Gas-generating composition

Abstract

A gas-generating composition is adapted for use in safety devices for vehicles. The composition essentially of 20 to 25% by weight of an organic fuel, 30 to 60% by weight ammonium perchlorate, 5 to 30% by weight of a chloride scavenger and conventional processing aids in a proportion of 0 to 10% by weight. An iron compound is used as said chloride scavenger, which is converted into gaseous iron(III)-chloride upon reaction of the composition.

Description

TECHNICAL FIELD OF THE INVENTION

The invention relates to a gas-generating composition for use in a safety device in vehicles.

BACKGROUND OF THE INVENTION

Safety devices for vehicles usually contain a gas generator with a pyrotechnic solid propellant as gas-generating composition which is activated as a result of a vehicle accident and releases a gas or gas mixture within a very short period of time for actuating the safety device, such as an inflatable gas bag, a belt tensioner or a pneumatic knee guard. Furthermore, so-called hybrid gas generators are known which, in addition to the pyrotechnic solid propellant, contain a pressurized gas stored in a pressure chamber, the pyrotechnic propellant tearing open an opening device of the pressure chamber, for example a rupture disc, in the case of activation, and thus releasing and heating up the gas which is stored in the pressure chamber.

In the past, in particular mixtures of sodium azide and inorganic nitrates were used as pyrotechnic propellant. Owing to the high toxicity of these mixtures, however, propellants on the basis of organic fuels and of inorganic oxidizing agents are recently coming into use. These azide-free propellants are distinguished by relatively high combustion temperatures which favour the production of liquid or gaseous by-products. Therefore, large quantities of so-called slag-forming materials or coolants must be added to these azide-free propellants, which, however, reduce the gas yield of the propellant.

A possibility for the production of pyrotechnic propellants having a higher gas yield lies in the use of ammonium perchlorate as oxidizing agent. When propellants containing ammonium perchlorate are reacted, however, hydrochloric acid is produced as a by-product which is undesirable, owing to its toxicity and liability to corrosion. Therefore, ammonium perchlorate has been used hitherto as oxidizing agent in pyrotechnic propellants for vehicle occupant restraint systems only in small proportions and/or mixed with suitable compounds such as, for example, alkali metal nitrates, for the neutralization of the hydrochloric acid in the propellant, which occurs during combustion. These mixtures, however, likewise have a reduced gas yield owing to the additives which are then necessary.

A gas-generating propellant is known from the EP-B1-0 750 599, which consists substantially of 55 to 75% by weight of guanidine nitrate, 25 to 45% by weight of an oxidizing agent which is selected from the group consisting of potassium perchlorate and ammonium perchlorate, 0.5 to 5.0% by weight of a flowability improving agent and up to 5% by weight of a binding agent. The example embodiments, however, merely describe the use of potassium perchlorate as oxidizing agent and therefore show an insufficient gas yield. The potassium chloride occurring as combustion product is only able to be retained with difficulty in a filter and therefore leads to the formation of fumes.

The U.S. Pat. No. 5,861,571 describes a gas-generating composition of ammonium perchlorate, a chloride scavenger and an inorganic fuel, with alkali salts and the salts of strontium and barium being named in particular as chloride scavengers. On the other hand, transition metal compounds of, for example, zinc and copper, are expressly regarded as not being suitable. Owing to the high proportion of the alkali- and alkaline earth compounds contained in the composition, when it is reacted the composition produces a high proportion of particles which, again, has to be retained in suitable filter devices. The gas yield of the known composition is therefore likewise insufficient.

However, in the motor vehicle industry requirements exist for a miniaturization of all systems, including the safety devices. Therefore, it is desirable to use pyrotechnic propellants which burn almost free of particles and have high gas yields, which allow filter devices in the gas generator to be dispensed with and allow a reduction in the amount of propellant, such that the overall size and the weight of the gas generators can be further reduced.

SUMMARY OF THE INVENTION

The present invention provides a physiologically harmless propellant for gas generators, which reacts to form a substantially particle-free and non-poisonous combustion gas with a high gas yield. In addition, the composition has a sufficiently high combustion rate and a high combustion temperature desirable for use in hybrid gas generators or belt tensioners.

According to the present invention, a gas-generating composition is provided, which consists essentially of 20 to 65% by weight of an organic fuel, 30 to 60% by weight of ammonium perchlorate, 5 to 30% by weight of a chloride scavenger and conventional processing aids in a proportion of 0 to 10% by weight, each based on the total weight of the composition, and which is characterized in that the chloride scavenger is an iron compound.

The use of ammonium perchlorate as sole oxidizing agent in combination with an iron compound as chloride scavenger makes possible the provision of gas-generating mixtures which are practically totally converted into gas when they are reacted. Hereby, a maximum efficiency of the propellant is achieved. Ammonium perchlorate is, moreover, compatible with almost all organic fuels.

When the compositions according to the invention are reacted, FeCl₃ is produced from the iron compounds, which is already sublimated at approximately 300 degrees C. and is therefore also vaporized upon the combustion of the gas-generating composition. The gaseous FeCl₃ is also included here in the gas yield of the composition. Iron(II) compounds are oxidized during combustion by the ammonium perchlorate to iron(III) compounds. The iron compounds are therefore able to bond a total of three mole of the hydrochloric acid released from the ammonium perchlorate per mole of iron. Owing to the high bonding capacity of the iron compounds for hydrochloric acid, the quantity of chloride scavengers can therefore also be reduced. An addition of further chloride scavengers in the form of alkali metal- or alkaline earth metal salts is no longer necessary.

As the hydrochloric acid resulting from the reaction of the ammonium perchlorate with the fuel is reliably neutralized by the iron compound with the formation of non-poisonous FeCl₃, and the FeCl₃ is gaseous under the reaction conditions, the gas generators can therefore be produced so as to be smaller and also lighter and at a more favourable cost by dispensing with costly and expensive filter materials.

The use of iron compounds as chloride scavengers has the further advantage that the iron compounds in addition promote the decomposition of the ammonium perchlorate and hence increase the combustion rate.

As iron compounds, both inorganic compounds such as FeO, FeO(OH), α-Fe₂O₃, γ-Fe₂O₃, Fe₃O₄ and FeCO₃ can be used, and also iron complexes, i.e. Fe(II)- and Fe(III) compounds with organic ligands. Examples of these iron complexes are iron oxalate, iron fumarate, iron citrate, iron lactate, iron gluconate and iron acetyl acetonate. The use of mixed valent iron compounds is also possible. However, inorganic iron oxides, in particular α-Fe₂O₃ and γ-Fe₂O₃ are particularly preferred.

In accordance with the function as chloride scavenger, the iron compounds are present in the compositions according to the invention at least in a quantity which is effective to neutralize the hydrochloric acid occurring from the conversion of the ammonium perchlorate largely as FeCl₃. When an iron oxide, in particular Fe₂O₃ is used as chloride scavenger, the weight ratio of ammonium perchlorate to iron oxide in the composition preferably lies in a range between about 3:1 to 10:1.

Compounds of the general total formula C_x/H_y/N_v/O_w wherein x≧1, y≧0 and v≧0 and w≧0 are suitable as organic fuels. The usability of organic compounds is almost universal. The only ones which are excluded are compounds which have characteristics which are not desired for the purpose of use in motor vehicles, such as, for example, those which are highly toxic, are carcinogenic, mutagenic or have other unacceptable characteristics with regard to the environment, such as, for example, a great danger to water or soil. Furthermore, compounds are not desired which have a chemical or thermal stability which is too low or have a low stability under storage in tests at 110 degrees C. over 400 hours. In addition, compounds are to be ruled out which do not permit a reliable processing during the production of the pyrotechnic propellants, such as sensitive, highly explosive substances or mixtures.

Fuels on the basis of polymeric substances, such as, for example, epoxy resins, which offer particular advantages in the production of extruded propellants, are also especially suitable. In addition, organic fuels which already have as high an oxygen content as possible per molecule are preferred, because hereby the required proportion of ammonium perchlorate and hence the proportion of hydrochloric acid which is to be neutralized can be further reduced.

As organic fuels, preferably nitrogenous compounds are used such as guanidine compounds and heterocyclic compounds of the group of triazoles, tetrazoles, triazines, imidazoles and azacycloalkanes and mixtures thereof. Examples of these compounds are 5-aminotetrazole, 1H-tetrazole, bistetrazole, azotetrazole, triazolone, nitrotriazolone, hexogen, octogen, guanidine carbonate, guanidine nitrate, guanidine perchlorate, aminoguanidine nitrate, diaminoguanidine nitrate, triaminoguanidine nitrate and nitroguanidine as well as salts, derivatives or mixtures thereof.

In addition, the organic fuel can be selected from the group consisting of nitrogenous heterocyclic organic acids and mixtures thereof. Examples of these nitrogenous heterocyclic organic acids are cyanuric acid, isocyanuric acid, cyamelide, urazole, uracile, uramine, urazine, alloxane, alloxanic acid, alloxantine, xanthine, allantoine, barbituric acid, orotic acid, dilituric acid, triazolone, violuric acid, succinimide, dialuric acid, isodialuric acid, hydantoine, pseudohydantoine, imidazolone, pyrazolone, parabanic acid, furazane, ammeline, creatinine, maleic acid hydrazide, uric acid, pseudouric acid, guanizine, guanazole and melamine, as well as salts, derivatives or mixtures thereof.

Further, an organic acid which is free of nitrogen can be used as organic fuel. In this respect, fumaric acid, maleic acid, malonic acid, tartaric acid, tartronic acid, citric acid, ascorbic acid, the salts or derivatives thereof, or mixtures of the organic acids which are free of nitrogen, are preferred.

Finally, a polymer compound can be used as organic fuel, which can be selected for example from the group consisting of the polyaklyl compounds, polalkylene compounds, polyamides, polyimides, polyesters, polyethers, polyurethanes, polyacetates, polyacrylic compounds and polyglycols and also the derivatives thereof containing —OH, CN, —COOH, —NH₂, —N₃, —ONO₂ or —NO₂— groups, and copolymers.

Fuels rich in energy having an oxygen balance of greater than −40%, in particular between −40% and 0% are especially preferred. In the following, the “oxygen balance” of a compound or of a composition is understood to mean the amount of oxygen in % by weight which is released upon complete conversion of the compound or of the composition to CO₂, H₂O, Al₂O₃, B₂O₃ etc. (oxygen excess). If the oxygen which is present is not sufficient for this, then the missing amount which is necessary for the complete conversion is indicated with a minus sign (oxygen deficiency).

Examples of such fuels which are rich in energy are guanidine nitrate, nitroguanidine, triaminoguanidine nitrate, urea nitrate, nitrourea, nitropenta, 3-nitro-1,2,4-triazole-5-one (NTO), hexogen, octogen, N,N′-dinitroammeline or mixtures thereof. The fuel guanidine nitrate and/or nitroguanidine is particularly preferred.

The fuels which are rich in energy having a slightly negative oxygen balance permit the further reduction of the proportion of ammonium perchlorate as oxidizer and hence also of the hydrochloric acid resulting therefrom upon the conversion of the composition according to the invention. In this case also less amounts of the iron compound have to be added as chloride scavenger.

Finally, the propellant can additionally contain conventional processing aids, such as e.g. lubricants, flowing aids, pressing aids and/or releasing agents. These conventional processing aids are used in a maximum proportion of 10% by weight in relation to the overall weight of the composition. Higher proportions lead to an undesirably low gas yield of the propellant. If the processing aids themselves are not converted at least partially into a gaseous reaction product, their maximum proportion preferably amounts to 5% by weight, particularly preferably a maximum proportion of 3% by weight.

Examples of such processing aids are polyethylene glycol, cellulose, methyl cellulose, soot, graphite, wax, calcium stearate, magnesium stearate, zinc stearate, boron nitride, talcum, bentonite, aluminium oxide, silicon dioxide or molybdenum sulphide. The use of these means is described in the prior art.

A particularly preferred composition according to the invention consists of 40 to 60% by weight guanidine nitrate, 35 to 50% by weight ammonium perchlorate, 5 to 15% by weight Fe₂O₃ and 0 to 0.5% by weight of a processing aid such as calcium stearate. Instead of the guanidine nitrate, nitroguanidine can be used as organic fuel.

The combustion temperatures of the compositions according to the invention preferably lie over 2500 K, particularly preferably in a range of between 2800 and 3200 K. Furthermore, the compositions according to the invention show high gas yields, in relation to mass, of over 95%, preferably at least 98%, because the FeCl₃ resulting from the conversion of the iron compound which is used as chloride scavenger is also vaporized and therefore also enters into the released gas volume. The gas yield related to mass is understood here to mean the mass of the generated gas in relation to the mass of the propellant which is used. Owing to the use of the compositions according to the invention, the gas generators can be operated with a smaller quantity of propellant than conventional gas generators. The high combustion temperature of the compositions likewise contributes to this, leading to a higher effective gas volume with an identical quantity of propellant.

The composition according to the invention therefore makes possible the provision of smaller and lighter gas generators, in which filter arrangements can be largely dispensed with, because no or only a few particles are present in the released hot gas mixture. Thereby, the further components of the gas bag module are also less highly stressed. A damage to the gas bag fabric caused by hot particles or pieces of slag is likewise ruled out.

A further object of the invention is the use of the compositions according to the invention in hybrid gas generators and belt tensioners. In hybrid gas generators, a rapid releasing of the stored compressed gas is desired, with a simultaneous heating up of the gas mixture. Belt tensioners require propellants with short reaction times and high gas volumes with a minimal quantity of propellant. Owing to the high combustion temperatures and the high gas yields, the compositions according to the invention are particularly suitable for these applications. Furthermore, the characteristics described above also permit a use of the compositions according to the invention as ignition means, in particular as amplifier charge or auto-ignition compositions, for igniting further propellants in a gas generator.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

The invention is described below by means of preferred embodiments which, however, are not to be understood in a restrictive sense.

EXAMPLE 1

47.25% by weight nitroguanidine, 42.5% by weight ammonium perchlorate, 10% by weight Fe₂O₃ and 0.25% by weight calcium stearate were ground, mixed with each other and pressed to form tablets with a density of 1.94 g/cm³. The propellant tablets which were obtained in this way were then filled into a conventional pressure bomb and ignited. The combustion temperature of the propellant in the bomb was 3192 K. At a chamber pressure of 300 bar, the combustion rates was 43.5 mm/s.

The gas yield of the composition related to mass was 99.6%. The proportion of toxic NO_x in the released gas mixture lay below the reference limit, the proportion of carbon monoxide was approximately 0.47%. There was also no longer evidence of hydrochloric acid in the released gas mixture.

EXAMPLE 2

51.5% by weight guanidine nitrate, 38.8% by weight ammonium perchlorate, 9.5% by weight Fe₂O₃ and 0.2% by weight calcium stearate were ground, mixed with each other and pressed to form propellant tablets with a density of 1.73 g/cm³. The tablets obtained in this way were filled into a conventional pressure bomb and ignited. The combustion temperature of the propellant which was determined during this test was 2877 K. The combustion rate was 29.3 mm/s at a chamber pressure of 300 bar.

The gas yield of the composition related to mass was 99.3%. In the released gas mixture, there was evidence of neither hydrochloric acid nor NO_x as toxic components. The proportion of carbon monoxide in the gas mixture was 0.34%.

The test results show that the compositions according to the invention are predestined for use in hybrid gas generators and belt tensioners because of their high gas yield and also the high combustion temperatures and combustion rates. In addition, they are chemically and thermally stable and are therefore well suited for civil applications, in particular for safety devices in vehicles.

Claims

1. A gas-generating composition adapted for use in a safety device for vehicles, consisting essentially of 20 to 65% by weight of an organic fuel, 30 to 60% by weight ammonium perchlorate, 5 to 30% by weight of a chloride scavenger and conventional processing aids in a proportion of 0 to 10% by weight, each with respect to the total composition, characterized in that the chloride scavenger is an iron compound.

2. The composition according to claim 1, characterized in that the iron compound is selected from the group consisting of iron oxides, FeCO3, Fe(II)- and Fe(III)-compounds with organic ligands and mixed valent iron compounds and also mixtures thereof.

3. The composition according to claim 2, characterized in that the iron oxides are selected from the group consisting of FeO, FeO(OH), Fe2O3 and Fe3O4.

4. The composition according to claim 1, characterized in that the fuel is selected from the group consisting of guanidine compounds, heterocyclic compounds and mixtures thereof.

5. The composition according to claim 4, characterized in that the guanidine compounds are selected from the group consisting of guanidine nitrate, nitroguanidine, guanidine carbonate, guanidine perchlorate, aminoguanidine nitrate, diaminoguanidine nitrate, triaminoguanidine nitrate, and mixtures thereof.

6. The composition according to claim 4, characterized in that the heterocyclic compounds are selected from the group of triazoles, tetrazoles, triazines, imidazoles and hexogen and mixtures thereof.

7. The composition according to claim 1, characterized in that the fuel is selected from the group of organic acids which are free of nitrogen.

8. The composition according to claim 1, characterized in that the fuel is selected from the group of polymer compounds.

9. The composition according to claim 1, characterized in that the chloride scavenger is an iron oxide and that the weight ratio of ammonium perchlorate and iron oxide in the composition is from 3:1 to 10:1.

10. The composition according to claim 1, characterized in that the combustion temperature of the composition is at least 2500 K.

11. The composition according to claim 1, characterized in that the composition has a gas yield of at least 95% based on the mass of the composition.

12. The composition according to claim 1, characterized in that the processing aids are selected from the group of pressing aids, releasing agents, lubricants and flowing aids.

13. The composition according to claim 1, characterized in that the composition consists of 40 to 60% by weight of guanidine nitrate, 35 to 50% by weight of ammonium perchlorate, 5 to 15% by weight of Fe2O3 and 0 to 0.5% by weight of calcium stearate.

14. The composition according to claim 1, characterized in that the composition consists of 40 to 60% by weight of nitroguanidine, 35 to 50% by weight of ammonium perchlorate, 5 to 15% by weight of Fe2O3 and 0 to 0.5% by weight of calcium stearate.

15. The gas-generating composition according to claim 1, wherein the safety device comprises at least one of a hybrid gas generator and a belt tensioner.

16. The gas-generating composition according to claim 1, wherein the safety device includes a gas generator, and wherein the gas-generating composition is adapted for use as an auto-ignition composition or amplifier charge for igniting a further propellant contained in said gas generator.