WO1997042142A1

WO1997042142A1 - Gas-generating, azide-free mixture

Info

Publication number: WO1997042142A1
Application number: PCT/EP1997/002222
Authority: WO
Inventors: Achim Hofmann; Roland Schropp; Karl-Heinz Rödig; Siegfried Zeuner
Original assignee: Trw Airbag Systems Gmbh & Co. Kg
Priority date: 1996-05-02
Filing date: 1997-04-30
Publication date: 1997-11-13
Also published as: DE19617538C1

Abstract

A mixture of substances for the pyrotechnic generation of propellants for motor vehicle passenger protection devices consists of: (a) 10 to 50 wt % of a fuel selected from the acid dihydrazide group and (b) 50 to 90 wt % of an inorganic oxidiser selected from the group of nitrates and peroxides or their mixtures, in relation to the total weight of the mixture of fuel and oxidiser, and (c) possibly up to 30 wt % in relation to the total weight of the mixture, of a metal oxide and (d) possibly further additives or auxiliaries. The mixture of the invention is azide-free, consists of non-toxic components, is highly stable thermally and chemically, has an adequate rate of combustion and good ignitibility, where the combustion temperatures are markedly lower than those of prior art gas-generating mixtures, the quantity of gaseously generated fine dust particles is smaller, the combustion residues are easily filtrable, the propellant gas composition is essentially benign, the concentrations of noxious gases like CO and NOx in the propellant are as small as possible and the raw materials are particularly cheap.

Description

Gas-generating, acid-free substance chemical

The invention relates to a mixture of substances for the pyrotechnic generation of gases, in particular propellant gases for occupant protection devices in motor vehicles, such as e.g. for driver, front passenger and / or side impact airbag systems.

Such a mixture of substances is known for example from DE 44 23 088 AI.

Passive safety devices for motor vehicles, such as airbag systems, serve to protect the vehicle occupants from injuries in the event of a collision of the vehicle. For this purpose, a gas generator in a combustion chamber contains a gas-generating substance mixture in the form of tablets, pellets or granules which, after electrical activation, generates a propellant gas which in turn inflates a gas bag, thereby avoiding that the vehicle occupant, for example, hits the Windshield, the steering wheel or the dashboard.

Various mixtures of substances are known as gas-generating mixtures of substances. For example, sodium azide is used as the main gas-supplying component, potassium nitrate as the oxidizer and silicon dioxide, the silicon dioxide binding the substances sodium and potassium formed in the reaction of the azide with the nitrate as slag. A major disadvantage of the use of fuels containing sodium azide is their high toxicity, which requires special measures, inter alia, in the manufacture, transport and disposal. Another disadvantage is the strongly alkaline reaction to combustion residues, which can cause a risk of chemical burns. Furthermore, gas-generating compositions are known which consist of an alkali metal azide and a metal oxide, usually iron oxide (cf. DE 24 59 667 AI). However, these known mixtures of substances also have a low burning rate and a poor willingness to ignite.

Furthermore, DE 43 17 727 AI discloses the use of a mixture of nitrocellulose and nitroglycerin. Such fuel mixtures based on nitrocellulose have poor temperature stability, with the consequence of a limited lifespan and the impossibility of recycling this substance. In addition, these fuel mixtures contain heavy metal salts as combustion controllers, which additionally complicates disposal. The greatest disadvantage, on the other hand, is the enormous amounts of carbon monoxide that arise during combustion.

Sodium-azide-free fuel mixtures have been proposed in recent years (see, for example, US Pat. No. 4,948,439) which contain nitrogen-rich organic compounds such as tetrazoles or tetrazole derivatives or tetrazolates as the main component. However, such nitrogenous, organic fuel mixtures have the disadvantage that, in addition to carbon monoxide, nitrous gases NOχ are also released in considerable quantities during combustion, so that the risk of combined poisoning cannot be ruled out.

The document US 3,880,595 discloses mixtures of substances which are based on a nitrogen-free organic compound, such as citric acid. The disadvantage of these fuels lies in their low thermal stability and their high hygroscopicity, as well as in their poor processability, in particular the compression into tablets or pellets which can only be carried out with great difficulty. No. 3,839,105 describes a curable propellant based on combustible plastics and mainly ammonium perchlorate (similar to rocket propellants), to which oxalyl dihydrazide is added as a coolant. The use of the substance combination mentioned is mandatory. Combinations with low combustion temperatures can also be produced, but these are generally heavily underbalanced by oxygen, which leads to the formation of high amounts of toxic carbon monoxide on combustion.

The use of ammonium perchlorate also generates excessive amounts of toxic and corrosive hydrogen chloride. Both are highly undesirable and are not permitted for use in vehicle occupant protection systems (airbags).

A disadvantage of the gas-generating mixtures described in US Pat. No. 5,035,757 is the general use of 5-aminotetrazole (5-ATZ) as fuel. 5-ATZ is usually available with one mole of water of crystallization (CH3N5. H2O), which is released relatively easily depending on the ambient temperature. For use in airbag gas generators, this results in an uncertainty point, so that 5-ATZ must always be used in dried form (without water of crystallization). In this form, however, the material is extremely hygroscopic and requires a great deal of additional effort in production, processing and assembly to ensure the 15-year guarantee of life that is usually required today.

In the case of the mixtures disclosed in DE 42 09 878 A1, the use of inorganic peroxides is again disadvantageous. These are known to be only moderately stable thermally and chemically. It is doubtful whether these mixtures can meet the high requirements placed on airbag gas generators (for example temperature storage over 400 h at 105 ° C.). DE-Al-195 16 818 relates to a gas developer composition which contains, as main components, hydrazodicarbonamide as a fuel and an oxygen acid salt as an oxidizing agent. The hydrazodicarbonamide content is 10 to 45% by weight and the oxygen acid salt content is 90 to 55% by weight, based in each case on the total weight of these main components. In particular, the alkali metal salts of halogeno-oxygen acids or perhalo-acids and especially potassium perchlorate are proposed as the oxygen-acid salt. However, nitrates such as potassium nitrate, sodium nitrate, strontium nitrate, barium nitrate, calcium nitrate and lead nitrate are also mentioned. Because of the restriction to hydrazodicarbonamide as fuel, however, the proposed gas developer composition has only limited applicability.

DE-Al-44 35 790 relates to the use of a gas generator fuel which comprises at least one carbonate, hydrogen carbonate or nitrate of guanidine, aminoguanidine, diaminoguanidine or triaminoguanidine. However, the fuel shows insufficient storage stability.

Finally, because of the use of nitrogen-free components, the mixtures described in DE 44 23 088 A1 cited at the beginning have the advantage that no harmful nitrogen oxides (NO 2) can be formed during combustion, but the relatively high combustion rates are disadvantageous temperatures (generally> 2100 K) and the generation of large amounts of non-filterable fine dust particles such as KC1 (Kp. 1686 K, from the decomposition of KCIO4), which arise in gaseous form under the reaction conditions and for the most part only after flowing out of the gas generator condense. In addition, the molar gas yields are relatively low (approx. 16 mol / kg).

In contrast, the invention is based on the object of providing a gas-generating, azide-free substance mixture which consists of non-toxic components, a high thermal and has chemical stability, is easy to process and is not hygroscopic, has a sufficient burn-up rate and is easy to ignite, the combustion temperatures being markedly reduced compared to known gas-generating mixtures, the proportion of fine dust particles produced in gaseous form is lower, and a combustible combustion residue is produced , the propellant gas composition is essentially harmless, the concentrations of harmful gases such as CO, NOχ in the propellant gas are as low as possible and the starting materials are as inexpensive as possible.

This complex problem is solved in a simple and surprisingly effective manner with a mixture of substances of the type mentioned at the outset in that the mixture of substances consists of

(a) 10 to 50% by weight of a fuel selected from the group of acid dihydrazides and

(b) 50 to 90% by weight of an inorganic oxidizer selected from the group of nitrates and peroxides or mixtures thereof, in each case based on the total weight of the mixture of fuel and oxidizer, and

(c) optionally up to 30% by weight, based on the total weight of the mixture, of a metal oxide and

(d) optionally further additives or auxiliaries

consists.

The main components of the mixture of substances according to the invention are non-toxic and inexpensive as well as recyclable and, moreover, very easy to process. Furthermore, a high burning rate is achieved with the mixture of substances according to the invention, but the combustion residues are non-toxic. In addition, the propellant gas itself has only a minimal harmful gas content. Finally, the mixture of substances according to the invention is free of heavy metals and can be produced at low production costs.

The mixture of substances according to the invention also has high thermal stability and great long-term stability. The starting materials for producing the mixture of substances according to the invention are non-toxic and in particular free of azides and heavy metals. The combustion temperatures of the mixture according to the invention are typically relatively low (below 2000 K) and do not lead to the generation of toxic propellant gases. Finally, the proportion of fine dust particles produced in gaseous form is extremely low when the substance mixture according to the invention is used to produce propellant gases.

An advantageous development of the mixture of substances according to the invention can additionally contain a metal oxide in an amount of up to 30% by weight, preferably between 5 and 20% by weight, based on the total weight of the mixture of substances. The metal oxide serves on the one hand as a coolant and on the other hand as a ballistic additive.

The metal oxide can be selected from the group consisting of Al2O3, B2O, SiO2, TiO2, MnO2, CuO, Fe2O3 and ZnO, or a mixture of the metal oxides mentioned can be used.

An embodiment of the substance mixture according to the invention is preferred, in which a substance from the group of the dihydrazides of carbonic acid, oxalic acid, organic dicarboxylic acids or a mixture of these is used as fuel.

Oxalic acid dihydrazide is particularly preferably used as fuel. A substance from the group of strontium nitrate, potassium nitrate, sodium nitrate or mixtures thereof is preferably used as the inorganic oxidizer.

Particularly preferably, only strontium nitrate or a mixture of strontium nitrate and potassium nitrate or of strontium nitrate and sodium nitrate is used as the inorganic oxidizer.

The inorganic oxidizer can also be selected from the group of alkali metal peroxides, alkaline earth metal peroxides and transition metal peroxides and mixtures thereof. Zinc peroxide in particular can be used as transition metal peroxide.

Additional auxiliaries and additives which may be present in the mixture of substances according to the invention can be, for example, pressing aids, preferably methyl cellulose ethers, or agents for improving the flowability, preferably silicates. These further auxiliaries and additives are particularly preferably present in a proportion of up to 5% by weight, based on the total weight of the mixture.

Further advantages of the invention result from the description and the drawings. Likewise, the features mentioned above and those listed further can be used according to the invention individually or individually or in any combination. The embodiments shown and described are not to be understood as an exhaustive list, but rather have an exemplary character for the description of the invention.

The invention is illustrated in the drawings and is explained in more detail using exemplary embodiments. It shows: 1 shows a diagram of a thermal long-term stability test of a substance mixture according to the invention;

2 shows a diagram of a hygroscopicity test of the substance mixture according to the invention from FIG. 1 at different relative atmospheric humidities; and

Fig. 3 is a comparative table with properties of a mixture of substances according to the invention according to embodiment 3 in comparison to the properties of a fuel mixture known from DE 44 23 088 AI.

According to a first exemplary embodiment, the substance mixture according to the invention can consist of 30% by weight oxalic acid dihydrazide as fuel and 70% by weight strontium nitrate as inorganic oxidizer.

In a second embodiment, the mixture of substances according to the invention can contain 32% by weight oxalic acid dihydrazide as fuel, 63% by weight strontium nitrate as an inorganic oxidizer and additionally 5% by weight iron (III) oxide as a cooling reagent, which is the combustion temperature reduced, have.

In a third exemplary embodiment, the substance mixture according to the invention contains 30% by weight oxalic acid dihydrazide as fuel and a mixture of 65% by weight strontium nitrate and 5% by weight sodium nitrate as inorganic oxidizer.

In general, the mixture of substances according to the invention can contain 10 to 50% by weight of a fuel from the group of the acid dihydrazides and correspondingly 90 to 50% by weight of an inorganic oxidizer from the group of the nitrates, peroxides and mixtures thereof. In addition, up to 30% by weight of a metal oxide and preferably up to 5% by weight of further auxiliaries and additives known in the art can be present in the mixture of substances according to the invention. One characteristic of the mixtures according to the invention is the composition, which is either balanced or oxygen overbalanced, so that harmful gas fractions such as carbon monoxide only occur in very small amounts in the propellant gas generated. The same applies to the production of nitrogen oxides, since the substance mixtures according to the invention have relatively low combustion temperatures (generally below 2000 K), so that the production of N0χ remains minimal and the corresponding harmful gases are formed only in amounts which are harmless to the vehicle occupants become. Hydrochloric acid and other harmful gases do not arise at all when the substance mixtures according to the invention are burned off.

Furthermore, when the mixture of substances according to the invention is burned, there is no emission of non-filterable, gaseous fine dust particles, but the combustion residues occur in condensed form (solid or liquid) and can therefore be retained by mechanical filtration.

Finally, a common property of the substance mixtures according to the invention described above results in a high molar gas yield, a low hygroscopicity and a high thermal and chemical long-term stability.

1 shows a stability test (so-called Holland test) of a substance mixture according to the third exemplary embodiment described above at a temperature of 110 ° C. over a period of more than 400 h. To carry out the test, the mixture of substances was first pre-dried for 8 hours, so that the volatile constituents, such as those caused by surface moisture of the pellets in the mixture of substances, had already been removed at the start of the long-term measurement. As can be seen from the measurement curve, the decrease in weight of the substance mixture according to the invention after 72 h was less than 0.05%. Only after very long periods of time over 400 h in the test chamber heated at 110 ° C. did the weight decrease by approximately 0.3%. In comparison, the Weight loss from known mixtures of substances after only 72 hours under comparable conditions already at approx. 1%. The mixture of substances according to the invention according to the third embodiment can therefore be described as particularly long-term stable.

2 shows the results of a hygroscopicity test with the substance mixture according to embodiment 3. The curve with the reference symbol 1 was recorded at a relative air humidity of 45%, the curve with the reference symbol 2 at a relative air humidity of 55%. After a measurement period of more than 140 h, there was only a weight gain of about 0.05%.

3 shows, in tabular form, a comparison of properties of the substance mixture according to the invention according to embodiment 3 with those of a substance mixture according to DE 44 23 088 AI cited above.

It shows that the gas yield of the substance structure according to the invention is considerably higher and the combustion temperatures are considerably lower. The proportion of evaporated solid particles (essentially KCl and K2CI2) is approximately 8.7% in the known mixture of substances, while no evaporated solid particles occur in the mixture of substances according to the invention.

Claims

claims

1. Mixture of substances for the pyrotechnic generation of gases, in particular propellant gases for occupant protection devices in motor vehicles, consisting of

(d) optionally further additives or auxiliaries.

2. A mixture of substances according to claim 1, characterized in that the mixture of substances contains up to 30% by weight of a metal oxide.

3. Mixture of substances according to claim 2, characterized in that the metal oxide is selected from the group consisting of Al2O3, B2O3, SiO2 _r TiO2, MnO2, CuO, Fβ2O3, ZnO or mixtures thereof.

4. Mixture of substances according to one of claims 2 or 3, characterized in that the metal oxide is contained in a proportion of between 5 and 20 wt .-%.

5. Mixture of substances according to one of the preceding claims, characterized in that the fuel is selected from the group consisting of the dihydrazides of carbonic acid, oxalic acid, organic dicarboxylic acids or mixtures thereof.

6. Mixture of substances according to one of claims 1 to 5, characterized in that the fuel is oxalic acid dihydrazide.

7. Mixture of substances according to one of the preceding claims, characterized in that the inorganic oxidizer is selected from the group consisting of strontium nitrate, potassium nitrate, sodium nitrate or mixtures thereof.

8. A mixture of substances according to claim 7, characterized in that the inorganic oxidizer is strontium nitrate.

9. A mixture of substances according to claim 7, characterized in that the inorganic oxidizer is a mixture of strontium nitrate with potassium nitrate or a mixture of strontium nitrate with sodium nitrate.

10. A mixture of substances according to one of claims 1 to 6, characterized in that the inorganic oxidizer is selected from the group consisting of the alkali metal peroxides, alkaline earth metal peroxides and transition metal peroxides.