.

16. A system according to any one of claims 7 to 15 characterized further in that it is part of an inflator for a vehicle passenger restraint system.