Pyrotechnic Grains of Large Dimensions, and Their Production and Use

Abstract

The present invention provides pyrotechnic grains of composition comprising at least one oxidizing charge and at least one reducing charge and no binder. In characteristic manner, said grains are in the form of substantially cylindrical blocks: with a thickness of more than 5 mm; with an equivalent diameter of 10 mm or more; and with porosity in the range 1% to 8%, limits included. Said grains, advantageously based on guanidine nitrate and basic copper nitrate, are suitable for the slow, low-pressure generation of gas over an extended period.

Description

The present invention relates to large pyrotechnic grains based on binder-free formulations, to their production, and to their use. More precisely, it relates to:

• • pyrotechnic grains which are in the form of blocks (monolithic blocks) with large dimensions;
  • a method of producing them; and
  • a method of generating gas comprising the combustion of said grains.

Gas generation has a number of applications, both in the civilian and in military fields.

In the automotive safety sector, gas generators have been developed with a short operational duration, which use compositions with a high combustion rate. Those compositions are in the form of thin pellets (1 mm [millimeter] to 3 mm) with a small diameter (3 mm to 7 mm). This provides them with a large combustion surface, encouraging a high rate over a short period (necessary for airbag applications in automotive safety, of the driver's airbag type). The porosity of such pellets is generally less than 4%. Pelletization operations carried out to produce said pellets are well known, similar to those used in medicinal formulations. Compositions based on guanidine nitrate and basic copper nitrate have been used in that context (United States patent U.S. Pat. No. 5,608,183). An attempt has even been made to improve the combustion rate (U.S. Pat. No. 6,143,102) since said combustion rate of such compositions is rather low, compared with that of other compositions used in the same field.

In similar sectors (automotive safety, to inflate curtain types of airbag which need to remain inflated for a long period), or in completely different sectors, gas generators have been proposed, especially for:

• • inflating or deforming a structure;
  • propelling a liquid or a gas;
  • actuating a jack or some other mechanical actuator;
  • causing a displacement.

Generating and supplying gas has also been exploited to operate fire extinguishing devices (FR-A-2 864 905 and FR-A-2 870 459).

That relates to the field of slow gas generation. For slowly generating and supplying gas under pressure, a gas stored under pressure is generally used. The use of chemical reactions or the combustion of pyrotechnic cartridges has also been mentioned. With the combustion of pyrotechnic cartridges, the flow rate of the gas that is generated is controlled by selecting the characteristics of the propellant, the geometry of the cartridges, and/or the conditions for ejecting said generated gas, etc.

The present invention falls within the context of generators operation providing long duration, low flow rate, and low operating pressure.

In such a context, the present invention proposes pyrotechnic grains that are simple in shape and that satisfy the following specifications:

a) a long combustion period (50 ms [millisecond] to 1 min [minute]) due to a slow combustion rate (to ensure a long period of operation) and/or to a ratio of combustion area over burning thickness that is low, and/or to a low operating pressure (for use in low pressure generators and to encourage a low combustion rate);

b) low dependency of the rate of combustion on the operating temperature of the grain, to ensure good ballistic behavior under extreme temperature conditions from −60° C. to 110° C. (hence an extension of the application range (see below), especially in aeronautics);

c) suitable mechanical behavior (without the intervention of a binder, the presence of which would deleteriously affect ballistic performance), especially encouraging the use of said grains in an aggressive vibratory environment.

The pyrotechnic grains of the invention are also advantageously non-aggressive for the gas generator in which they are used and for the associated system (i.e., on combustion, they generate neither particles nor corrosive species). In the same manner, the generated species are advantageously of low toxicity.

Referring to point a) of the above specifications, the skilled person will understand that the pyrotechnic grain of the invention must be thick and have a small combustion area. Thus, it must be a large grain, larger than the pellets mentioned above.

Referring to point b) of the above specifications, the skilled person will know that said grain must have low porosity.

Said skilled person will also be aware of the difficulties in fabricating a large grain of that type (pyrotechnic grain) with low porosity, with appropriate mechanical behavior, and without using a binder. He knows that pyrotechnic grains of large dimensions and without binder in their composition have generally porosity values which are not acceptable in reference to their ballistic properties, more particularly their combustion rate. This point is all the more crucial than their operating temperature is high.

Surprisingly, the Applicant has obtained pyrotechnic grains, from specific oxidizing and reducing charges, which are simple in shape and that satisfy the above specifications. Such pyrotechnic grains constitute the first aspect of the present invention.

Said pyrotechnic grains of the invention have a composition that comprises at least one (specific) oxidizing charge and at least one (specific) reducing charge, but that includes no binder. In this respect, they are pyrotechnic grains of the pellet type mentioned above. The composition of the pyrotechnic grains of the invention generally essentially consists of at least one such (specific) oxidizing charge and at least one such (specific) reducing charge. It generally essentially consists of a (specific) oxidizing charge (an oxidizer) and a (specific) reducing charge (a reducer).

Said at least one specific oxidizing charge is selected from ammonium, potassium, sodium, barium, strontium, and basic copper nitrates, and ammonium, potassium, and sodium perchlorates, and mixtures thereof. Advantageously, it comprises basic copper nitrate and/or potassium perchlorate. Highly advantageously, it consists of basic copper nitrate or potassium perchlorate. Preferably, it consists of basic copper nitrate.

Said at least one specific reducing charge is selected from nitroguanidine, guanidine nitrate, and mixtures thereof. Preferably, it consists of guanidine nitrate.

In characteristic manner, said pyrotechnic grains of the invention are in the form of substantially cylindrical blocks:

• • with thickness of more than 5 mm;
  • with an equivalent diameter of 10 mm or more; and
  • with porosity in the range 1% to 8%, limits included.

Said (monolithic) blocks are large blocks, which are simple in shape and of low porosity.

Said blocks have a substantially cylindrical shape. Generally, but not exclusively, they are circular cylinders or near (quasi) circular cylinders.

The shape of the grains of the invention is the signature of the last step of the method by which they are obtained: a compacting step (see below).

Said blocks may in particular consist of solid (quasi) cylinders or (quasi) cylindrical sleeves.

The grains in question are large objects (thickness more than 5 mm, equivalent diameter (or, clearly, diameter if it is a perfect circular cylinder) of 10 mm or more), and are relatively dense (porosity in the range 1% to 8%).

Non-limiting dimensions of said grains are detailed below.

Generally and independently:

• • the thickness of the blocks is 75 mm or less; or
  • the equivalent diameter of said blocks is 75 mm or less.

In general, said thickness and said equivalent diameter are both 75 mm or less.

According to a first advantageous embodiment, independently

• • the thickness of the blocks is 7.5 mm or more; or
  • the equivalent diameter of said blocks is 20 mm or more.

In general, in the context of said first advantageous embodiment, said thickness and said equivalent diameter are both as precised above.

According to a second advantageous embodiment, independently:

• • the thickness of the blocks is in the range 10 mm to 60 mm (values of 10 mm and 60 mm being included); or
  • the equivalent diameter of said blocks is in the range 10 mm to 60 mm (values of 10 mm and 60 mm being included).

In general, in the context of this second advantageous embodiment, said thickness and said equivalent diameter are both in the range 10 mm to 60 mm (values of 10 mm and 60 mm being included).

In particular, pyrotechnic blocks of the solid circular cylinder type and with the dimensions given below, have been produced in the context of the invention:

• • diameter of 25 mm and thickness of 10 mm;
  • diameter of 50 mm and thickness of 50 mm.

Further, it has been stated that the porosity of the blocks is in the range 1% to 8% (it should be noted here that this parameter, expressed as a percentage, corresponds to the ratio between the actual density and the theoretical density; it is in fact the difference from the theoretical density).

Said porosity is advantageously 5% or less. It should be recalled that the lower said porosity, the lower the dependency of the rate of combustion on the operating temperature of the grain.

It should be indicated here that, in a non-limiting manner, blocks, which are of the circular cylinder type with the dimensions described above, have been respectively obtained with the porosities indicated below:

• • porosity of about 5% (with diameter=25 mm and thickness=10 mm);
  • porosity of about 7% (with diameter=thickness=50 mm).

As indicated above, the pyrotechnic grains of the invention, which are simple in shape, being substantially cylindrical, may in particular consist of solid cylinders or sleeves. The equivalent internal diameter of said sleeves is advantageously more than 10 mm; preferably, it is in the range 12 mm to 35 mm (that implies obviously a greater equivalent diameter of the sleeve).

Referring to the composition of the pyrotechnic grains of the invention, the following non-limiting details can furthermore be given.

The said composition of the pyrotechnic grains of the invention advantageously includes basic copper nitrate as the oxidizing charge and guanidine nitrate as the reducing charge. Highly advantageously, it comprises basic copper nitrate as the only oxidizing charge and guanidine nitrate as the only reducing charge. In the automotive safety field (driver type airbags), said basic copper nitrate/guanidine nitrate combination is known to have a combustion rate that is rather low relative to that of other compositions used in this field. In the context of this highly advantageous embodiment, said composition generally comprises:

• • 45% to 55% by weight of guanidine nitrate;
  • 40% to 50% by weight of basic copper nitrate; and
  • 0 to 5% by weight of additives.

The additives in question, which are optionally present, are chemical compounds that are known per se as well as for their beneficial involvement in the general production of pyrotechnic grains. In particular, they may be:

• • slagging agents supplied with the starting materials: oxidizing charge(s) and reducing charge(s) (for example: alumina);
  • processing aids of the pressing aid or unmolding aid type (for example: silica, calcium stearate, mica, etc).

In a second aspect, the present invention provides the production of large pyrotechnic grains, as described above.

The method in question comprises a series of steps that are known per se.

Surprisingly, it has been possible to find operating conditions for this series of steps (which are known per se) that enable, from the selected oxidizing and reducing charges, grains of the invention to be obtained without the involvement of a binder, and having simultaneously large dimensions, low porosity, and also acceptable mechanical properties (which can tolerate the vibrational environment of an automobile or aircraft).

The method of producing pyrotechnic grains of the invention comprises carrying out the following steps in succession:

• • intimately dry or wet mixing said oxidizing and reducing charges used in the powder state;
  • dry or wet granulating the powdered mixture obtained;
  • sizing the granules obtained; and
  • shaping the retained sized granules by compacting under conditions that produce the anticipated result, i.e. the production of pyrotechnic grains having the dimensional and porosity characteristics defined above.

The conditions in question are principally

• • the characteristics of the starting powders;
  • the characteristics of the retained sized granules; and
  • the compacting parameters.

The Applicant has discovered that suitable conditions may be combined to obtain the desired result.

Before detailing said conditions (parameters) of the method of the invention, it should in general be indicated:

• • that the powders are advantageously intimately dry mixed
  • that granulation is also advantageously a dry granulation (independently of the manner of said intimate mixing);
  • that the granules are advantageously sized by sieving; and
  • that at least one shaping additive (a pressing aid, such as calcium stearate or mica) may advantageously be added to said sized granules before shaping them by compacting.

In characteristic manner, to obtain the anticipated result, more particularly to obtain a slow rate of combustion, at least one of the charges used in the powder state has a median diameter that is substantially larger than that which is used in the prior art to prepare pellets (of small dimensions). Thus, it is recommended in the context of carrying out the method of the present invention to use at least part of the reducing charge(s) with a median diameter that is substantially higher (×4, ×6) than that known to be used in the preparation of prior art pellets. It is recommended that a reducing charge with a median diameter of 200 μm [micrometer] or more be used. In particular, it is recommended that guanidine nitrate with a median diameter of 200 μm to 300 μm be used. Said guanidine nitrate is conventionally used for the fabrication of pellets, with a median diameter of 50 μm.

In characteristic manner, to obtain the anticipated result, more particularly to obtain low porosity, the granules selected are those that have a median diameter of more than 200 μm, preferably in the range 500 μm to 1000 μm. It is thus recommended, in the context of the present invention, that basic copper nitrate/guanidine nitrate granules with a median diameter in the range 500 μm to 700 μm or potassium perchlorate/guanidine nitrate granules with a median diameter in the range 500 μm to 1000 μm be selected.

The skilled person is capable of optimizing this parameter (granule size) as well as the others (powder mean diameter and compacting parameters) as a function of the exact nature of the products in question.

Regarding the compacting parameters, in general, to obtain the anticipated result, more particular to obtain low porosity, a pressure in the range 15×10⁶ Pa [pascal] to 35×10⁶ Pa is applied for 0.1 s [second] to 30 s.

Compacting is clearly carried out in a mold the shape of which corresponds to that envisaged for the prepared pyrotechnic grain (substantially cylindrical shape).

The method of the invention is very advantageously carried out with all the above advantageous embodiments, i.e.

• • a median diameter of the at least one reducing charge of 200 μm or more;
  • a median diameter of the granules of more than 200 μm; and
  • a compacting step carried out at 15-35×10⁶ Pa during 0.1 to 30 s.

The method of the invention is advantageously carried out with a combination of basic copper nitrate or potassium perchlorate/guanidine nitrate; highly advantageously, it is carried out with a basic copper nitrate/guanidine nitrate combination. In the context of the present invention, the combustion rate of compositions based on these two elements (BCN/NG) is reduced. The teaching of the present invention thus goes against that of U.S. Pat. No. 6,143,102.

In a third aspect, the present invention provides a method of generating gas, the method comprising, in conventional manner, the combustion of at least one pyrotechnic grain. In characteristic manner, said method of the invention comprises the combustion of at least one pyrotechnic grain as described above (large pyrotechnic grain) and/or produced using the method described above.

Said gas generation method is carried out in a suitable device which, in particular, has a combustion chamber adapted to the dimensions of the original pyrotechnic charge, based on large pyrotechnic grains. In common with currently known devices, said device comprises:

• • elements for retaining, positioning, and adjusting the pyrotechnic charge based on large pyrotechnic grains (which constitutes the first aspect of the present invention);
  • ignition elements;
  • at least one safety cap ensuring depressurization of the generator in the event of accidental overpressure;
  • at least one nozzle; and
  • a filter to retain the majority of the solid combustion residues.

Interesting applications for the pyrotechnic grains of the invention are specified below, i.e. fields in which gas generation must be maintained for a relatively long period.

The following applications are particularly contemplated

• • the generation of gas intended to propel a fluid, either directly or via a separating membrane, for example, to extinguish fire in civilian or military aircraft engines, to inert bays in civilian or military aircraft, to extinguish fire in a building, in all types of civilian vehicles (automobiles, etc) or military vehicles (tanks, etc), to inject grease or active principle, etc;
  • the generation of gas in a pyrotechnic actuator intended to actuate a jack, for example (to open an aircraft door, for example);
  • the generation of gas to inflate or deform inflatable structures that are deformable, such as aircraft escape chutes, automotive safety anti-submarining structures, or releasers.

Finally, examples of the present invention are described below.

EXAMPLE 1

Pyrotechnic grains of the invention were prepared from the following starting materials:

• • basic copper nitrate (BCN): powder with a median diameter of 8 μm;
  • guanidine nitrate (GN): powder with a median diameter of 218 μm;
  • alumina (slagging agent): powder with a median diameter of 1 μm.

Said powders were dry mixed in a Sofraden® type powder mixer (a Guédu® type mixer would also have been suitable) in the following proportions:

BCN 44.87%

GN 52.44%

Alumina 2.69% (weight %).

The intimate mixture of powders obtained was dry granulated in a Sahut Conreur® type granulator (an Alexanderwerk® type granulator would also have been suitable). Said intimate mixture of powders was forced through a screen. The granulometric distribution of the granules obtained was from 10 μm to 1000 μm.

The granules obtained were then sieved, and those with a mean diameter of more than 200 μm were selected.

Said selected granules were then compacted on a 35 (metric) tonne single action press. The compacting pressure applied was 230 bars (23×10⁶ Pa). It was applied for 10 s.

Pyrotechnic grains in accordance with the invention were obtained, which grains were cylindrical monoliths (shape of mold) with a thickness of 10 mm, a diameter of 24.6 mm, and porosity of 3%.

EXAMPLE 2

In similar manner, cylindrical grains were prepared as follows:

• • with a thickness of 50 mm;
  • with a diameter of 50 mm;
  • with porosity of 7%.

EXAMPLE 3

Prior Art

In similar manner, prior art pellets, which were cylindrical grains, were prepared as follows:

• • with a thickness of 1.8 mm;
  • with a diameter of 6.3 mm;
  • with porosity of 2%.

The grains of said Examples 1 to 3 were tested in generators with the same architecture. The graphs of the operating pressures of said generators as a function of time are given in the accompanying FIGS. 1 to 3.

With the prior art pellet, as expected, an operating peak was obtained (FIG. 3). With the blocks of the invention, the desired spread over time was observed (FIGS. 1 and 2).

Further, the mechanical behavior of the grains of the invention as regards aeronautical vibrations was tested, with satisfactory results (standard RTCA DO-160D: “Environmental Conditions and Test Procedures for Airborne Equipment”).

Claims

1. A pyrotechnic grain, the composition of which comprises at least one oxidizing charge selected from ammonium, potassium, sodium, barium, strontium, and basic copper nitrates, and ammonium, potassium, and sodium perchlorates, and mixtures thereof;

at least one reducing charge selected from nitroguanidine, guanidine nitrate, and mixtures thereof; and

no binder,

wherein it is in the form of a substantially cylindrical block: with a thickness of more than 5 mm; with an equivalent diameter of 10 mm or more; and with porosity in the range 1% to 8%, limits included.

2. The pyrotechnic grain according to claim 1, wherein the thickness of said block is 7.5 mm or more and/or the equivalent diameter of said block is 20 mm or more.

3. The pyrotechnic grain according to claim 1, wherein the thickness and/or equivalent diameter of said block is in the range 10 mm to 60 mm, limits included.

4. The pyrotechnic grain according to claim 1, wherein the porosity of said block is 5% or less.

5. The pyrotechnic grain according to claim 1 wherein it is in the form of a sleeve of internal equivalent diameter that is advantageously more than 10 mm.

6. The pyrotechnic grain according to claim 1, wherein said at least one oxidizing charge consists of basic copper nitrate.

7. The pyrotechnic grain according to claim 1, wherein said at least one reducing charge consists of guanidine nitrate.

8. The pyrotechnic grain according to claim 1, wherein its composition comprises:

45% to 55% by weight of guanidine nitrate;

40% to 50% by weight of basic copper nitrate; and

0 to 5% by weight of additives.

9. A method of producing a pyrotechnic grain according to claim 1, the method comprising: the characteristics of the starting powders, the retained sized granules and the compacting parameters being selected to produce said pyrotechnic grains having said dimensional and porosity characteristics.

intimately dry or wet mixing said oxidizing and reducing charges used in the powder state;

dry or wet granulating the powdered mixture obtained;

sizing the granules obtained; and

shaping the retained sized granules by compacting;

10. The method according to claim 9, wherein the intimate mixture or (and) granulation is (are) carried out by a dry method.

11. The method according to claim 9, wherein the granules are sized by sieving.

12. The method according to claim 9, wherein it comprises:

using at least one reducing charge with a median diameter of 200 μm or more; and/or, advantageously and

sieving the granules to retain those with a median diameter of more than 200 μm; and/or, advantageously and

shaping by compacting at a pressure in the range 15×106 Pa to 35×106 Pa for 0.1 s to 30 s.

13. The method according to claim 9, wherein it is carried out with guanidine nitrate as the reducing charge and basic copper nitrate or potassium perchlorate as the oxidizing charge.

14. A method of generating gas, comprising the combustion of at least one pyrotechnic grain, wherein said at least one pyrotechnic grain is a grain according to claim 1 and/or a grain produced by a method comprising: the characteristics of the starting powders, the retained sized granules and the compacting parameters being selected to produce said pyrotechnic grains having said dimensional and porosity characteristics.

intimately dry or wet mixing said oxidizing and reducing charges used in the powder state:

dry or wet granulating the powdered mixture obtained:

sizing the granules obtained: and

shaping the retained sized granules by compacting: