FR2781878A1 - PROCESS FOR IMPLEMENTING A PYROTECHNIC SUBSTANCE AND PYROTECHNIC INITIATOR OBTAINED WITH SUCH A METHOD - Google Patents

Abstract

The invention concerns a method for preparing at least one pyrotechnic substance to produce a pyrotechnic component, in particular an initiator with resistive bridge. Said method is characterised in that it consists in first suspending at least a pyrotechnic substance in powder form in an inert liquid or pasty binding agent, capable of being hardened by polymerisation; then placing the resulting pyrotechnic material in liquid or pasty form into the component; carrying out in situ a polymerisation step by radiation or bombardment completed by heating the composition to harden the binding agent carrying the pyrotechnic substance(s).

Description

PROCESS FOR IMPLEMENTING A PYROTECHNIC SUBSTANCE AND

  The field of the present invention is that of methods of using pyrotechnic substances to produce a pyrotechnic component.

  Different methods are known for producing the pyrotechnic components.

  Such a component generally comprises a pyrotechnic substance sensitive to shock or to heating (primary explosive) which is intended to initiate a reinforcing pyrotechnic composition (flame for an igniter or shock for a detonator). This primary explosive can be mixed with the reinforcing composition (as in patents EP600791 and

  FR2720493) or adjacent to it (as in patent GB960186).

  The sensitive substance most often includes a primary explosive, such as lead trinitroresorcinate, lead azide or silver azide.20 Electrically initiated pyrotechnic components use a resistive element, such as a hot wire or a wafer semi conductive. Patents EP600791 and FR2720493 describe such electrical initiators intended more particularly to be used for triggering automobile safety devices (inflatable safety cushions commonly called "airbags").

  Known initiators have drawbacks.

  Thus the implementation of primary explosives generates

  significant risks to the safety of personnel.

  In fact, the current components impose the contact of very sensitive primary explosive substances with the resistive element. This contact, which is desired as intimately as possible, requires the use of explosive materials35 of small particle size and therefore of poor flowability. This makes loading difficult and increases the risk of accidental initiation which is further increased by excessive pollution of industrial means of loading with the primary substance. In addition, the loading of a pulverulent material implies a minimum of filling of the bottom of the cell of the component in order to completely cover the resistive element. This results in an overabundant placement of the primary substance which penalizes both the manufacturing cost and safety.10 Finally, the pulverulent material must be compressed. The resistive element can undergo during this compression considerable stresses which can cause its rupture. Another known embodiment provides for a suspension of the sensitive pyrotechnic substance in a liquid solvent in which a binder (of active type or not) is dissolved. The usual solvents are water, alcohol or

  acetone. The solvent is removed by heating or drying after placing the composition. Such a method has drawbacks.

  When the solvent used is water, it is difficult to remove, which leads to long drying stages and which can degrade the performance of the compositions. When the solvent is volatile, its elimination generates toxic and / or explosive vapors (alcohol, acetone ...)

dangerous for personnel.

  Modern pyrotechnic components, in particular those used in the field of motor vehicle safety, bring still other constraints. The resistive element is now produced in the form of a semiconductor bridge, the dimensions of the resistive bridge are therefore very small (of the order of 50 micrometers x 150

  micrometers) and it therefore becomes essential to use a sensitive pyrotechnic substance which is of a very fine particle size, less than the dimensions of the bridge (particle size 35 less than 50 micrometers).

  Such a choice increases the difficulties of placing the primary substance, the sensitivity of which is increased by the fineness of the particle size. Finally, we are now seeking to make the components used in the automotive field non-polluting and non-toxic, which excludes the use of primary lead-based explosives. Now known and non-toxic energetic pyrotechnic compositions (such as zirconium / potassium perchlorate or boron / nitrate of

  potassium) cannot be, in their current industrial forms, directly initiated with a semiconductor bridge.

  Indeed, the common and inexpensive forms of these compositions have a large particle size and are coated. The use of a very fine particle size for these pyrotechnic compositions is not satisfactory industrially (cost too high) and increases the risks (increased sensitivity and pollution of tools by active dust). It is the object of the invention to overcome such

disadvantages.

  Thus, the invention provides a method of using a pyrotechnic substance which allows both

  reduce the risks for the personnel involved in its manufacture and reduce the costs of producing the pyrotechnic components such as the initiators.

  The method according to the invention also makes it possible to define a pyrotechnic component for automobile safety systems whose reliability is improved and whose non-toxicity is ensured. Thus the subject of the invention is a method of implementing at least one pyrotechnic substance for producing a pyrotechnic component, in particular a resistive bridge initiator, process characterized in that at least one powdery pyrotechnic substance is first suspended in an inert binder, liquid or pasty, and capable of hardening by polymerization , the pyrotechnic material thus formed then being placed in the liquid or pasty state

  in the component and an in situ polymerization step being finally carried out to harden the binder carrying the pyrotechnic substance or substances.

  Preferably, the binder will be chosen to be polymerizable by radiation or bombardment, in particular by

  ultraviolet radiation. The polymerization of the binder can also be ensured or supplemented by heating.

  The binder may be constituted by a photosensitive resin based on natural or synthetic resin, the

  base resin can then be chosen from the following resins: acrylic resin, epoxy resin, polyurethane resin, silicone resin, anaerobic resin, polyester resin.

  According to a particular embodiment, the pyrotechnic substance may comprise at least one primary explosive.

  Preferably, the pyrotechnic substance will contain as primary explosive a dinitro-benzo-furoxane salt.

  The pyrotechnic material may comprise from 40 to 60% by mass of pyrotechnic substance in suspension in 60% to 40% by mass of resin. According to another characteristic of the process according to the invention, a portion of the liquid or pasty pyrotechnic material is deposited on a resistive element of a pyrotechnic initiator, then the binder is cured by subjecting this material to ultraviolet radiation. Another subject of the invention is a pyrotechnic initiator with a resistive element using at least one pyrotechnic substance thus used, an initiator which is characterized in that the resistive element is completely

  covered by a pyrotechnic initiation material formed of the pyrotechnic substance or substances incorporated in an inert binder hardened by polymerization.

  This pyrotechnic initiator may include a pyrotechnic composition placed in contact with the initiating pyrotechnic material and compressed thereon. Advantageously, the resistive element may be a semiconductor bridge.

  The average particle size of the pyrotechnic substance will then be less than or equal to 50 micrometers.

  The pyrotechnic substance incorporated into the binder will preferably be a dinitro benzo furoxane salt, for example

  potassium dinitro benzo furoxanate.

  The pyrotechnic composition may be chosen from the following compositions: Zirconium / potassium perchlorate, Boron / potassium nitrate. The invention will be better understood on reading the

  description which follows of particular embodiments, description made with reference to the accompanying drawing

  which represents an example of a pyrotechnic component obtained with the process according to the invention. Referring to this figure, a pyrotechnic initiator 1 according to the invention comprises a metal casing 2 (for example made of stainless steel) on which a plastic molding 3 of the polyamide or polycarbonate type is produced. The housing 2 is closed by a plug 4 also in

plastic material.

  The housing 2 comprises a cylindrical wall 5 welded to a bottom 6 which is traversed completely by an electrode 7a and partially by an electrode 7b. The electrode 7b is in electrical contact with the bottom 6

  and the electrode 7a is electrically insulated from the bottom by an insulating sleeve 8 (for example made of glass).

  A semiconductor wafer 14 is formed from an insulating substrate 9 based on undoped silicon which is bonded to the bottom 6. This wafer comprises a semiconductor bridge 10 (for example made of doped silicon) which is partially covered by two conductive pads lia, llb,

for example aluminum.

  The spacing between the pads is between 60 and 100 micrometers and preferably of the order of 80 micrometers. The stud llb is connected to the electrode 7a by a connecting wire 12 fixed by welding. The pad 11a is connected to the electrode 7b via the metal bottom 6 through a semiconductor well 13 (doped silicon) which passes through the substrate

insulator 9.

  Such a structure is described in detail by patent FR2720493.

  According to the invention, an initiating pyrotechnic material 15 (which is therefore sensitive to heating) is deposited on the wafer 14 and also covers the wire 12 and 15 the welds. This pyrotechnic material consists of at least one pulverulent pyrotechnic substance mixed with a hardened inert binder. The average particle size of the pyrotechnic substance will be chosen to be of the same order of magnitude as the dimensions of the semiconductor bridge. Such an arrangement ensures thermal transfer by convection and / or projection during the rise in temperature of the semiconductor bridge. This ensures reliable initiation of the pyrotechnic substance by the resistive bridge. The bridge generally has dimensions of the order of 50 micrometers x 150 micrometers. We therefore adopt for the composition a particle size less than or equal to 50 micrometers. The inert binder chosen will be at room temperature a

  liquid or paste and will likely harden by in situ polymerization.

  Once cured the material 15 formed of the binder carrying the pyrotechnic substance covers and protects both

  mechanically and chemically the wafer 14 as well as the wire 12 and its welds.

  A reinforcing pyrotechnic composition 16 fills the entire housing 2. It is therefore in contact with the initiating pyrotechnic material 15 and can be initiated

by the latter.

  The pyrotechnic composition may be compressed in the casing, in particular by means of the plug 4. The material 15 protects the resistive bridge as well as the plate, the wire and the welds which cannot therefore be damaged.

  The reliability of the initiator according to the invention is therefore increased.

  Alternatively, and to minimize the quantities of material used, it is of course possible to deposit the initiation material 15 only on the wafer 14.15 The method of using the pyrotechnic substance according to the invention allows to obtain the initiating pyrotechnic material 15. This process is as follows: During a first step, the

  pulverulent pyrotechnic substances with the inert binder.

  In the example described above, a primary explosive will be chosen as the substance, for example trinitro-

  lead resorcinate, lead azide, silver azide, or a dinitro benzo furoxane salt such as potassium dinitro benzo furoxanate (or KDNBF).

  Dinitro benzo furoxane salts will be preferred because of their non-toxicity (absence of lead or heavy metals). The polymerizable binder will be chosen to be chemically compatible with the primary explosive (s) used (not inducing a reaction with it during the aging phases). The binder will preferably be chosen from photosensitive resins (which incorporate a photosensitizer in a known manner) and which are produced from synthetic resins. These resins are therefore (depending on the choice made) polymerizable by radiation (ultraviolet, X, microwave) or by bombardment (electronic). Photosensitive resins are known and commercially available and it is possible in particular to choose a photosensitive resin, the base resin of which will be chosen from the following resins: acrylic resin, epoxy resin, polyurethane resin, silicone resin, polyester resin, anaerobic resin (resin not polymerizing only in the absence of air). If a drop of such a photosensitive resin is exposed to UV, the radiation causes a surface polymerization isolating the rest of the resin. The core polymerization will be obtained (depending on the resin chosen) by heating, or chemical reaction (in particular photosensitive anaerobic resins will be able to completely polymerize after exposure to UV rays because they will be isolated from the air by the crust resulting from the UV polymerization of the surface of the drop). The viscosity of the binder will be chosen according to the characteristics of the system for depositing the material in the

  component. Rather liquid resins are preferred to facilitate the distribution of the material.

  The relative proportions of binder / primary pyrotechnic substance will be chosen by a person skilled in the art according to the sensitivity sought for the final material and that of the primary explosive used. We can adopt proportions of between 40% and 60% by mass for the pyrotechnic substance and 60% and 40% by mass for the binder. An equal 50/50 proportion may be adopted. A first advantage of this first stage of implementation is that the primary explosive, the particle size of which is extremely fine, is coated with a binder which desensitizes it. The preservation of the material can thus be ensured away from light (to avoid hardening) and in complete safety.35 A second advantage of this implementation is that the liquid or pasty mixture (depending on the viscosity of the resin used ) can be easily distributed mechanically in the

  housing of the pyrotechnic component to be produced, for example using a syringe or a worm hopper (second step of the process).

  A third advantage is that it is possible with the method according to the invention to minimize the quantity of primary pyrotechnic substance with reduced particle size disposed in each component while precisely dosing said quantity. It is therefore no longer necessary to fill the bottom of the case with a sensitive composition to ensure the

  overlap of the resistive bridge. By way of example, a single drop of approximately 1 milligram ensures the recovery of this bridge and the proper functioning of the component, whereas it was necessary with the prior methods to deposit nearly 30 milligrams of a pulverulent composition.

  After placing the pyrotechnic material in the component, it is hardened by polymerization (3 step). For example, a commercially available source of ultraviolet radiation (wavelength 365 nanometers) will be used for this, which will be placed at a distance from the material to 35 mm. The exposure time of the material will be easily determined according to the characteristics of the resin chosen and the total mass to be polymerized. This exposure time is for example less than 10 minutes for a mass of approximately 1.1 mg of material (deposited drop thickness of approximately 1 mm). The radiation polymerization may be completed by heating the composition (or post-curing) which will ensure the complete polymerization of the composition. This heating will be obtained for example by using a UV source also emitting in the infrared range. This heating can alternatively be provided by conventional means, such as passage through the oven. 35 In all cases, the heating means will be chosen so as not to overheat the pyrotechnic component so as not to initiate it (generally the rate temperature rise should be less than 0.5 C / second). Once the hardening has been carried out, the pyrotechnic composition 16 is loaded in a conventional manner (placing in the granular state by hopper, plugging of the component, compression in situ, welding of the plug 4 on the overmolding 3). For example, a pyrotechnic material was thus combined combining 53% by mass (or 0.54 mg) of a polyurethane resin with a viscosity of 110 centipoises with 47% by mass of KDNBF (ie 0.48 mg). The polymerization under UV under the operating conditions described above led to a solidification of the

  pyrotechnic material on the resistive semiconductor bridge.

  The initiators produced were then loaded with 120 mg of a pyrotechnic composition 16 associating Zirconium and

  potassium perchlorate in the conventional proportions of 60% by mass of zirconium for 40% by mass of perchlorate.

  The initiation tests conducted made it possible to measure an initiation time Tf of less than 500 microseconds and a

  component outlet pressure greater than 5 MPa (tests conducted in a 3 cm3 pressure bomb with an initiation current intensity of 1.5 amperes. No misfire25 was found for 50 components tested.

  This component uses only 1.1 milligrams of initiation pyrotechnic material. The performance obtained is however similar to that of a conventional component in which nearly 30 milligrams of a powdery initiation composition of lead styphnate are used and associated with 50 mg of reinforcing composition Bore / potassium nitrate. To make an initiator intended for automotive applications, it will be preferable to combine a pyrotechnic material incorporating a dinitro benzo furoxane salt (lead-free primary explosive) with an energetic unleaded pyrotechnic composition (such as Zirconium (60%) / potassium perchlorate (40 %) or Boron (25%) / nitrate

potassium (75%)).

  Dinitro benzo furoxane salts are interesting because of their non-toxicity, but they have the disadvantages of being expensive and not very energetic. Thanks to the invention it is possible to minimize the necessary quantity of this explosive in the form of a suspension in a drop of solidifiable binder (less than 1 mg of primary explosive) which is in intimate contact with the resistive bridge. Its low energy level is nevertheless sufficient

  to allow the initiation of the composition Zr / KC104 or B / KN03 which in its industrial form (high particle size and coating) cannot be initiated directly by the semiconductor bridge15.

  The structure of the initiator (shape of the housing, of the cap, presence or not of a molding) can of course

be different.

  As a variant it is also possible to put

  implementing the method thus described with other types of initiators, for example hot wire or percussion.

  In all cases the method according to the invention makes it possible to simplify and make safer the manufacture of the components

  pyrotechnic, while protecting the resistive element from the mechanical stresses undergone in particular during the compression of the pyrotechnic composition.

  It is also possible to implement the method according to the invention with other pyrotechnic substances,

  for example mixtures associating at least one oxidant with at least one reducing agent and with or without primary explosive.

Claims (7)

  1-Process for using at least one pyrotechnic substance to produce a pyrotechnic component, in particular an initiator with a resistive bridge, characterized in that first of all at least one pulverulent pyrotechnic substance is suspended in an inert binder , liquid or pasty, and capable of hardening by polymerization, the pyrotechnic material thus formed is then placed in the liquid or pasty state in the component and an in situ polymerization step is finally carried out to harden the   binder carrying the pyrotechnic substance or substances.   2-implementation method according to claim 1 characterized in that the binder is polymerizable by   radiation or bombardment, in particular by ultraviolet radiation.   3-Method of implementation according to one of claims   1 or 2, characterized in that the binder can be polymerized by heating.   4-Method of implementation according to claim 2   characterized in that the binder consists of a photosensitive resin based on natural or synthetic resin.   -Implementation method according to claim 4, characterized in that the binder is a photosensitive resin whose base resin is chosen from the following resins: acrylic resin, epoxy resin, polyurethane resin, silicone resin, anaerobic resin, resin   polyester. 6-Method of implementation according to one of claims   1 to 5, characterized in that the pyrotechnic substance   contains at least one primary explosive.   7-Method of implementation according to claim 6, characterized in that the pyrotechnic substance comprises   as a primary explosive a dinitro-benzo-furoxane salt. 8- Method according to one of claims 5 to 7 characterized   in that the pyrotechnic material comprises 40 to 60% by mass of pyrotechnic substance in suspension in 60% to 40%   by mass of resin. 9-Method according to one of claims 1 to 8, characterized in that is deposited on a resistive element of an initiator   pyrotechnic part of the liquid or pasty pyrotechnic material, then the binder is cured by subjecting this material to ultraviolet radiation. 10-Pyrotechnic initiator (1) with resistive element using at least one pyrotechnic substance used   according to the method of claims 1 to 9, characterized in that the resistive element (10) is completely covered by   a pyrotechnic initiation material (15) formed of the pyrotechnic substance or substances incorporated in an inert binder hardened by polymerization. 15 11-Pyrotechnic initiator according to claim 10, characterized in that it comprises a composition   pyrotechnic (16) arranged in contact with the initiating pyrotechnic material (15) and compressed thereon. 12- Pyrotechnic initiator according to one of claims   10 or 11, characterized in that the resistive element is a semiconductor bridge (10).   13-Pyrotechnic initiator according to claim 12, characterized in that the average particle size of the   pyrotechnic substance is less than or equal to 5025 micrometers.   14-Pyrotechnic initiator according to one of the claims   11 to 13, characterized in that the pyrotechnic substance incorporated into the binder is a dinitro benzo furoxane salt, for example potassium dinitro benzo furoxanate. 30 15-Pyrotechnic initiator according to claim 14, characterized in that the pyrotechnic composition is   chosen from the following compositions: Zirconium / potassium perchlorate, Boron / potassium nitrate.