EP0404651B1 - Solid gas-generating composition and its application in gas generators for inflatable safety bags in motor vehicles - Google Patents

Description

The present invention relates to the field of protection in the event of an impact on the passengers of a motor vehicle by an airbag by means of a pyrotechnic generator of non-toxic cold gases. More specifically, the invention relates to a new solid gas-generating composition based on molybdenum sulfide and alkali or alkaline earth azide and its use in such a gas generator.

It is known to provide impact protection for passengers of a motor vehicle by means of cushions which inflate at the time of impact under the effect of gas coming from a pyrotechnic gas generator initiated by a shock detector. The generation of the gases takes place by combustion in the said generator of a solid composition generating the gases which are cooled and filtered before entering the said cushion.

• a) elle doit avoir une vitesse de combustion très élevée de manière à assurer le gonflement du coussin dans des durées de quelques dizaines de millisecondes, sans pour cela être explosive,
• b) elle doit générer des gaz dits "froids" de manière à ce que, après passage rapide dans le dispositif de refroidissement du générateur, ces gaz refroidis entrent dans le coussin à une température supportable par l'organisme du passager. On admet en général que ces gaz "froids" doivent avoir une température de combustion qui ne doit pas être supérieure à 500°C-600°C,
• c) elle doit générer des gaz non toxiques pour les passagers,
• d) enfin pour éviter tout risque inutile d'incendie, elle doit générer des gaz inertes.

To ensure satisfactory operation of the gas generator, the solid composition, which is generally in the form of pellets or small blocks, must satisfy several imperatives:

• a) it must have a very high combustion speed so as to ensure the inflation of the cushion over periods of a few tens of milliseconds, without being explosive,
• b) it must generate so-called "cold" gases so that, after rapid passage through the cooling device of the generator, these cooled gases enter the cushion at a temperature that can be tolerated by the body of the passenger. It is generally accepted that these "cold" gases must have a combustion temperature which must not be higher than 500 ° C-600 ° C,
• c) it must generate gases which are non-toxic to passengers,
• d) finally to avoid any unnecessary risk of fire, it must generate inert gases.

For these last two reasons, nitrogen-generating compositions are particularly sought after. Among the nitrogen-generating compositions, the solid compositions based on alkali or alkaline earth azide and on molybdenum sulfide, optionally in the presence of sulfur, have been found to meet the first two requirements, namely the high combustion rate, particularly well. and a moderate combustion temperature. Such compositions are for example described in patents US-A-3,741,585, US-A-4,203,787 and US-A-4,547,235. These compositions are entirely satisfactory in terms of the generation of non-toxic cold gases for automobile safety but have certain disadvantages of implementation and storage linked to the presence of metal azides which are explosive bodies and sensitive to humidity, the hydrolysis products of which are themselves dangerous explosives.

Similar compositions have then been proposed based on molybdenum sulphide and an alkali azide or similar compounds in which the reactive ingredients are coated with an oxygen-rich polyurethane binder and obtained, by hot molding, from solid compounds. . This solution, which is for example described in patent US-A-3,779,823, however, has the disadvantage of being difficult to apply, given the pyrotechnic nature of the ingredients to be ground and hot molded.

These implementation disadvantages, which are set out, for example, in US Pat. No. 4,370,181, have prompted those skilled in the art to seek compositions based on molybdenum sulfide but without azide. The aforementioned US Pat. No. 4,370,181 thus proposes compositions which generate non-toxic cold gases based on molybdenum sulfide and a non-hydrogenated tetrazole compound. However, such compositions have the drawback of using a product which is not very available on the market, the non-hydrogenated tetrazole compounds such as bitetrazole are not in fact products of wide industrial distribution, and moreover these compositions are not easy. to pellet.

It is also known that the azide functional group can be introduced into a composition by certain polymers carrying this group such as, for example, glycidyl polyazide.

US Pat. No. 4,601,344 describes, for example, gas-generating compositions for fire extinguishers which are formed on the one hand by tetrazole compounds and on the other hand by a binder based on poly (glycidyl azide). However, such compositions have the drawback of generating toxic gases at contents incompatible with the requirements of motor vehicle safety.

In the field of the protection of the passengers of a motor vehicle by an airbag, the skilled person is always looking for a gas-generating composition which has all the advantages of compositions based on molybdenum sulfide and d alkali or alkaline earth azides without having their disadvantages of implementation and storage.

The object of the present invention is precisely to propose such a composition. The object of the invention relates to a solid composition generating non-toxic cold gases comprising molybdenum sulfide, an alkali or alkaline earth azide, characterized in that it contains between 3% and 6% by weight relative to the weight total of said composition of a binder consisting of the reaction product of a glycidyl polyazide with hydroxyl endings with at least one polyisocyanate chosen from the group consisting of diisocyanates and triisocyanates.

According to a first preferred embodiment of the invention, the said azide is sodium azide.

According to a second preferred embodiment of the invention, part of the molybdenum sulphide is replaced by sulfur.

The invention also relates to the use of such a composition in a gas generator for an airbag intended for motor vehicles.

It was indeed found, surprisingly, by the plaintiff that when replacing in a traditional composition based on molybdenum sulfide and alkali or alkaline earth azide, a small proportion -between 3% and 6% by weight relative to the total weight of the composition- of metal azides by a polyurethane binder based on glycidyl polyazide, a composition is obtained which generates cold, non-toxic gases at high combustion speed which no longer has the disadvantages of processing and storing said traditional compositions.

The compositions according to the invention are thus particularly suitable for pyrotechnic gas generators intended to inflate the airbags of motor vehicles.

A detailed description of the invention is given below.

The invention relates to compositions based on molybdenum sulfide and alkali or alkaline earth azide such as for example sodium azide, lithium azide, strontium azide, calcium azide.

According to a first preferred embodiment of the invention, sodium azide will be used.

Typically, the compositions according to the invention contain between 3% and 6% by weight relative to the total weight of said composition of a binder consisting of the reaction product of a glycidyl polyazide with OH hydroxyl endings with at least a polyisocyanate chosen from the group consisting of diisocyanates and triisocyanates.

dans laquelle C, H, O, N représentent respectivement le carbone, l'hydrogène, l'oxygène et l'azote et où n représente un nombre entier compris entre 5 et 100.The glycidyl polyazides with OH hydroxyl endings which can be used in the context of the present invention are polyethers of general formula:

in which C, H, O, N represent carbon, hydrogen, oxygen and nitrogen respectively and where n represents an integer between 5 and 100.

These polymers are generally obtained by reaction of sodium azide on a polyepichlorohydrin as described for example in US Patents 4,268,450, 4,379,894 or 4,486,351. In general, glycidyl polyazides with molecular weights between 600 and 7000 and we prefer those whose molecular mass is between 1500 and 2500.

As polyisocyanate, it is possible to use the usual aliphatic, cycloaliphatic or aromatic diisocyanates and triisocyanates such as, for example, methyl-1 cyclohexane-2,4 diisocyanate, methyl-1 cyclohexane-2,6 diisocyanate, dicyclohexylmethane-4,4'diisocyanate, l isophorone diisocyanate, methylene diisocyanate, hexane-1,6 diisocyanate, trimethyl-2,2,4 hexane-1,6 diisocyanate, toluene diisocyanate, biuret trimer of hexamethylene diisocyanate, polymethylene polyphenyl isocyanate.

et les polyméthylène polyphénylisocyanates de formule :

dans laquelle x représente un nombre entier ou nul et qui ont une fonctionnalité moyenne en groupe NCO de 2,7.However, the trimer biuret of hexamethylene diisocyanate of formula: polyisocyanate is preferred:

and polymethylene polyphenylisocyanates of formula:

in which x represents an integer or zero and which have an average functionality in group NCO of 2.7.

In the formulation of the binder according to the invention, the NCO: OH ratio is close to 1: 1.

As was said above, a characteristic of the invention lies in the fact that the weight content of the binder composition is within a very precise range, between 3% and 6% relative to the total weight of the composition , so that the composition retains the advantages of traditional compositions based on molybdenum sulfide and sodium azide in terms of the non-toxicity of the gases while being easier to use and store thanks to the fact that the azide grains are, at least partially, coated with a binder. According to a preferred embodiment of the invention, the weight content of the binder composition is between 4% and 5% relative to the total weight of the composition.

Under these conditions the weight content of the azide composition is advantageously between 55% and 71% relative to the total weight of the composition and the weight content of molybdenum sulfide is advantageously between 25% and 40% relative to the total weight of composition.

According to a last preferred variant of the invention, part of the molybdenum sulphide can be replaced by sulfur. In this case the weight content of the sulfur composition must be less than 8% relative to the total weight of the composition, the content overall of the sulfur and molybdenum sulfide composition being between 25% and 40% relative to the total weight of the composition.

The formulation of the compositions according to the invention is advantageously carried out in the following manner.

In a powder mixer, the solid constituents, namely the azide, the molybdenum sulphide and possibly the sulfur, are mixed. In a liquid mixer, a solution of glycidyl polyazide and isocyanate in a non-reactive volatile anhydrous solvent such as methylene chloride is prepared. Advantageously, a 50:50 volume solution is produced between the solvent on the one hand and the constituents of the binder on the other hand. The mixture of solid constituents and said solution is then introduced into a kneader, catalysts intended to promote the reaction of the isocyanate on the glycidyl polyazide, such as dibutyltin dilaurate or acetylacetonate, are optionally added and kneaded under vacuum to remove the solvent. A powder is then obtained which can be shaped by tableting or compression. The pellets or blocks thus obtained are then baked in an oven heated to a temperature between 60 ° C and 100 ° C to complete the crosslinking of the binder.

The pellets or blocks thus obtained burn at a moderate temperature with high combustion rates, releasing a large volume of non-toxic gases within the meaning of the standards imposed by the manufacturers of motor vehicles, these gases being essentially constituted by nitrogen. Furthermore, it has been found that the pellets or blocks of compositions according to the invention are much less sensitive to external aggressions, in particular moisture, than the pellets or blocks of traditional compositions based on alkali or alkaline earth azide and molybdenum sulfide.

The compositions according to the invention can thus advantageously be used to constitute the pyrotechnic charge of non-toxic cold gas generators intended to inflate the airbags of motor vehicles.

Example

• . sulfure de molybdène   : 35 % en poids
• . azoture de sodium   : 60 % en poids
• . liant   : 5 % en poids

We made pellets with the following composition:

• . molybdenum sulfide: 35% by weight
• . sodium azide: 60% by weight
• . binder: 5% by weight

The binder was obtained by reaction of a glycidyl polyazide with OH hydroxyl terminations of average molecular weight 2000 on the trimer biuret of hexamethylene diisocyanate. Dibutyltin dilaurate was used as the catalyst.

The pellets obtained burn with a speed of 7 mm / s at 7 MPa in a pressure bomb. The combustion temperature is 867 ° K or 594 ° C.

Analysis of unfiltered combustion gases gives a percentage of nitrogen greater than 99%, a rate of carbon monoxide CO less than 120 ppm and a rate of nitrogen oxides (NO + NO₂) less than 0.5 ppm.

Claims (12)

1. Solid composition generating nontoxic gases, comprising molybdenum sulphide and an alkali or alkaline-earth metal azide, characterized in that it contains between 3 % and 6 % by weight relative to the total weight of the said composition of a binder consisting of the product of reaction of a polyglycidyl azide containing hydroxyl ends with at least one polyisocyanate chosen from the group consisting of diisocyanates and triisocyanates.
2. Composition according to Claim 1, characterized in that the said azide is chosen from the group consisting of sodium azide, lithium azide, strontium azide and calcium azide.
3. Composition according to Claim 2, characterized in that the said azide is sodium azide.
4. Composition according to any one of Claims 1 to 3, characterized in that the weight content of azide is between 55 % and 71 % relative to the total weight of the said composition.
5. Composition according to any one of Claims 1 to 4, characterized in that the said polyglycidyl azide containing hydroxyl ends has an average molecular mass of between 1000 and 7000.
6. Composition according to Claim 5, characterized in that the said polyglycidyl azide containing hydroxyl ends has an average molecular mass of between 1500 and 2500.
7. Composition according to any one of Claims 1 to 6, characterized in that the said polyisocyanate is chosen from the group consisting of the hexamethylene diisocyanate biuret trimer and polymethylenepolyphenyl isocyanate.
8. Composition according to any one of Claims 1 to 7, characterized in that the weight content of binder is between 4 % and 5 % relative to the total weight of the said composition.
9. Composition according to any one of Claims 1 to 8, characterized in that the weight content of molybdenum sulphide is between 25 % and 40 % relative to the total weight of the said composition.
10. Composition according to any one of Claims 1 to 9, characterized in that part of the molybdenum sulphide is replaced with sulphur.
11. Composition according to Claim 10, characterized in that the weight content of sulphur is lower than 8 % relative to the total weight of the said composition.
12. Use of a composition according to any one of Claims 1 to 11 in a gas generator for an inflatable safety cushion intended for motor vehicles.