US20110041969A1

US20110041969A1 - Gas-generating pyrotechnic compound and production process

Info

Publication number: US20110041969A1
Application number: US12/598,323
Authority: US
Inventors: Frederic Marlin; Bernard Thibieroz
Original assignee: SNPE Materiaux Energetiques SA
Current assignee: Safran Ceramics SA
Priority date: 2007-05-02
Filing date: 2008-04-30
Publication date: 2011-02-24
Also published as: JP2010526010A; CN101679139B; FR2915746A1; EP2139828A2; FR2915746B1; WO2008145935A3; WO2008145935A2; EP2139828B1; ATE527227T1; CN101679139A; JP5483732B2

Abstract

The present invention provides pyrotechnic gas generator compounds and a method of obtaining them. Said compounds have a composition which comprises:

- an oxidizing charge which comprises at least one inorganic perchlorate and which does not comprise a chlorine scavenger; and
- a crosslinked, oxygen-containing hydrocarbon binder obtained by crosslinking an elastomer in the presence of at least one crosslinking agent and at least one oxygen-containing plasticizer for said elastomer; said elastomer having a molecular weight of more than 200 000 and being preferably selected from polyesters and polyacrylates; said at least one oxygen-containing plasticizer being composed of a plasticizer of which the molecular weight is greater than 350 g/mol and the oxygen balance equal to or greater than −230 %, of a mixture of such plasticizers with one another or of a mixture of at least one such plasticizer with at least one other oxygen-containing plasticizer.

Said compounds can be obtained continuously by extrusion.

Description

The present invention provides pyrotechnic gas generator compounds and also a method of obtaining them. Said pyrotechnic compounds have:

- a high inflation power (the intrinsic inflation power of a formulation is given by the product of the molar yield of said formulation (in mol/kg) and the temperature T (in K). This product is equivalent to the product of pressure times volume, and hence conveys the capacity of a formulation to generate a quantity of gas); and
- a high inflation rate per unit area (the inflation rate per unit area is estimated by the product ρ×n×T×Vc, where ρ is the density of the material (in g/cm³), n is the molar gas yield of combustion (in mol/g), T is the combustion temperature (in K), and Vc is the combustion rate in cm/s. Accordingly the parameter of inflation rate per unit area is expressed in mol.K/cm².s).

Furthermore, the combustion of said pyrotechnic compounds does not generate many condensed particles.
The present invention is situated in the technical field of pyrotechnic gas generation. It finds application more particularly in vehicle occupant protection systems offering protection by means of airbags, which are inflated by the combustion gases from pyrotechnic compounds.

- The technical field of airbags inflated by gas generators, including pyrotechnic compounds based on pyrotechnic compositions, has experienced very extensive development over the last twenty years. This field is presently reaching maturity, and the majority of the systems which are produced and sold are based on formulations which exhibit high similarities. The pyrotechnic compositions employed in these systems are “cold” compositions, whose combustion temperature is generally between 1800 and 2200 K.

The use of this type of composition has the following advantages:

- the low temperature allows the thickness of the bag to be limited;
- the gaseous effluents are generally of high quality in relation to the admissible toxicity standards in force in the field 1(USCAR).

Conversely, these compositions give rise to drawbacks, which may prevent some applications:

- the gas yield is generally low. The weight of product required is therefore substantial;
- since the temperature is relatively low and the gas yield is low, the inflation power is limited;
- since combustion produces solid residues, filters are needed in order to reduce the emission of particulates. These filters also reduce the inflation power, owing to the thermal losses to which they give rise. The filters also give rise to an increase in the volume and cost of the generator;
- in almost all cases, cold compositions exhibit low combustion rates. This point, combined with the low inflation power, gives rise to low inflation rates per unit area. These low inflation rates per unit area are compensated by the use of fine pellets with small diameters, which give rise to a high initial surface area of a pyrotechnic product and to a low charge density.

Generally speaking, the high initial surface area of “cold” product with a low combustion rate gives rise, in addition to the aforementioned drawbacks, to the following difficulties:

- the combustion of the product is initiated slowly, and a high ignition charge is required;
- the manufacture of small pellets needed in order to obtain the rate over the duration of combustion gives rise to a high initial rate and hence to a large increase in pressure at the start of combustion in the bag. This is injurious for said bag within the first few milliseconds, when the bag is folded, and the bag is therefore very subject to the stresses of positioning in the deployment phase;
- the combustion of pellets with decreasing surface development gives rise to a reduction in rate in line with the progress of combustion, whereas the functional requirement, conversely, is for a constant or even increasing rate over time.
- The high inflation power of a composition is associated with the pairing of molar gas yield and temperature, and so compositions meeting these requirements have a high combustion temperature and a low level of particulates produced in the course of combustion. The low level of particulates is made necessary in order to have the maximum of gases emitted, and so as to prevent attack on the bags. Indeed, tests in airbag modules have demonstrated that the damage to the bags by the combustion products is exacerbated by the presence of particles when “hot” compositions are used.

For all of the aforementioned reasons, a composition having a high inflation power, a high inflation rate per unit area, and without particulates, has many advantages in terms of operation and in terms of the miniaturization of gas generators for an airbag system. This miniaturization is required for reasons of cost, weight, and integration within vehicles.
Compounds obtained from solventlessly extrudable compositions with a high inflation power have already been employed in airbag systems. Compositions of this kind, referred to as double base compositions, containing nitrocellulose and nitroglycerine, were employed in the 1990s in the form of monolithic blocks. Details of formulations and of double base propellant compounds are found in the Davenas publication “Technologie des propergols solides” [Solid propellant technology], Masson edition 1989, and an example of use of these compounds is found in patent application FR 2 853 872. The formulations have a high gas yield, a relatively high combustion temperature, and a low level of particulates. Tests conducted in airbag modules with these compounds showed that operation was good in an uncoated bag with a relatively low thickness (˜470 dtex). The attack on the bag is acceptable in spite of the high temperature, because of the low level of particulates produced, and because of the operation caused by the monolithic block, which limits the attack on the bag at the start of deployment. These compounds, however, are no longer employed in new systems, since they are not sufficiently resistant to high-temperature aging as required by automakers. In addition to the problem of temperature aging, these compounds have an oxygen balance which is shifted toward negative values (oxygen deficit), which gives rise to a level of carbon monoxide markedly greater than the maximum standards in force for the application in question. The oxygen balance is the weight percentage of oxygen given off (positive value) or absorbed (negative value) in the combustion reaction of an ingredient. For example, a CH₂group, a basic component in aliphatic polymers, has an oxygen balance of −343%. This is because three oxygen atoms, with a molar weight of 48 g, are needed to balance the combustion of the 14 g/mol of CH₂.
In order to overcome these difficulties, compositions based on silicone binder were subsequently proposed. Compositions based on “RTV” (“room-temperature vulcanizable”) binder and potassium perchlorate were described in patent applications FR 2 190 776 and FR 2 213 254. Patent application FR 2 728 562 describes an improved version of these products, which can be manufactured by a continuous mixing and extrusion method. In the latter document, the silicone binder is in combination with a mixture of ammonium perchlorate and sodium nitrate, said sodium nitrate serving as a chlorine scavenger. The compositions in accordance with these documents exhibit an acceptable gas yield, a high combustion temperature, and a high combustion rate. The inflation rate per unit area is therefore relatively high. Moreover, relative to the compositions based on nitrocellulose and nitroglycerine, these compositions have the advantage of being very temperature-stable and of having an oxygen balance which can be regulated in order to observe the standards relating to toxicity of gaseous effluents. These compositions, on the other hand, have the disadvantage of emitting a high level of particulates (approximately 30%), more particularly sodium chloride and silica, and this necessitates the use of a bag with a high thickness and a protective coating on the internal face of said bag.
Patent application EP 1 216 977 describes a solventlessly extrudable composition essentially comprising an oxygen-containing hydrocarbon binder having two components (an elastomer (rubber) and a plasticizer), a nitrogenous organic compound, and oxidizing charges. The oxidizing charges are ammonium perchlorate and a chlorine scavenger, which may be sodium nitrate. The sodium nitrate is present at approximately 15% to 20% by weight in the composition. Its role is to trap the chlorine from the ammonium perchlorate to form sodium chloride. This composition has a high combustion temperature and also an inflation power and a rate per unit area that are of the same order of magnitude as those of the silicone bases, owing to a slightly lower combustion rate and a lower level of particulates. In spite of this improvement, the gas yield, and therefore the inflation power, is limited by the presence of the chlorine scavenger, which gives rise to the formation of a solid chlorine salt. The overall amount of solid combustion residues of these compositions is approximately 14%. The combustion rates of these compositions are approximately 20 to 40 mm/s at a combustion pressure of 20 MPa.
The person skilled in the art is continually looking for high-combustion-temperature compositions which produce few particulates and little toxic effluent (with the aim of increasing the inflation power and the inflation rate per unit area, in order to reduce the volumes, weights and costs of the gas generators for airbag systems), it being possible for these compositions to be advantageously shaped by solventless extrusion, which allows the production of a monolithic form which facilitates charging, improving the charge density of the generator. Moreover, the absence of solvent prevents the emission of VOCs (volatile organic compounds).
On the basis of the teaching of patent application EP 1 216 977, the inventors wished to provide improved pyrotechnic gas generator compounds. The objectives they set themselves were more particularly as follows:

- to increase the gas yield;
- to suppress particulates (solid combustion residues);
- to have a high combustion rate, allowing the thickness to be burnt, and hence the compactness of the generator, to be increased; for the purpose of:
- reducing the required volume and weight of propellant, and
- eliminating the filters used for the filtration of said particulates.

In this way the gas generator can be smaller, compact, and less expensive. Moreover, in spite of a high temperature of the combustion gases, the absence of particles, the progressive nature of the deployment of the gas rate, which is associated with the operation of a monolithic block, and the moderation of the energy released, owing to the low weight of pyrotechnic product required, ought to allow the use of bags with a moderate or low thickness.
The set objective of improvement has been attained. The inventors presently propose pyrotechnic compounds which are highly performing, with reference to the above specifications, and pyrotechnic compounds whose performance is better than that of those compounds corresponding to the compositions of patent application EP 1 216 977.
The pyrotechnic compounds of the invention have a composition which is like that of the compounds according to said patent application EP 1 216 977, in that it comprises:

- an oxidizing charge which comprises at least one inorganic (i.e., without carbon) perchlorate; and
- a crosslinked, oxygen-containing hydrocarbon binder, obtained by crosslinking an elastomer in the presence of at least one crosslinking agent and at least one oxygen-containing plasticizer for said elastomer; said elastomer having a molecular weight of more than 200 000 and being preferably selected from polyesters and polyacrylates. The plasticizer or plasticizers are obviously perfectly miscible in the elastomer, so as not to be exuded at temperature and thus to lower the glass transition temperature of said elastomer.

The pyrotechnic gas generator compounds of the invention accordingly comprise, in their composition, an oxidizing charge (without carbon) and an oxygen-containing hydrocarbon binder, of the above types.
Characteristically, the oxidizing charge in question, which is inorganic (without carbon), which comprises at least one inorganic perchlorate, does not contain a chlorine scavenger, and said at least one oxygen-containing plasticizer for the elastomer is composed of:

- a plasticizer of which the molecular weight is greater than 350 g/mol (its vapor tension is therefore negligible at 120° C.) and the oxygen balance is equal to or greater than −230% (said oxygen balance may thus be balanced in combustion with the charges selected); or
- a mixture of such plasticizers with one another (of at least two plasticizers of this type); or
- a mixture of at least one such plasticizer with at least one other oxygen-containing plasticizer (of another type).

Said at least one present original oxygen-containing plasticizer is composed advantageously of at least one plasticizer having the properties set out above. Mixtures of plasticizers of this kind with other oxygen-containing plasticizers (such as dioctyl adipate and/or dioctyl azelate) may, however, also be present.
The inventors have demonstrated that it is possible to compensate the suppression of the complementary oxidizing charge constituted by the chlorine scavenger (more particularly sodium nitrate), in accordance with the teaching of patent application EP 1 216 977, by the presence of a new oxygen-containing plasticizer, which provides the amount of oxygen needed for effective combustion, thereby preventing the production of CO (whose concentration must be limited in accordance with the standards in force, for the type of application in question). This result was in no way gained beforehand.
The compounds of the invention generally have the disadvantage of emitting hydrogen chloride. At the end of combustion, said hydrogen chloride undergoes condensation with the water formed by the combustion, and may be trapped by the bag. The level of hydrogen chloride emitted by the compounds according to the invention is approximately 0.2 g/g. In order to limit the spreading of the hydrogen chloride within the passenger compartment of the vehicle, the bag may be coated with chlorine scavengers.
The pyrotechnic gas generator compounds of the invention are therefore characterized in that their composition does not contain a chlorine scavenger (complementary oxidizing agent), but comprises at least one specific oxygen-containing plasticizer (which compensates for the absence of said complementary oxidizing agent).
Said at least one specific oxygen-containing plasticizer is advantageously selected from the class of oxygen-containing diesters, more particularly diesters containing ether functions.
Said at least one oxygen-containing plasticizer is very advantageously selected from:

- dibutoxyethoxyethyl adipate (DBEEA),
- dibutoxyethoxyethoxyethyl adipate (DBEEEA),
- dibutoxyethoxyethoxyethyl glutarate (DBEEEG),
- dibutoxyethoxyethoxyethyl phthalate,
- bis(ethylene glycol monobutyl ether) adipate (dibutyl Cellosolve® adipate),
- bis(ethylene glycol monobutyl ether) phthalate (dibutyl Cellosolve® phthalate)
- mixtures of said diesters with one another, and
- mixtures of at least one of said diesters with dioctyl adipate (DOA) and/or dioctyl azelate.

Specified above is the original nature of the second main component (plasticizer) of the crosslinked, oxygen-containing hydrocarbon binder (binder=crosslinked elastomer+plasticizer) of the pyrotechnic compounds of the invention. The aim below is to provide, in a way which is in no sense limitative, specific information on each of the constituents of said compounds, and also on the amount in which they are each employed.
The oxidizing charge comprises at least one inorganic perchlorate. It represents generally from 64% to 94% by weight of the total weight of the compound, advantageously from 72% to 82% by weight of said total weight. This is the main constituent of the compounds of the invention.
Said oxidizing charge advantageously comprises potassium perchlorate and/or ammonium perchlorate. Said oxidizing charge very advantageously comprises ammonium perchlorate. With preference it is composed of ammonium perchlorate. Said ammonium perchlorate is advantageously employed at a plurality of particle sizes, in order to enhance incorporation and distribution in the polymer and to reduce the toxicity of the gaseous effluents during combustion.
The crosslinked, oxygen-containing hydrocarbon binder represents generally from 6% to 20% by weight of the total weight of the compound. It represents advantageously from 8% to 16% by weight of the total weight. It comprises the crosslinked elastomer and said at least one original oxygen-containing plasticizer of the invention. The elastomer in question is an elastomer of high molecular weight: Mw>200 000 g/mol. It is preferably selected from the class of polyesters and polyacrylates. The polyacrylates may have reactive chlorine/carboxyl, chlorine, hydroxyl or epoxy end groups. The polyesters may have reactive hydroxyl end groups. Advantageously they have such reactive hydroxyl end groups. The elastomer present in the composition of the compounds of the invention has been crosslinked via its reactive functions by at least one crosslinking agent (i.e., crosslinker). A crosslinking agent of this kind is composed advantageously of a diisocyanate or triisocyanate.
Generally speaking: said at least one elastomer and said at least one crosslinking agent represent from 3% to 10% by weight, advantageously from 4% to 8% by weight, of the total weight of the compounds of the invention; and/or, advantageously, and said at least one plasticizer represents from 3% to 10% by weight, advantageously from 4% to 8% by weight, of the total weight of said compounds.
In addition to the two constituents described above—the oxidizing charge and the crosslinked, oxygen-containing hydrocarbon binder (elastomer+at least one crosslinking agent+at least one original oxygen-containing plasticizer)—the compounds of the invention may comprise, in their composition, the following additional constituents:

- at least one organic nitro compound (nitrogenous compound) of which the oxygen balance is greater than −30% (which makes it possible in particular to increase the gas yield). An at least one compound of this kind is advantageously selected from nitrogenous nitrates and dinitramides. It is composed very advantageously of guanidine nitrate (NG) or guanylurea dinitramide (GUDN). In general, such at least one compound is employed at between 0 (1) % and 10%, advantageously between 3% and 8%, by weight within the compounds of the invention;
- at least one ballistic catalyst (which makes it possible, among other things, to improve the combustion rate). A person skilled in the art is aware that a ballistic catalyst of this kind is composed of a transition metal oxide having a high specific surface area (which accelerates the decomposition of the oxidizing charge). Such at least one ballistic catalyst is advantageously selected from copper oxide, iron oxide, manganese oxide, and cobalt oxide. In general, such at least one ballistic catalyst is used at between 0 (0.1) % and 6%, advantageously between 0 (0.1) % and 2%, very advantageously between 1% and 2%, by weight within the compounds of the invention;
- at least one wetting agent selected from organosiloxanes and titanates. Such at least one wetting agent is advantageously selected from vinyltris(2-methoxyethoxy)silane, tris(3-trimethoxysilylpropyl)isocyanurate, γ-glycidoxypropyltrimethoxysilane, diethoxydiacetoxysilane, diacetoxy-diethoxysilane, and dibutoxyethoxymethylsilane. Such at least one wetting agent allows a reduction in the viscoelasticity and the residual porosity of the compound. In general, such at least one wetting agent is employed at between 0 (0.1) % and 4%, advantageously between 0.2% and 3%, by weight within the compounds of the invention.

In the light of the above proposals, it is understood that the weight composition of the compounds of the invention is generally as follows:

- 64% to 94%, advantageously 72% to 82%, of an oxidizing charge;
- 6% to 20%, advantageously 8% to 16%, of at least one crosslinked, oxygen-containing hydrocarbon binder;
- 0% to 10%, advantageously 3% to 8%, of at least one organic nitro compound of which the oxygen balance is greater than −30%;
- 0% to 6%, advantageously from 1% to 2%, of at least one ballistic catalyst; and
- 0% to 4%, advantageously 0.2% to 3%, of at least one wetting agent.

The ingredients (oxidizing charge+crosslinked, oxygen-containing hydrocarbon binder+optional ingredients selected from those identified above) of the composition of the compounds of the invention, which are listed above, represent at least 95% by weight of said compounds, generally at least 98% by weight of said compounds. The remainder, if there is a remainder, is generally made up of additives, such as manufacturing auxiliaries (aids).
The compounds of the invention, which are obtained by the method specified hereinafter, may be of various types. They are monolithic compounds which may be solid or mono- or multi-perforated. Said compounds characteristically comprise in their composition an original oxygen-containing plasticizer as described above.
The performance of said compounds is particularly good. Their composition has a high gas yield (approximately 36 mol/kg) with combustion temperatures in the region of 3000 K (these are indeed hot compositions). Their combustion rate is rapid, approximately 45 mm/s at a pressure of 20 MPa. By virtue of their composition, they generate very little solid particulate matter (<2%). According to advantageous variants, moreover, their composition is liable to exhibit an oxygen balance value of between −2% and −3%, which means that they generate only very little toxic gas and that they are entirely suitable for application in the airbags field. The inflation rates per unit area of the compositions of the compounds of the invention are very advantageous. They are approximately 830 mol.K/cm².s.
The method of obtaining the compounds of the invention is advantageously a method by analogy, like that described in patent application EP 1 216 077. Very advantageously it is a method implemented continuously and solventlessly in a twin-screw mixer-extruder.
Said method, more generally, comprises:

- continuously implementing a solventless mixing of the oxidizing charge, the elastomer, the at least one crosslinking agent and the at least one plasticizer, and also, optionally, the at least one organic nitro compound of which the oxygen balance is greater than −30% and/or the at least one ballistic catalyst and/or the at least one wetting agent, and an extrusion of the paste resulting from said mixing;
- heat-treating said extruded paste to carry out the crosslinking of said elastomer.

Characteristically, said at least one plasticizer is composed of or comprises an original plasticizer of the invention.
As indicated above, said mixing and said extruding are advantageously implemented in a twin-screw mixer-extruder.
Furthermore, said method advantageously comprises cutting the extruded paste into charges, and heat-treating said charges (crosslinked charges=compounds of the invention).
Without any limitation whatsoever, one embodiment of the method of the invention may be specified below.
The twin-screw mixer-extruder comprises a mixing and blending compartment, a compression compartment, and an extrusion head. The solid and liquid constituents are introduced into the mixing and blending compartment via two different feed apertures, one feed aperture for the solids and one feed aperture for the liquids, and then are transported and blended, after which the homogeneous paste thus formed is degassed in the compression compartment and then extruded, by means of an extrusion head, into the form of strands, and, lastly, the strands thus formed are cut into charges by means of a chopping apparatus, after which these charges are crosslinked at a temperature, generally, of between 100° C. and 150° C.
The charges formed in this way find their preferred application as a pyrotechnic charge in gas generators intended for inflation of an airbag for occupants of an automotive vehicle. Indeed, the combustion rate of these charges, and also the level of solid residues produced and the level of carbon monoxide and of oxides of nitrogen that are produced, are particularly suitable (see above).
The presently claimed invention will now be illustrated, without any limitation whatsoever.
The compositions of pyrotechnic compounds were evaluated by means of thermodynamic calculations and physical measurements.
These compositions and compounds correspond to compositions and compounds of the prior art (compositions A and B below and corresponding compounds (pellets and extruded charges respectively)) or to compositions and compounds of the invention (compositions 1 and 2 below and corresponding compounds=extruded charges).

Composition a, Cold Type, Prior Art

This composition, with no binder, comprises:

- 49% by weight of guanidine nitrate,
- 48% by weight of basic copper nitrate, and
- 3% by weight of manufacturing auxiliary or auxiliaries.

This composition is of the type according to patents U.S. Pat. Nos. 6,143,102 and 5,608,183.
Pellets were prepared from said composition.

Composition B, Prior Art

This composition, with binder, comprises:

- 59% by weight of ammonium perchlorate,
- 21% by weight of sodium nitrate,
- 5% by weight of guanidine nitrate, and
- 15% by weight of acrylic elastomer, crosslinker, and plasticizer.

This composition is a composition in accordance with patent application EP 1 216 077. It is extruded and crosslinked to generate compounds.

Composition 1 of a Compound of the Invention

It comprises:

- 79.9% by weight of ammonium perchlorate,
- 6.2% by weight of acrylic elastomer+crosslinking agent,
- 6.4% by weight of DBEEEG (plasticizer),
- 1% by weight of vinyltris(2-methoxyethoxy)silane,
- 5.5% by weight of guanidine nitrate, and
- 1% by weight of ferric oxide.

The oxygen balance of this composition is −2.3%. Its theoretical density is 1.73 g/cm³.
Its combustion temperature is approximately 3000 K at 20 MPa and its molar yield is 36.1 mol/kg.
The level of particulates is less than 2%.
According to the particle size of the charges employed, the rate of combustion of the compounds (extrudates) is between 40 and 50 mm/s at 20 MPa (see table below).

Composition 2 of a Compound of the Invention

It comprises:

- 79.9% by weight of ammonium perchlorate,
- 6% by weight of acrylic elastomer+crosslinking agent,
- 6% by weight of DBEEA (plasticizer),
- 1.6% by weight of vinyltris(2-methoxyethoxy)silane,
- 5.5% by weight of guanidine nitrate, and
- 1% by weight of ferric oxide.

The oxygen balance of this composition is −2.4%. Its theoretical density is 1.72 g/cm³.
Its combustion temperature is approximately 3000 K at 20 MPa and its molar yield is 36 mol/kg.
The level of particulates is less than 2%.
According to the particle size of the charges employed, the rate of combustion of the compounds (extrudates) is between 40 and 50 mm/s at 20 MPa (see table below).
The performance characteristics of compositions A, B, 1 and 2 and those of the corresponding compounds are given in table 1 below.
The majority of the values reported in said table 1 are calculated values. The combustion rates and porosities for their part were measured on the compounds obtained from the compositions.

	TABLE 1

	Prior art

	A	B	Invention
	(without binder,	(extruded, according	Examples 1 and 2
Compositions	pelletized)	to EP 1 216 077)	(extruded compounds)

Combustion temperature at 20 MPa (K)	1870	2800	2900 < T < 3100
Gas yield at 1000 K and 20 MPa (mol/kg)	29.6	31.5 (at 0.1 MPa)	34 < Y < 37
Level of solid residues at 1000 K and 1 bar (%)	26.7	16	<2
Combustion rate at 20 MPa (mm/s)	22	38	40 < Vc < 50
Density (g/cm³)	1.98	1.7	1.68 < ρ < 1.75
Porosity %	3	/	<0.2
Inflation power per unit volume (mol · K/cm³)	106.3	151	~185
Inflation rate per unit area (mol · K/cm²· s)	233.9	575	830 < r_i< 840

The results in table 1 above indicate that the inflation power per unit volume of the compositions of the compounds of the invention is approximately 45% to 75% greater than that of the reference compositions of the prior art. In view of the respective densities of the various compounds, the weight can be virtually divided by two to fulfill the same inflation function.
Moreover, the markedly higher combustion rate of the compounds of the invention gives rise to an inflation rate per unit area that is greater than that of the reference compounds. Relative to the pellets of composition A, the characteristic dimensions of the article to be burnt can be doubled in order to provide an equivalent operational duration. The extruded block produced from the compositions of the invention can therefore be very compact and allow the development of small-sized generators containing a low pyrotechnic weight.
Moreover, in the absence of particulates emitted by combustion, the gas generators employing the compounds of the invention can to a certain extent be devoid of particulate filters. By this means the thermal gas losses are reduced.
It is also appropriate to note that the shaping of these compositions in the form of a monolithic block, which has a very high combustion rate, allows low-rate ignition to be realized. The monolithic geometry of the charge also allows the design of forms which deliver a progressive or constant rate, something which is not possible with pelletized charges commonly used in the case of type A compositions.
The compounds of the invention generally have the disadvantage of emitting hydrogen chloride. At the end of combustion, said hydrogen chloride undergoes condensation with the water formed by combustion, and may be trapped by the bag. The level of hydrogen chloride emitted by the compounds of the invention is approximately 0.2 g/g. In order to limit the spreading of the hydrogen chloride within the passenger compartment of the vehicle, the bag may be coated with chlorine scavengers. Moreover, in view of the nature of the composition of the gases, which contain 50% of condensable (H₂O) when the temperature reduces, the events may be reduced in order to increase the scavenging rate by condensation of acidified water within the bag. Following operation, the bag deflates naturally under the effect of the drop in temperature and the condensation of the condensables.
A gas generator for a driver's (side) airbag containing a compound according to the invention (14 g) emits 2.8 g of hydrogen chloride. In the unlikely absence of scavenging, the level of hydrogen chloride is of the order of 700 ppm in a passenger compartment measuring 2.8 m³.

Claims

1. A pyrotechnic gas generator compound whose composition comprises:

an oxidizing charge which comprises at least one inorganic perchlorate; and

a crosslinked, oxygen-containing hydrocarbon binder obtained by crosslinking an elastomer in the presence of at least one crosslinking agent and at least one oxygen-containing plasticizer for said elastomer; said elastomer having a molecular weight of more than 200 000 and being preferably selected from polyesters and polyacrylates;

wherein:

said oxidizing charge does not contain a chlorine scavenger; and

said at least one oxygen-containing plasticizer is composed of a plasticizer of which the molecular weight is greater than 350 g/mol and the oxygen balance equal to or greater than −230%, of a mixture of such plasticizers with one another or of a mixture of at least one such plasticizer with at least one other oxygen-containing plasticizer.

2. The pyrotechnic compound of claim 1, wherein said plasticizer or plasticizers of which the molecular weight is greater than 350 g/mol and the oxygen balance equal to or greater than −230% are selected from diesters having ether functions.

3. The compound of claim 1, wherein said at least one oxygen-containing plasticizer is selected from:

dibutoxyethoxyethyl adipate,

dibutoxyethoxyethoxyethyl adipate,

dibutoxyethoxyethoxyethyl glutarate,

dibutoxyethoxyethoxyethyl phthalate,

bis(ethylene glycol monobutyl ether) adipate,

bis(ethylene glycol monobutyl ether) phthalate

mixtures of said diesters with one another, and

mixtures of at least one of said diesters with dioctyl adipate and/or dioctyl azelate.

4. The compound of claim 1, wherein said oxidizing charge represents 64% to 94% by weight, advantageously 72% to 82% by weight, of the total weight of said compound.

5. The compound of claim 1, wherein said oxidizing charge comprises ammonium perchlorate; in that advantageously said oxidizing charge is composed of ammonium perchlorate.

6. The compound of

claim 1, wherein said crosslinked, oxygen-containing hydrocarbon binder represents 6% to 20% by weight, advantageously 8% to 16% by weight, of the total weight of said compound.

7. The compound of claim 1, wherein said at least one elastomer and said at least one crosslinking agent represent from 3% to 10% by weight, advantageously from 4% to 8% by weight, of the total weight of said compound; and/or, advantageously, and,

said at least one plasticizer represents from 3% to 10% by weight, advantageously from 4% to 8% by weight, of the total weight of said compound.

8. The compound of claim 1, wherein its composition further comprises at least one organic nitro compound of which the oxygen balance is greater than −30%; in that its composition advantageously further comprises at least one organic nitro compound selected from nitrogenous nitrates and dinitramides.

9. The compound of claim 1, wherein its composition further comprises at least one ballistic catalyst, advantageously selected from copper oxide, iron oxide, manganese oxide, and cobalt oxide.

10. The compound of claim 1, wherein its composition further comprises at least one wetting agent selected from organosilanes and titanates, advantageously selected from vinyltris(2-methoxyethoxy)si lane, tris(3-trimethoxysilylpropyl) isocyanurate, γ-glycidyloxypropyltrimethoxysilane, diethoxydiacetoxysilane, diacetoxydiethoxysilane, and dibutoxyethoxymethylsilane.

11. The compound of claim 1, wherein its composition comprises, expressed in percentages by weight:

64% to 94%, advantageously 72% to 82%, of an oxidizing charge;

6% to 20%, advantageously 8% to 16%, of at least one crosslinked, oxygen-containing hydrocarbon binder;

0% to 10%, advantageously 3% to 8%, of at least one organic nitro compound of which the oxygen balance is greater than −30%;

0% to 6%, advantageously from 1% to 2%, of at least one ballistic catalyst; and

0% to 4%, advantageously 0.2% to 3%, of at least one wetting agent.

12. The compound of claim 1, of monolithic type, which is solid or mono- or multi-perforated.

13. A method of obtaining a compound of claim 1, wherein it comprises:

continuously implementing a solventless mixing of said oxidizing charge, said elastomer, said at least one crosslinking agent, and said at least one plasticizer and also, optionally, said at least one organic nitro compound of which the oxygen balance is greater than −30% and/or said at least one ballistic catalyst and/or said at least one wetting agent, and an extrusion of the paste resulting from said mixing;

heat-treating said extruded paste to carry out the crosslinking of said elastomer.

14. The method of claim 13, wherein said mixing and said extruding are implemented in a twin-screw mixer-extruder.

15. The method of claim 13, wherein it comprises cutting the extruded paste into charges and heat-treating said charges.