EP2616413A2

EP2616413A2 - Pyrotechnic gas generator compounds

Info

Publication number: EP2616413A2
Application number: EP11773097.8A
Authority: EP
Inventors: Frédéric MARLIN; Stéphane BESOMBES
Original assignee: Herakles SA
Current assignee: ArianeGroup SAS
Priority date: 2010-09-15
Filing date: 2011-09-15
Publication date: 2013-07-24
Anticipated expiration: 2031-09-15
Also published as: KR20140135087A; MX2013002988A; WO2012035271A3; EP2616413B1; FR2964656B1; JP2013541487A; US20130228254A1; EP2616413B8; BR112013006065A2; WO2012035271A2; CN103180271A; FR2964656A1

Abstract

The main subject of the present invention is a solid pyrotechnic gas generator compound, the composition of which contains, expressed as weight percentages: - from 60 to 70% of guanidine nitrate, - from 26 to 33%, advantageously 26 to 30%, of potassium perchlorate, - from 2.5 to 6% of at least one combustion modifier chosen from transition metal oxides, the precursors of such oxides and mixtures thereof, from 0 to 6% of at least one additive, and does not contain an explosive ingredient. Such a compound is particularly effective for the inflation of side airbags.

Description

- -

PYROTECHNIC COMPOUNDS GENERATORS OF GAS

The present invention relates to pyrotechnic compounds gas generators, suitable for use in protective systems of occupants of motor vehicles, more particularly for the inflation of damping cushions (so-called "airbags") and particularly for the inflation of side damping cushions (so-called "side airbags" (see below)).

The technical field relating to the protection of occupants of motor vehicles has grown considerably over the last twenty years. The latest generation of vehicles now include in the cabin several airbag-type airbag safety systems whose functioning is provided by the combustion gases of pyrotechnic compounds. Among the cushioning type of systems, there are mainly frontal airbags for frontal impact and side airbags for side impact.

Side airbag systems differ from frontal airbag systems primarily in the time required for deployment and placement of the airbag. Typically, this time is shorter for a side airbag (of the order of 10-20 ms, against 40-50 ms for a front airbag). For a side airbag, the functional need for inflation of the bag over a short time requires the use of a pyrotechnic composition having a sufficiently high combustion rate (typically equal to or greater than 30 mm / s, or even 35 mm / s, at 20 MPa ) over the operating pressure range in the generator combustion chamber, in order to obtain an inflated surface flow rate value (product pxnx Te x Vc) sufficient. Moreover, in order to ensure satisfactory system operation, the pyrotechnic composition must also have good reliability characteristics. Also, given the generally decreasing surface profile of the loadings employed (pellet type), the composition should ideally have a stable and sufficiently high combustion rate at low pressure. In fact, those skilled in the art are aware that the . - specifications of the side airbags is more restrictive than that of the front airbags. Of course, any technological advance in the field of said side airbags can also advantageously be declined in the field of frontal airbags.

In the present text, a low pressure is defined as a pressure P such that: 0.1 <P <10 MPa, by medium pressure, a pressure P such that: 10 MPa <P <30 MPa, by high pressure, a pressure P such that: P> 30 MPa.

It is also customary to compensate for the low combustion rates of the pyrotechnic compounds used in current airbag gas generators, by using loadings composed of pellets of very small dimensions. Although it is not economically attractive because of the low mass flow rate of pelletizing machines and the resulting tooling costs, this makes it possible, to a certain extent, to overcome part of the lack of combustion speed. This solution, however, highlights two other disadvantages:

- The difficulty of ignition, which is increased because of the large initial surface of the load, which then requires the use of a reinforced igniter or the addition of an additional load acting ignition relay;

- A strong degression of the burning surface due to the reduced size of the pellets, which induces a long tail of combustion at low pressure. This long tail of low pressure combustion is the source of the emission of the majority of the toxic species present in the gases used to inflate the cushion.

It should therefore be noted that the desired increase in the rate of combustion of the pyrotechnic compound in question over the entire pressure range, including at low pressure, is therefore not only necessary to increase the gas flow to reach the inflation time specifications, but also to ensure the ignitability of the compound without resorting to the use of a relay charge and the safety of the products of combustion.

In addition, there are constraints with reference to the combustion temperature. . .

In general, said combustion temperature should not be too high (it must at least remain below 2400 K, more favorably below 2350 K) so that the temperature of the gases in the cushioning cushion does not affect the integrity physical of the occupant. A low combustion temperature makes it possible, on the one hand, to limit the thickness of the bag and, on the other hand, to simplify the design of the gas generator by making it possible to reduce the presence of baffles and filters within it. this.

Side airbag systems can use two types of gas generators: those called fully pyrotechnic (the gas generation being then ensured exclusively by the combustion of a pyrotechnic charge) and those called "hybrids" (the gases then coming from joint combustion of a pyrotechnic charge and a volume of neutral gas stored under pressure in a sealed tank). For "hybrid" type generators, the pyrotechnic charge must not have a low combustion temperature so that the combustion gases are sufficiently hot to compensate for the temperature drop caused by the expansion of the pre-compressed neutral gas volume. . Ideally, combustion temperatures above 2000 K are required.

Thus, those skilled in the art are in search of pyrotechnic compounds which are suitable for use in fully pyrotechnic gas generators or in hybrid generators, more particularly intended for side airbags, that is to say simultaneously presenting a moderate combustion temperature of the order of 2000-2400 K, more favorably 2000-2350 K, and a high combustion rate over the entire operating pressure range (especially greater than 30 mm / s at 20 MPa, more favorably higher at 35 mm / s at 20 MPa, v included at low pressure.

In addition, pyrotechnic compounds for airbags must also tend to meet the following requirements together:

the gases generated by the combustion of the pyrotechnic charge (comprising a compound or n compounds) must be non-toxic, that is to say have a low content of carbon monoxide, oxides of nitrogen and chlorinated compounds; . _

the gas yield (ie the quantity of gas generated by the combustion) must be high in order to lead to a high inflation power;

the quantity of solid particles generated by the combustion, which may constitute hot spots that can damage the wall of the airbag, must remain low;

- The pressure exponent must be as low as possible, especially at medium and high pressure (typically less than 0.35 as described in the prior art) but also at low pressure. A low pressure exponent makes it possible to significantly reduce the variability of operation between the extreme low temperature (around -40 ° C) and the extreme high temperature (around 90 ° C) required in the field of use. The reproducibility of the operation is thereby improved and the size of the metal structure of the generator can be advantageously reduced;

- their limiting combustion pressure must be as close as possible to the atmospheric pressure.

It is also highly desirable that the basic ingredients of the compounds be pyrotechnically non-hazardous. The presence of ingredient (s) belonging to the class of explosives such as nitroguanidine, hexogen (RDX), octogen (H X) is advantageously avoided. Explosive ingredients are ingredients classified in risk division 1.1 according to standard NF T 70-502 (see also UN - Recommendations on the Transport of Dangerous Goods - Manual of Tests and Criteria, Fourth revised edition, ST / SG / AC.10 / ll / Rev.4, ISBN 92-1-239083-8ISSN 1014-7179 and STANAG 4488). Guanidine nitrate and potassium perchlorate, taken separately, are not ingredients in this risk division. They are not explosive ingredients, especially in the sense of the invention.

It should be pointed out at this stage that the incorporation of explosive high energy ingredients such as nitroguanidine, even at reduced levels, contributes detrimentally to increasing the combustion temperature of the compounds, beyond the threshold of 2350 K set by the need for technical improvement sought by the inventors. Thus, compounds as described in US Pat. No. 6,893,517, consisting mainly of a mixture of a guanidine derivative (preferentially guanidine nitrate), an explosive nitrogen compound (preferably nitroguanidine) and a inorganic oxidant (such as ammonium perchlorate or potassium perchlorate) do not meet the requirements of the specification of the present invention. These compounds also include in their composition a low level of a ballistic catalyst, consisting of an oxygen compound of a transition metal, advantageously with a high specific surface area, traditionally used in the field of propellants to increase the combustion rate at medium and high pressure (this catalyst accelerates the decomposition of the oxidizing charge). The micro-gas generators for belt retractor devices as described in said US Pat. No. 6,893,517 (and in its priority application EP 1 275 629) operate in pulses, which requires a high rate of combustion at medium and high pressure. . A high exponent of low-pressure pressure and the non-combustion at atmospheric pressure of the compounds in question are not a problem, since the pressure does not fall back, in the context of use of said compounds, to a low level before the end of pyrotechnic operation. This application for belt retractor devices does not require, for the gas generator, requirements as severe as those required in the context of the present invention (airbags, and especially side airbags), especially a high rate of combustion. at low pressure, a lowering of the limit threshold of combustion pressure (closer to atmospheric pressure) and a low pressure exponent over the entire range of combustion (especially at low pressure).

Currently, for front airbags, pyrotechnic compounds that offer a good compromise, in terms of gas temperature, gas yield, emitted particulate levels and toxicity, contain, as main ingredients, guanidine nitrate (NG) in as a reducing filler and basic copper nitrate (BCN) as an oxidizing filler. US 5,608,183 and US 6,143,102 disclose such compounds. . _

These compounds, however, have relatively low rates of combustion, less than or equal to 20 mm / s at 20 MPa, and a low gas yield. They are also difficult to light.

With a view to improving the ignitability of this type of compounds, it has been proposed, according to the prior art, the addition of perchlorate in their composition based on guanidine nitrate (NG) and basic nitrate of copper ( BCN). Thus, patent application EP 1 526 121 describes the addition of a perchlorate (especially potassium perchlorate), at a low level (less than 5% by weight), to improve the ignition of these compounds, However, The incorporation of perchlorate at such a low rate does not sufficiently increase the rate of combustion of the compound for satisfactory use in gas generators for side airbags.

Applications WO 2007/042735 and WO 2009/126702 describe compounds having compositions of the same type which contain guanidine nitrate (NG), as a reducing charge, of basic copper nitrate (BCN), as a main oxidizing charge and, in addition, a second oxidizing charge, which advantageously consists of potassium perchlorate (KCIO ₄ ). These documents combine the good performances of said compounds, in particular a high combustion rate at high pressure, with the composition but also with the specific manufacturing process of said compounds (process which includes a step of dry roller compaction for WO 2007/042735 and two successive stages of spray drying and compression drying for WO 2009/126702).

With a view to improving the gas yield and the rate of combustion, it has also been proposed, according to the prior art, compounds based on one or more associated nitrogen reductants. to a strong oxidant perchlorate type.

In patent application US 2006/0137785, there is described the combination of a guanidine reducing agent (nitroguanidine or guanidine nitrate) and ammonium perchlorate, the latter necessarily being incorporated at a significantly high level (30% 60% by weight). The incorporation of ammonium perchlorate at such a high rate leads to two major drawbacks which are, on the one hand, a - a significant increase in the combustion temperature (above 2800 K) and, on the other hand, the generation of hydrogen chloride (which is a toxic and highly corrosive gas), said hydrogen chloride being then present in the gaseous effluents. To overcome this problem, it is described in said patent the need to add to the mixture of guanidine + ammonium perchlorate type a metal compound of iron oxide type in order to neutralize the hydrochloric acid present in the combustion gases, which has the consequence of reducing the gas yield value of the compound.

Incorporation of potassium perchlorate instead of ammonium perchlorate would have the advantage of leading to the formation of potassium chloride (KCl) in place of hydrogen chloride (HCl) (but the disadvantage of reducing the gas yield). In any case, the incorporation of KCl0 ₄ at such high levels (up to 60% by mass) would produce an increase in the prohibitive combustion temperature in the context of the intended application.

Logically, those skilled in the art therefore turned to compounds consisting of a mixture containing guanidine nitrate (NG, alone or in combination with a co-reductant) and potassium perchlorate (KClO ₄ ) at intermediates from about 25% to 45% by weight, as described in the patent application WO 95/25709 and US 5,854,442, US 5,997,666, said mixture to obtain compounds partially meeting the essential requirements scope of application of the present invention, namely:

- a good gas performance;

- a moderate combustion temperature;

an intrinsic non-toxicity of the particulate effluents; and

- A combustion rate to 20 MPa slightly increased compared to compositions formulated based on guanidine nitrate (NG) and basic copper nitrate (BCN), but which remains insufficient for use in side airbags.

The thermodynamic and ballistic characteristics of such a compound (reference compound 1), whose composition ("binary") contains only guanidine nitrate (NG) and potassium perchlorate (KCIO ₄ ), are presented in Table 1 below. - -

Table 1

(1): the given value is a relative pressure. A zero combustion pressure limit corresponds to the atmospheric pressure.

Reference compound 1 has many of the advantages expected from a gas generator compound for an airbag system. The basic ingredients are simple and easily available, inexpensive, non-hazardous on the pyrotechnic aspects (no component of the class of explosive compounds) and non-toxic. The thermodynamic performances (gas yield, particle rate) are good and the combustion temperature remains moderate and therefore acceptable. The particles emitted by combustion are non-toxic (essentially KCl).

However, such a compound does not have all the expected performance, especially for a side airbag application. Firstly, the burning rate of the order of 26 mm / s to 20 MPa is only 20 to 30% higher than that of a compound based on _ Guanidine nitrate (NG) and basic copper nitrate (BCN), and remains low compared to the set specifications. Then, in tests with this reference formulation, it appeared that the total substitution of the basic copper nitrate oxidant (BCN) with potassium perchlorate (KCIO4), if it increases the rate of combustion at above 5 MPa and also improve the ignitability, induced in return, and in a highly detrimental way, a very high pressure exponent at low pressure (greater than 0.55 over the range 6 to 10 MPa and a non-combustion at atmospheric pressure (additional tests have shown that the operating limit pressure is around 1.7 MPa, whereas a compound formulated based on guanidine nitrate (NG) and basic copper nitrate (BCN) advantageously has a combustion non-zero at atmospheric pressure).

Starting from the known performance of the guanidine nitrate (NG) / potassium perchlorate (KClO ₄ ) mixture, the inventors wished to propose improved pyrotechnic gas-generating compounds, which are particularly suitable for use in side airbags. In particular, they have set themselves the objective, while maintaining or improving the other features, of significantly improving the following three points:

- reduction of the combustion limit pressure,

- reduction of the pressure exponent (<0.26), advantageously a large decrease (<0.2), very advantageously a very large decrease (<0.1) from 6 MPa,

- Increasing the combustion rate over the entire pressure range, especially at low pressure.

In a totally unexpected manner, it has been found that the presence, within the composition of the compounds of the invention, of a low level of (at least) an oxygen compound of a transition metal (an oxide of a transition metal or a precursor compound for such an oxide), preferably with a high specific surface area (traditionally used as a ballistic catalyst in the field of propellants to increase the rate of combustion at high pressure (by accelerating the decomposition of the oxidizing charge)), has (also) major effects on the three improvement points sought above (i.e., the increase of the combustion rate (also) at low pressure, the reduction of the combustion limit pressure and the decrease of the pressure exponent over the entire pressure range).

The compositions of the pyrotechnic gas-generating compounds of the invention (particularly suitable for airbag applications, especially lateral applications) contain:

guanidine nitrate, and

potassium perchlorate.

They are characterized in that, expressed in mass percentages, they contain:

60 to 70% of guanidine nitrate,

26 to 33%, advantageously 26 to 30%, of potassium perchlorate,

2.5 to 6% of at least one combustion modifier chosen from transition metal oxides, precursors of such oxides and mixtures thereof,

0 to 6% of at least one additive, and

do not contain any explosive ingredient.

According to one variant, compositions of compounds of the invention consist (exclusively) in the ingredients listed above (NG + KCI0 ₄ + at least one combustion modifier + optionally at least one additive) taken at the levels indicated below. above.

The ingredients of the first three types above (guanidine nitrate, potassium perchlorate and specific combustion modifier) generally represent more than 90% by weight of the total mass (of the composition) of the compounds of the invention, very generally at less than 94% by weight, or even more than 98% by weight. The possible presence of additive (s), such as production auxiliaries (calcium stearate, silica, for example), is expressly provided for. The ingredients of the three types above can be quite 100% by weight of the total mass of the compounds of the invention.

Guanidine nitrate, representing 60 to 70% of the total mass, is also selected for reasons of pyrotechnic safety and for its rheoplastic behavior, suitable for the implementation of compaction phases and possible pelletization of the process. dry (see - - hereinafter), ensuring a good densification of the starting pulverulent pyrotechnic composition while limiting the compressive force to be applied. The manufacture of the compounds by the dry process process comprises up to four main steps (see below), which have been described in particular in the patent application WO 2006/134311.

Potassium perchlorate is present in the composition of the compounds of the invention at a moderate intermediate level (from 26 to 33% by weight, advantageously from 26 to 30% by weight), particularly with reference to the combustion temperature, "ignitability" and the target high-pressure combustion rate.

Within the NG + KCIO ₄ mixtures, the combustion modifiers, selected by the inventors, develop particularly interesting (unexpected) properties with reference to the three improvement points sought (see above).

Said at least one combustion modifier is selected from transition metal oxides, precursors of such oxides and mixtures thereof. A precursor of such an oxide leads to the formation of such an oxide (generates such an oxide) during its decomposition in temperature during the combustion of the pyrotechnic compound. Thus, the basic copper nitrate (Cu (NO ₃ ) ₂ '3Cu (OH) ₂ ) is decomposed into copper oxide (CuO) (see below).

Said at least one combustion modifier is present in a sufficient amount (> 2.5% by weight), to be effective (with reference to the three improvement points above), and not excessive (<6% by weight) in order not to harm the gas yield. A single combustion modifier is generally present but the presence of at least two such additives is expressly provided for in the context of the present invention.

Preferably, said at least one combustion modifier is selected from zinc oxide (ZnO), iron oxide (Fe ₂ O ₃ ), chromium oxide (Ο 2Ο ₃ ), manganese dioxide ( MnO ₂ ), copper oxide (CuO), basic copper nitrate (Cu (NO ₃ ) 2-3Cu (OH) ₂ ) and mixtures thereof. The copper oxide and the basic copper nitrate, precursor of said copper oxide (in that the BCN leads to the formation of copper oxide CuO during its decomposition in temperature), are - - particularly powerful. In a particularly preferred manner, the compounds of the invention therefore contain, as a combustion modifier, copper oxide and / or basic nitrate of copper. The use of these combustion modifiers makes it possible to obtain compounds of the invention having a pressure exponent value of less than or equal to 0.1 over the pressure range 6-52 Pa.

Preferably, said at least one combustion modifier according to the invention has a specific surface area greater than 3 m ² / g _/ advantageously greater than 10 m ² / g, very advantageously greater than 25 m ² / g.

It is understood that the function of said at least one specific combustion modifier (selected from transition metal oxides, their precursors and mixtures thereof) within the composition of the compounds of the invention is not only as in the art previous (see in particular the teaching of US Patent 6,893,517 mentioned above) to increase the combustion rate at high and medium pressure, but also, surprisingly, to give the pyrotechnic compounds:

a stable and self-sustaining combustion at low pressure (or at a pressure almost equal to atmospheric pressure),

a lower combustion rate at a lower pressure than that of the compositions of the prior art,

a low or almost no pressure exponent at low, medium and high pressure, significantly lower than that of the compositions of the prior art;

and this, with a "good" ignitability "of said compounds, without generating too much solid particles for combustion, and a combustion temperature of around 2300 K.

It can be stated here that the compounds of the invention, the composition of which has been specified above, exhibit:

a combustion temperature below 2350 K,

a pressure (relative, that is to say with reference to atmospheric pressure) combustion limit less than or equal to 1.5 MPa, preferably less than 0.2 MPa, very advantageously equal to 0.1 MPa, - -

an exponent of pressure less than or equal to 0.25, advantageously less than or equal to 0.2, very advantageously less than or equal to 0.1, for a pressure between 6 and 52 MPa,

- a burning speed:

+ greater than 24 mm / s, advantageously greater than 36 mm / s, at low pressure,

+ greater than 30 mm / s, advantageously greater than 35 mm / s, at medium pressure,

+ greater than 37 mm / s, advantageously greater than 45 mm / s, at high pressure.

It is necessary to insist here on the low or even very low pressure exponent values of the compounds of the invention.

The best results indicated above (advantageous variants and very advantageous variants) have been obtained in particular with copper oxide and basic copper nitrate as a combustion modifier. In support of this statement, reference may be made to the following examples.

In the context of the present invention, in the composition of the compounds of the invention, it is therefore proposed an original use (particularly with reference to the above parameters) to the oxides and precursors of oxides in question (original, compared to that conventional known ballistic catalyst, in different compositions).

In addition to the above constituents (NG + KCIO ₄ + at least one combustion modifier of the specified type), the pyrotechnic compounds of the invention may contain, at a low mass level (less than or equal to 6%, generally at least 0.1%), at least one additive, especially at least one additive making it easier to obtain (shaping, when they are obtained), such as calcium or magnesium stearate, graphite and / or at least an additive improving the aggregation of the solid products of their combustion chosen from refractory oxides with a softening or melting temperature adapted to the composition, such as silica or alumina. It is advantageously silica, generally introduced in fine powder form (advantageously of micrometric size, very advantageously of nanometric dimension) having a high specific surface area (Advantageously 100 m ² / g or more) or in the form of silica fibers of small diameter (1 to 20 microns) and a few tens or hundreds of microns (20 to 500 microns) in length. Surprisingly, it has been found that the presence, within the pyrotechnic compounds of the invention, of silica, at levels of between 0.5 and 6% by weight, advantageously between 0.5 and 3.5% in mass, also has a very significant effect of lowering the combustion limit pressure.

It is therefore also the merit of the inventors to have demonstrated this effect of the silica within compositions of NG + KCIO _{4 type} (see Table 3 below), and therefore within the compositions of the invention (type NG + KCI0 ₄ + at least one combustion modifier) wherein said effect is cumulative with those (including that of lowering the combustion limit pressure) of the at least one combustion modifier present.

The at least one additive intervenes with the constituent ingredients

(NG, KCI0 ₄ + at least one combustion modifier of the aforementioned type) (at the beginning of the manufacturing process) or is added, further downstream, in the manufacturing process of the compounds of the invention.

It is recalled that the compositions of the compounds of the invention do not contain any explosive ingredient (see the NF standard and the UN recommendations specified above), and this, in particular with reference to the parameters: pyrotechnic safety and combustion temperature. It should also be noted that the masses of pyrotechnic compounds required for the inflation of an airbag, in particular of a lateral airbag, are greater than those required for the inflation of a retractor device according to US Pat. 6,893,517 (said inflations not being of the same type: inflation time greater than 10-20 ms / per pulse).

The pyrotechnic compounds of the invention can be obtained by a wet process. According to one variant, said process comprises the extrusion of a paste containing the constituents of the compound. According to another variant, said process includes a step of aqueous dissolution of all (or some) of the main constituents comprising solubilization of at least one of said main constituents (oxidant and / or reducing agent) and then obtaining a powder by spray-drying, the addition to the powder obtained of the constituent (s) which would not have been dissolved, and the shaping of the powder in the form of objects by the usual processes in the dry process.

The pyrotechnic compounds of the invention may also be obtained in the dry process, for example by simply pelletizing the powder obtained by mixing their constituents.

The preferred method of obtaining the pyrotechnic compounds of the invention includes a dry compaction step of a mixture of powder constitutive ingredients of said compounds (except for said at least one additive which may be added during the process). Dry compaction is generally carried out, in a manner known per se, in a roller compactor, at a compacting pressure of between 10 ⁸ and 6.10 ⁸ Pa. It can be implemented according to different variants (with a characteristic step compaction "simple" followed by at least one complementary step, with a compaction characteristic step coupled to a shaping step). Thus, the pyrotechnic compounds of the invention are likely to exist in different forms (especially over the manufacturing process leading to the final compounds):

- after dry compaction coupled with a shaping

(By use of at least one compacting cylinder, the outer surface of which has cavities), plates with relief patterns are obtained which can be broken in order to directly obtain pyrotechnic objects formed;

- After dry compaction followed by granulation, granules are obtained;

- After dry compaction followed by granulation and then pelletizing (dry compression), pellets are obtained;

- After dry compaction followed by granulation and then mixing the granules obtained with an extrudable binder and the extrusion of said binder loaded into said granules, we obtain extruded monolithic blocks (loaded with said granules) .

The pyrotechnic compounds of the invention are therefore particularly likely to exist in the form of objects of the type:

- granules, -.

- lozenges,

- monolithic blocks.

In no way limiting, we can indicate here that:

the granules of the invention generally have a particle size (a median diameter) of between 200 and 1400 μm (and an apparent density of between 0.8 and 1.2 cm ³ / g);

the pellets of the invention generally have a thickness of between 1 and 3 mm.

When the compounds of the invention are obtained by a dry process, the constitutive ingredients of the compounds of the invention advantageously have a fine particle size, less than or equal to 20 μm. Said granulometry (value of the median diameter) is generally between 3 and 20 pm. The compounds described in the present invention express their full potential if they are obtained by a dry process from powders having a median diameter of between 10 and 20 pm for KCl0 ₄ and 5 to 15 pm for guanidine nitrate. .

According to another of its objects, the present invention relates to a powdery composition (mixture of powders), precursor of a compound of the invention, the composition of which corresponds to that of a compound of the invention (see above ).

According to another of its objects, the present invention relates to gas generators containing at least one pyrotechnic compound of the invention. Said generators are ideal for airbags, especially side airbags (see above).

It is now proposed to illustrate, in a non-limiting way, the invention.

Table 2 below gives examples of compositions of compounds of the present invention, as well as the performances of said compounds compared with those of the compound of the prior art of reference 1. The compounds were evaluated by means of thermodynamic calculations or from physical measurements on pellets or pellets made from the compositions via the mixing process of . . powders - compaction - granulation - and possibly dry pelletisation.

The reference compound 1 of the prior art (see Table 1 above) contains guanidine nitrate and potassium perchlorate and does not contain a combustion modifier within the meaning of the invention. The compounds of Examples 1 to 7 contain in their composition, in addition to the two constituents of the reference compound 1, such a combustion modifier.

The levels of the major constituents have been adjusted in order to maintain an oxygen balance value close to -3%, so that the performances of the compounds of Table 1 can be directly compared.

The results in Table 2 show, as expected according to the teaching of the prior art (teaching of US Pat. No. 6,893,517 and that of the field of propellants), that the addition of a combustion modifier within the meaning of the invention in a composition of the type of that of the reference compound 1 leads to an increase in the combustion rate at medium and high pressure without significant modification of the combustion temperature.

Surprisingly, said addition conjointly leads to a very significant lowering of the pressure exponent, a very low pressure exponent over the entire operating pressure range (beyond 6 MPa), of the combustion limit pressure and a significant increase in the low-pressure combustion rate.

CuO is the compound which, added to the reference composition

1, provides the most significant improvements (see Example 2). The pressure exponent is almost zero over the entire operating range, the operating limit pressure is almost equal to the atmospheric pressure.

Since a metal complex such as BCN decomposes during exothermic combustion reactions by generating in situ high surface area CuO (which has been verified experimentally), CuO can therefore be replaced by BCN as burn modifier with results that are equivalent to those of CuO (see Example 7). - -

CuO and BCN allow, when incorporated at a reduced rate (5% in the examples), to retain an advantageous value of gas yield (> 32 g / mol) and ultimately lead to a very significant improvement in the surface flow rate value of inflation (by more than 40%) relative to the reference composition NG / KCIO4 of the reference compound 1.

Table 2

Table 2 (continued)

Table 3 below shows the second surprising effect demonstrated by the inventors, namely the very significant reduction in the combustion limit pressure (measured on granules) when silica is introduced at a moderate rate into the composition of the compounds of the invention. This same effect, obtained with another refractory metal oxide such as alumina, is not of sufficient magnitude to be of interest.

TABLE 3

Claims

1. A pyrotechnic solid gas generating compound, the composition of which contains:

guanidine nitrate, and

potassium perchlorate,

characterized in that its composition, expressed in percentages by weight, contains:

60 to 70% of guanidine nitrate,

26 to 33%, advantageously 26 to 30%, of potassium perchlorate,

0 to 6% of at least one additive, and

does not contain any explosive ingredient.

2. Compound according to claim 1, characterized in that its composition consists of said guanidine nitrate, potassium perchlorate, at least one combustion modifier, and optionally at least one additive.

3. Compound according to claim 1 or 2, characterized in that its composition is constituted for at least 94% by weight, preferably at least 98% by weight, or even 100% by weight, said guanidine nitrate, potassium perchlorate. and at least one combustion modifier.

4. Compound according to any one of claims 1 to 3, characterized in that said at least one combustion modifier is selected from zinc oxide (ZnO), iron oxide (Fe ₂ O 3), chromium oxide (Cr ₂ O 3), manganese dioxide (MnO ₂ ), copper oxide (CuO), basic copper nitrate (Cu (NO ₃ ) ₂ "3Cu (OH) ₂ ) and mixtures thereof.

5. Compound according to any one of claims 1 to 4, characterized in that said at least one combustion modifier consists of copper oxide and / or basic copper nitrate.

6. Compound according to claim 5, characterized in that it has a pressure exponent of less than or equal to 0.1, for a pressure of between 6 and 52 MPa.

7. Compound according to any one of claims 1 to 6, characterized in that said at least one combustion modifier has a specific surface area greater than 3 m ² / g, preferably greater than 10 m ² / g, very advantageously greater than 25 m ² / g.

8. A compound according to any one of claims 1 to 7, characterized in that its composition contains, as an additive, silica.

9. Compound according to claim 8, characterized in that said silica is in pulverulent form with a high specific surface area advantageously of 100 m ² / g or more, and of a micrometric size, advantageously of nanometric dimension, or in the form of silica fibers. 1 to 20 microns in diameter and 20 to 500 microns in length.

10. A compound according to any one of claims 1 to 9, characterized in that it is obtained by a dry process, which comprises a step of compacting a powder mixture containing the constituent ingredients of said powder compound, possibly followed a granulation step, itself followed, optionally, a pelletizing shaping step.

11. Compound according to any one of claims 1 to 10, characterized in that it is in the form of granules, pellets or monolithic blocks.

12. powder composition, precursor of a compound according to any one of claims 1 to 11, the composition of which corresponds to that of a compound according to any one of claims 1 to 9.

13. Gas generator, suitable for airbag, characterized in that it contains at least one compound according to any one of claims 1 to 11.