Patent Application
20090308509
The present invention relates to pyrotechnic gas generation, especially for inflating cushions used in systems for protecting occupants of an automobile. The present invention more precisely relates to pyrotechnic compositions known as cold pyrotechnic compositions rapidly generating clean and non-toxic gases at temperatures acceptable for automobile safety (temperatures qualified as low, that is, under 2200 K). The present invention also relates to a method for producing such pyrotechnic compositions.
Within the scope of automobile safety, the pyrotechnic compositions employed in gas generators must provided the quantity of gas necessary for putting in place the inflatable cushion in an extremely short time, typically between 10 and 40 milliseconds. Also, the gases generated must be clean, that is exempt from solid particles (likely to constitute hot points which might damage the wall of said cushion) and non-toxic, that is having low content of carbon monoxide, nitrogen oxides and chlorinated products.
Various types of pyrotechnic compositions have already been proposed to date.
Currently, those pyrotechnic compositions appearing to offer the best compromise in terms of gas temperature, gas yield, rate of particles emitted and toxicity contain, as principal ingredients, guanidine nitrate (GN) and basic copper nitrate (BCN). U.S. Pat. No. 5,608,183 describes such compositions, obtained by wet method.
These compositions do however have the disadvantage of having relatively slow combustion speeds, less than or equal to 20 mm/s at 20 MPa and being difficult to ignite. Similarly, they are very difficult to use in hybrid and side generators which need very brief operating time, between 10 and 20 milliseconds.
It has been proposed, according to the prior art, to add perchlorates to such pyrotechnic compositions based on guanidine nitrate (GN) and basic copper nitrate (BCN):
In reference to the technical problem of a speed increase in combustion of pyrotechnic compositions based on guanidine nitrate (GN) and basic copper nitrate (BCN), the addition of perchlorate is therefore not a satisfactory solution per se.
The roller compacting and dry granulation method is also known, implemented in different contexts, for treating powders, generally mixtures of powders. Such mixtures of powders, coming from a dry powder mixer, are transported, for example by a metering screw, to supply a cylinder compacter. Such a compacter is composed of two rotating cylinders, placed in rotation at a defined speed, in the opposite direction. The powder mixture is pushed by the metering screw between said two cylinders. A known effort is applied to the cylinders. Accordingly, the material passing between them, at a given rate, is compacted in the form of a flat plate. Such an operation places a strong rate of compression and shearing on the mixture which improves the closeness between the constituents. The compacted product, originating from the compacter is then broken and forced through a grater to generate granules. Such granules generally turn out to be easier to handle than the starting powders. A single device generally successively manages both compacting and granulation.
Within the scope of the present invention, the inventor has shown, in reference to the technical problem of the increase in combustion speed, that the major interest is to carry out dry roller compacting in the context of the making of pyrotechnic compositions of guanidine nitrate and basic copper nitrate type. A real synergy has surprisingly been observed: the positive effect (on the combustion speed) due to intervention of a limited quantity of perchlorate (without harmful effect on the combustion temperature) is potentialised by carrying out dry roller compacting; use of which has per se no substantial effect in the absence of perchlorate. In support of this affirmation, comparative data hereinbelow can be supplied on combustion speeds at 20 MPa:
GN+BCN≦20 mm/s
GN+BCN+compacting 20-22 mm/s
GN+BCN+KClO4 (example 4 hereinbelow) 32 mm/s
GN+BCN+KClO4+compacting (example 5 hereinbelow) 38.2 mm/s.
According to its first object, the present invention therefore relates to pyrotechnic compositions generating gases which combine two characteristics. Said pyrotechnic gas compositions:
said charges representing at least 95% by mass (generally at least 98 by mass) of the charges present; and
The pyrotechnic compositions of the invention, including a specific reducing charge (in the form of at least one organic nitrogen compound) and a specific oxidizing charge (in the form of at least one basic metal nitrate), include a limited quantity of a specific second oxidizing charge (in the form of at least one alkaline metal perchlorate) and are obtained on completion of a method including a dry roller compacting step of the pulverulent mixture including said specific reducing and oxidizing charges.
Dry roller compacting is carried out, as known per se, in a cylinder compacter, generally at a compacting pressure of between 108 and 6.108 Pa.
The method for producing the pyrotechnic compositions of the invention which characteristically includes a dry roller compacting step, is described in detail further on in the present text.
It can be carried out according to different variants (with a characteristic step of “simple” roller compacting followed by at least one additional step, with a characteristic step of roller compacting coupled with a forming step . . . ) and the pyrotechnic compositions of the invention therefore exist in different forms.
In fact:
The pyrotechnic compositions of the invention are therefore likely to exist in the form of:
In a non-limiting manner it can be indicated here that:
It can also be pointed out here that the followings especially form part of the first object of the invention:
It is now proposed to give some precise details, non-limiting, on the ingredients making up the pyrotechnic compositions of the invention and their occurrence rate within said compositions.
The at least one organic nitrogen compound, making up the reducing charge, can especially be selected from guanidine nitrate, nitroguanidine, guanyl urea dinitramide and their mixtures. It consists advantageously of guanidine nitrate (GN).
The at least one basic metal nitrate, constitutive of the (first) oxidizing charge can especially be selected from basic copper nitrate, basic zinc nitrate, basic bismuth nitrate and their mixtures. It consists advantageously of basic copper nitrate (BCN).
The at least one alkaline metal perchlorate can especially be selected from potassium perchlorate, sodium perchlorate and their mixtures. It consists advantageously of potassium perchlorate (KClO4).
According to a preferred variant, the principal constitutive ingredients of the compositions of the invention are therefore:
With respect to the respective quantities of each of said ingredients, there is generally, independently and advantageously in combination:
According to a preferred variant, the pyrotechnic compositions of the invention include:
The pyrotechnic compositions of the invention, in the form of shaped objects, granules and pellets, consist essentially (at least at 95% by mass, generally at least at 98% by mass), even exclusively (at 100% by mass), of the principal constitutive ingredients (charges) identified hereinabove: said at least one organic nitrogen compound, said at least one basic metal nitrate and said at least one alkaline metal perchlorate. Said ingredients can in fact themselves constitute at 100% the charges of said pyrotechnic compositions (this is generally the case), or even constitute said pyrotechnic compositions at 100%. However, the presence, within the compositions of the invention, of other charges in minimal quantities could not be excluded (in any case, the charges identified hereinabove represent at least 95% by mass, generally at least 98% by mass of charges present) and/or that of at least one additive (fabrication auxiliary type).
The pyrotechnic compositions of the invention, in the form of extruded monolithic blocks, include those principal constitutive ingredients (charges) identified hereinabove (plus possibly, other charges in minimal quantities) in a dry gel. This gel, extrudable per se or mixed with a solvent, entered upstream to allow extrusion. It occurs in an efficient quantity (to allow extrusion), though limited so as not to substantially affect the performances of the compositions of the invention.
The extruded monolithic blocks of the invention generally include no more than 10% by mass of such a dry gel. They advantageously include from 4 to 6% by mass. In their midst, the synergy of the invention develops with the same intensity.
The presence of at least one additive is also not excluded from this context. The principal constitutive ingredients (charges) and dry gel generally represent at least 95% by mass, very generally at least 98% by mass (or even 100% by mass) of said compositions.
With respect to the nature of said gel, it is not per se original. Said gel is generally selected from cellulosic gels, gels obtained from acrylic elastomers, ethylene-vinyl-acetate copolymers with a high acetate rate (including over 60% by mass of acetate units), polyester polymers, and their mixtures. Said gel advantageously consists of a sodium carboxymethyl cellulose gel.
According to its second object, the present invention relates to the method for obtaining pyrotechnic compositions such as described hereinabove; a method which characteristically comprises dry compacting of powders.
Said method actually comprises:
Precise details on the nature of the ingredients in question and their rate of respective presence are specified hereinabove in the present text.
The ingredients making up the desired pyrotechnic compositions occur in the powdered state. Advantageously, said powders have fine granulometry, less than or equal to 40 μm. Said granulometry (value of the median diameter) is generally between 3 and 40 μm.
The steps for dry mixing of the powders and dry roller compacting of the mixture obtained are executed conventionally. With respect to dry roller compacting, it has been explained that it is carried out by passing the mixture of the powders between two cylinders, the pressure exerted being generally between 108 and 6.108 Pa. “Simple” compacting with two cylinders having non-machined external surfaces or compacting coupled with forming with cylinders, whereof the external surface of at least one of the two is machined to present alveoli, is carried out.
It is incidentally noted here that the originality of the invention as claimed is not based on the originality per se of the method in question, but rather the originality of the execution of said method with particular mixtures of powders.
The method of the invention can be limited to these two successive steps of mixing and dry roller compacting, in the context hereinbelow: that of directly obtaining shaped objects, in the hypothesis of carrying out roller compacting coupled with forming (the external surface of at least one of the compacting cylinders presenting alveoli).
The method of the invention, in addition to said two mixing and (“simple”) dry roller compacting steps, can include:
a) a dry granulation step (the compacted mixture of powders is forced mechanically by a rotor via a grater acting as a screen whereof the mesh generally varies from 500 μm to 3 mm). Roller compacting and granulation can be carried out inside a single device or in two independent devices. The resulting pyrotechnic compositions are then in the form of granules (see above);
b) a dry granulation step (see above) followed by pelleting (dry compression during which the granules undergo pressure generally of between 4.108 and 109 Pa. It is incidentally noted here that feeding the pelleting mould impressions is much more easily carried out with granules than with starting powders). The resulting pyrotechnic compositions are thus in the form of pellets (see above);
c) a dry granulation step (see above) followed by mixing the granules obtained with an extrudable binder and extrusion of said mixture. The pyrotechnic compositions finally obtained are in the form of extruded monolithic blocks loaded with granules.
The variants of the method of the invention which include steps b) and c) hereinabove are particularly preferred. Within the scope of said two variants, the method of the invention includes the steps of (“simple”) roller compacting and dry granulation of the mixture of the starting powders.
Irrespective of the exact variant of execution of the method of the invention, it has proven opportune, especially in reference to increasing the combustion speed of the prepared pyrotechnic composition, to recycle at least in part the fines generated at the compacting step and/or at that of granulation, when said granulation step is carried out. Carrying out a fines recycling rate of between 10 and 30% is specified.
The invention will now be illustrated, in no way limiting, by the examples hereinbelow. More precisely, examples illustrating two variants of the invention (pyrotechnic compositions in the form of pellets (example 5) and extruded monolithic blocks (example 7)), are proposed, to be considered in parallel with comparative examples.
The powders (raw materials) utilised have fine granulometry: a median diameter of around 20 μm for KClO4, 4.5 μm for BCN, 10 μm for GN.
Such powders do not flow and cannot therefore per se be used in industrial pelleting (it is very difficult to fill the pelleting mould impressions).
Table I hereinbelow shows formulation examples as well as the thermodynamic and ballistic performances of pellets (of around 2 mm in thickness) obtained by pelleting (performed at 5.108 Pa) of the powder mixtures not pre-compacted.
Table II hereinbelow shows the contribution of the roller compacting method on the ballistic performances of the composition.
Applying the roller compacting and dry granulation method to the composition according to example 4 causes a rise in combustion speed to 20 MPa of the order of 20%. This increased speed is attributed to improved closeness of the ingredients after passing through the compacter. The roller compacting phase causes compression and shearing stresses on the mixture, improving the quality of the mixture. Assays conducted at various pressures on the compacter have confirmed this point. To a certain extent, the ballistics of the formulation is therefore adjustable by the pressure applied to the rollers during the compacting phase.
Also, the roller compacting and granulation phase generates fines (of low granulometry), which can be reintroduced in the system. This reintroduction also generates greater increase in combustion speed, which may reach 40 mm/s to 20 MPa, in the case of the composition of example 5 with 20% of recycling fines.
According to example 6, the powders are directly introduced (with the binder) to the device.
According to example 7, said powders were pre-compacted and granulated under the conditions specified hereinabove for example 5 and the resulting granules are introduced (with the binder) to the device.
The performances of the two compositions, prepared in the form of monolithic blocks, are also specified in Table III hereinbelow.
The intervention of the binder is definitely harmful to performance, in terms of combustion speed, of the composition (of example 7 relative to that similar to example 5). All the same, also in this context of extruded product (as for the pellet product), carrying roller compacting on dry powders results in significantly improving said combustion speed. The gain obtained is of the same order of magnitude, specifically around 20%.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0510437 | Oct 2005 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FR2006/051026 | 10/12/2006 | WO | 00 | 8/6/2009 |
