EP0394103B1 - Process for the production of solid non-toxic gas-generating charges and charges obtained thereby - Google Patents

Description

The present invention relates to pyrotechnic generators of non-toxic cold gases. More precisely, the invention relates to a new process for manufacturing pyrotechnic charges producing non-toxic cold gases as well as the charges obtained which can be used as a charge in such a gas generator, in particular in a gas generator intended to deploy an airbag provided in place in motor vehicles.

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 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 generator cooling device, these gases enter the cushion at a temperature that can be tolerated by the passenger's body. 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 metallic sulfides such as molybdenum sulfide, possibly in the presence of sulfur, have been found to satisfy the first two requirements particularly well, namely the high burning speed and moderate burning temperature. Such compositions are for example described in US Patents 3,741,585, 4,203,787, 4,547,235 and 4,734,141. These compositions are entirely satisfactory in terms of the generation of non-toxic cold gases, but nevertheless have the drawback when they are put in the form of compact loads not to have good mechanical properties. However, the loads thus produced are called upon to be able to be kept for many years in a motor vehicle subjected to numerous mechanical stresses without being altered.

The present invention aims precisely to improve the mechanical properties of the loads produced from the compositions mentioned above so as to guarantee good mechanical preservation of these loads over time once they are subjected to mechanical stresses such as vibrations, repeated accelerations, especially when they are willing to the interior of a motor vehicle.

The invention therefore relates to a method of manufacturing fillers for gas generators, in particular intended for motor vehicle safety, from solid compositions generating non-toxic gases, the constituents of which comprise at least (i) an alkali or alkaline earth azide, (ii ) a metal sulphide chosen from the group consisting of molybdenum sulphide or mixed molybdenum sulphides, and (iii) sulfur, by mixing the said constituents and shaping by compression, characterized in that, after shaping, the said charges are brought to a temperature at least equal to 120 ° C. for a duration at least equal to 5 minutes.

According to a first preferred variant of implementation of the invention, said charges are brought to a temperature between 120 ° C and 130 ° C for a period of between 5 minutes and 20 minutes.

According to a second preferred variant of implementation of the invention, said charges are brought to a temperature between 120 ° C and 125 ° C for a time between 10 minutes and 15 minutes.

According to another preferred variant of implementation of the invention, the sulfur content by weight in the said compositions is between 2% and 9%.

The invention also relates, as new products, to the fillers of solid composition generating non-toxic gases obtained by the process according to the invention.

A detailed description is given below of the invention and of its possibilities of implementation.

The invention therefore essentially relates to a method of manufacture of loads for gas generators, in particular intended for automobile safety. There are traditionally two types of loadings for cold gas generators intended to allow the inflation of airbags for motor vehicles.

The first type includes the charges, generally in the form of a cylindrical or annular block, placed inside the combustion chamber of the gas generator. In this case the loading can be of a single piece or in several elementary pieces whose juxtaposition constitutes the loading.

The second type includes the charges constituted by a stack of pellets inside the combustion chamber of the gas generator.

For the purposes of the present invention, the expression "charges for gas generators" is understood to mean the constituent elements of the generator charge, whether pellets, elementary parts making it possible to constitute the cylindrical or annular block or the block itself if it is in one piece.

These charges are obtained by dry or wet mixing of the constituents of a solid composition generating non-toxic gases and by shaping the mixture thus formed. This shaping is generally carried out by compression. It can be a compression at the press in the case where the load constitutes a block or an elementary piece of block, or a compression in pelletizer when the load has the form of a pellet.

The solid compositions generating non-toxic cold gases which can be used in the context of the present are compositions whose constituents comprise at least: (i) an alkali or alkaline earth metal azide, (ii) a metal sulfide chosen from the group consisting of by molybdenum sulfide and sulfides mixed molybdenum, and (iii) sulfur.

According to a preferred embodiment of the invention, the sulfur content by weight in the said composition is between 2% and 9% of the total weight of the said composition.

As azide, sodium azide will be preferred in the context of the present invention.

Mo

représente un atome de molybdène,

M

représente un atome de fer ou de cuivre,

S

représente un atome de soufre,

x

représente un indice de valeur comprise entre 0,7 et 3,0,

y

représente un indice de valeur comprise entre 1,0 et 5,0,

z

représente un indice de valeur comprise entre 4,0 et 12,0.

As mixed molybdenum sulfides, it is advantageous to use mixed molybdenum and copper or iron sulfides and in particular those corresponding to the formula Mo _x M _y S _z in which:

Mo

represents a molybdenum atom,

M

represents an iron or copper atom,

S

represents a sulfur atom,

x

represents a value index between 0.7 and 3.0,

y

represents a value index between 1.0 and 5.0,

z

represents a value index between 4.0 and 12.0.

In addition to the constituents indicated above, the presence of which is compulsory, the compositions according to the invention may contain other additives and in particular mineral nitrates such as potassium nitrate or oxides such as iron, cobalt, nickel, palladium or silicon. These additives then form part of the composition and are to be taken into account in the total weight of the composition when reference is made to the weight contents indicated in the present application.

However, the compositions preferred in the context of the present invention are the compositions constituted by an alkali or alkaline earth metal azide, by molybdenum sulfide and by sulfur and particularly those based on sodium azide, molybdenum and sulfur sulfide. Among these latter compositions, preference will be given to those in which the content by weight of sodium azide is between 60% and 82%, the content by weight of molybdenum sulfide between 15% and 35% and the content by weight of sulfur between 2% and 9% .

As mentioned above, the charges for gas generators are generally obtained by mixing the constituents of the said compositions in a solid mixer and by compression shaping of the said mixture. Compression is generally carried out at ambient temperature under an average pressure of 4,000 bar or 400 MPa if the compression is carried out with a press and of 2,000 bar or 200 MPa if the compression is carried out in a pelletizer.

Typically in the context of the present invention, after shaping said charges are brought to a temperature at least equal to 120 ° C for a period at least equal to 5 minutes. Heating takes place in a dry atmosphere, generally in an oven.

The Applicant has also observed that, for the usual fillers, the best results are obtained when the said fillers are brought to a temperature between 120 ° C. and 130 ° C. for a period of between 5 minutes and 20 minutes.

Preferably, the usual charges such as pellets or pieces of around one centimeter thick will be brought to a temperature of between 120 ° C. and 125 ° C. for a period of between 10 and 15 minutes.

After heating, the charges are cooled in the open air, at ambient temperature. Under these conditions, it has been discovered that the charges treated by the process according to the invention have, compared to identical charges which have not undergone the heat treatment according to the invention, a combustion speed which is approximately 20% higher. and very high mechanical strength properties superior, which can in some cases be approximately three times superior.

Although the Applicant does not intend to be limited by theoretical considerations, it believes that the treatment according to the invention allows a fusion and a recrystallization of the sulfur which begins to play the role of a binder in the charge, thus significantly improving its mechanical strength and its combustion speed.

The charges obtained by the process according to the invention thus constitute new products which make it possible to carry out charges for cold gas generators. These cold gas generators can be suitable for numerous applications and in particular as cold gas generators intended for automobile safety. The loadings according to the invention have all the advantages of traditional loadings of the same type from the point of view of the rate of combustion and the non-toxicity of the gases but have, compared to the latter, a much higher mechanical strength.

The loads made from the loads according to the invention thus have better mechanical preservation over time when they are mounted inside a motor vehicle or more generally in a support subjected to vibrations.

The examples which follow illustrate various possibilities for implementing the invention without limiting its scope.

Examples 1 to 3

Cubes with a side length of one centimeter were produced by compression at room temperature from the following three compositions, in which the percentages are expressed by weight:

• lot A : aucun traitement complémentaire
• lot B : chauffage en étuve à 120°C pendant 10 minutes.

For each example, these cubes were divided into two lots:

• lot A: no additional treatment
• lot B: heating in an oven at 120 ° C for 10 minutes.

After cooling the cubes in batch B, the combustion rates and the mechanical properties of the cubes in each batch were compared. The combustion speeds were measured in a pressure canister of 70 bar or 7 MPa and the mechanical properties in compression were measured with an apparatus of the "INSTRÖN" ® brand at 20 ° C, the compression piston having a speed of 1 mm / min.
The results were as follows:

It is found that the samples which have undergone the heat treatment according to the invention have, compared to the samples which have not undergone it, an improved combustion rate and considerably higher mechanical properties.

Examples 4 to 10

Cylindrical pellets with parallel faces 8 mm in diameter and 3.4 mm thick were manufactured by tableting from the following seven compositions:

The contents of sodium azide (NaN₃), molybdenum sulfide (MoS₂) and sulfur (S) are weight contents.

For each example the pellets were divided into two lots, one lot not undergoing heat treatment, another lot undergoing heating at 120 ° C for 10 minutes.

The breaking strengths were measured on a device of the "ERWEKA" ® brand for the pellets of the different batches. The results were as follows:

These examples confirm the spectacular improvement in the mechanical properties of the pellets having undergone the heat treatment according to the invention.

Example 11

With a composition analogous to that of Example 4, cylindrical pellets with parallel faces with a diameter of 6.26 mm, a thickness of 2.0 mm and a density of 2.0 were manufactured by tableting.

These pellets were divided into two lots, one lot not undergoing heat treatment, another lot undergoing heating at 120 ° C for 10 minutes.

For each batch of pellets, the combustion speed was measured in a pressure bomb of 70 bar, i.e. 7MPa as well as the tensile strength under the same conditions as those of Examples 4 to 10.

The results were as follows:

Examples 4 to 11 confirm, for the tablet geometry, the improvement in the results observed for the cube geometry in Examples 1 to 3.

Claims (10)

1. Method for manufacturing charges for gas generators from solid compositions which generate non-toxic gases, the constituents of which comprise at least (i) an alkali metal or alkaline earth metal nitride, (ii) a metal sulphide chosen from the group consisting of molybdenum sulphide or mixed sulphides of molybdenum, and (iii) sulphur, by mixing the said constituents and shaping by compression, characterized in that, after shaping, the said charges are raised to a temperature at least equal to 120°C for a time at least equal to 5 minutes.
2. Method according to Claim 1, characterized in that the said charges are raised to a temperature of between 120°C and 130°C.
3. Method according to Claim 1, characterized in that the said charges are raised to a temperature of between 120°C and 130°C for a time of between 5 minutes and 20 minutes.
4. Method according to Claim 1, characterized in that the said charges are heated to a temperature of between 120°C and 125°C for a time of between 10 minutes and 15 minutes.
5. Method according to Claim 1, characterized in that the sulphur content by weight in the said composition lies between 2% and 9%.
6. Method according to Claim 1, characterized in that the said mixed sulphides of molybdenum correspond to the formula Mo_xM_yS_z, in which:

   Mo   represents molybdenum

   M   represents copper or iron

   S   represents sulphur

   x   has a value lying between 0.7 and 3.0

   y   has a value lying between 1.0 and 5.0

   z   has a value lying between 4.0 and 12.0.
7. Method according to Claim 1, characterized in that the said solid compositions consist of an alkali metal or alkaline earth metal nitride, molybdenum sulphide and sulphur.
8. Method according to Claim 7, characterized in that the said solid compositions consist of sodium nitride, molybdenum sulphide and sulphur.
9. Method according to Claim 8, characterized in that the sodium nitride content by weight lies between 60% and 82%, the molybdenum sulphide content by weight between 15% and 35%, and the sulphur content by weight between 2% and 9%.
10. Charges of solid composition which generate non-toxic gases, obtained by the method according to any one of Claims 1 to 9.