FR2885684A1 - Electrical pyrotechnic initiator e.g. for motor vehicle airbag gas generator has composition base of fiber-reinforced plastic matrix - Google Patents

Abstract

The initiator with plastic to metal seal pyrotechnic layers comprises a cap (2) containing at least one pyrotechnic charge, a base (3) and a resistance (13) made e.g. from a mixture of zirconium and potassium perchlorate. The cap and base are made from a composition material with dispersed fibers in a plastic matrix having a melting point of between 260 and 320 degrees C and a moisture intake rate preferably below 0.03 per cent in 24 hours. The matrix is reinforced by 20-45 per cent by weight of glass, carbon or mixed fibers.

Description

The present invention relates to an electro-pyrotechnic initiator, that is

i.e. an electrical initiator intended to initiate the combustion of a charge of a pyrotechnic device, and more precisely an electrical initiator with compressed pyrotechnic layers of the PTMS type with

fast operation. The initiator according to the invention is in particular intended to be used in a gas generator where the combustion of said pyrotechnic charge triggers the combustion of a gas-generating composition. The invention relates more particularly to the field of motor vehicle safety.

Airbags intended for the safety of motor vehicle passengers are generally pyrotechnically triggered. They are deployed under the action of the gas generated by the combustion of a pyrotechnic composition, called a gas-generating composition, contained in a gas generator comprising a corrosion-resistant metal casing, generally aluminum. The inflation gas released by the exothermic decomposition reaction is typically nitrogen. Examples of inflators are described in US patents US 4923212, US 4907819 and US 4865635. When gas generators are intended for automobile safety, they must remain stable under high temperatures for long periods of time. Perfect preservation of the characteristics of the generator at 107 C for 408 hours has been a classic requirement for motor vehicle safety since the early 1990s. The combustion of the gas-generating composition is generally initiated by an electrical initiator contained in the gas generator. By electric initiator (or electric detonator) is meant a device with an energy amplification function whose role is to convert an input energy of an electrical nature (electrical ignition signal) into an output energy of an electrical nature. pyrotechnic, much higher than the input energy. This electrical initiator typically comprises two electrical current supply pins inserted into an electrically insulating base and connected to at least one collision detection device, a metallic or semiconductor resistive element establishing an electrical connection between the two pins and a cap electrically insulating material containing at least one pyrotechnic charge, composed of a pyrotechnic substance or a mixture of pyrotechnic substances with an adequate self-initiation temperature. By "resistive element" is meant in the present invention a filament electrically connecting the two current supply pins of an electrical initiator. The resistive element may be a cylindrical metallic filament, or a flat filament in the form of thin or thick metallic layers or layers of semiconductor material, or a structure combining metallic elements and semiconductors.

Such initiators can contain three types of pyrotechnic charges. They necessarily include a primary load, (most of the time) in contact with the resistive element, and optionally a main load (called a secondary load), and possibly an additional load which can be a self-initiating load. Most initiators operate by thermal exchange between the resistive element or electrothermal bridge and the load in contact with the resistive element, to which it supplies heat under the effect of an electric current by Joule effect and thermal exchange. This pyrotechnic charge is brought to an exothermic decomposition reaction by combustion and / or deflagration or detonation, triggered when the temperature reaches its self-initiation temperature. The auto-initiation temperature can, in general, be assimilated to the auto-ignition temperature. There are also initiators whose primary load is not in contact with the resistive element. These initiators generally use a secondary explosive as the primary charge and their resistive element is subjected to high voltage. They operate by explosion of the resistive element, which projects a plate (or spangle) initiating the primary load by shock. This type of initiator is less advantageous for an application in the automotive field because it only operates under high electrical energy.

Electrical initiators generally comprise a second pyrotechnic composition, called the main charge, which is as energetic as the charge in contact with the resistive element, but which reacts less violently, in particular for the initiation of gas-generating compositions. By "self-initiating charge" is meant a substance or a mixture of substances whose self-ignition temperature is lower than the self-ignition temperature of the gas-generating composition 2885684 3 employed. The self-initiating charges are intended to prevent the explosion of the gas generator in the event of a significant rise in temperature, in particular in the event of a fire. Indeed, gas generators are only designed to withstand the temperatures that can be reached in an automobile under normal use (-40 C; 90 C). At much higher temperatures, the burning rate of the gas-generating compositions tends to become excessive. The pressure generated is then such that the casing of the gas generator, generally made of aluminum, can lose its mechanical integrity and explode with projections of metallic bodies dangerous for people in the vicinity. The role of the self-initiating charges is to ignite during an abnormal rise in temperature and thus to trigger directly or by cooperation with an initiating reinforcing charge the combustion of the gas-generating composition in a controlled manner, before its auto-ignition temperature is reached. The term “initiation reinforcing charge” or “power increasing charge” (booster) is understood to mean a charge serving as a complement to the main charge and therefore playing the role of initiating reinforcement of the gas-generating composition. It is a load external to the initiator, which is generally placed between the initiator and the gas-generating composition.

The pyrotechnic compositions making up the charges are generally mixtures of oxidants and organic or inorganic reducing agents, and in particular mixtures of oxidants and inorganic reducing agents to which are optionally added a more powerful explosive or, on the contrary, a phlegmatizer, a binder or any other additive intended to obtain particular properties or to facilitate the use of the composition.

The design of electric initiators differs depending on the objective set. For automotive safety applications, initiators must operate under low electrical energy, which limits the mass of the resistive element. This excludes the use of exploded wire or plate projection (slapper) initiators, or even, semiconductor bridge (SCB) initiators when these are used to initiate the detonation of secondary explosives. In the most conventional initiators, a suspension of primary explosive and binder is deposited on the filament. After drying, the substance adheres to the filament and its support thanks to the binder. However, the substance thus deposited on the filament is porous and its thermal conductivity is limited, which has the consequence of reducing the intensity of the non-operating safety current for a given operating reliability current intensity. It is largely to ensure good thermal conductivity to the pyrotechnic substance in contact with the resistive element and thus limit the difference between the operating reliability intensity and the operating safety intensity, that initiators to Compressed pyrotechnic layers have been developed for automotive safety applications. By “initiator with compressed pyrotechnic layers” is meant an initiator whose charge in contact with the resistive element, at least, is in the form of a compressed layer. Compressed pyrotechnic layer initiators differ, among other classifications, in the materials providing the structure of their base. There are two categories: GTMS (Glass To Metal Seal), where the pins ensuring the passage of current pass through a base of glass (or ceramic) and metal, and PTMS (Plastic To Metal Seal) called "plastic initiators. ", where the pins pass through a base of plastic material. The glass-to-metal base of the GTMS and the plastic base of the PTMS provide the same functions, namely electrical insulation and hermeticity.

In GTMS, pyrotechnic substances are contained in a metal case welded to the metal part of the base. A coating generally made of a plastic material isolates the GTMS initiator from the metal body of the gas generator where the initiator is introduced. The GTMS configuration constitutes a reliable, well-proven means of ensuring an airtight seal between the pyrotechnic compositions contained in the initiator and the exterior. Initiators have been produced industrially according to this principle with different design variants.

PTMS, which use a plastic material rather than a glass-to-metal assembly, were designed to reduce the manufacturing costs of the initiators, while ensuring satisfactory levels of quality. In such initiators, the pyrotechnic substances are contained in a plastic case or cap, welded to the plastic of the base supporting the resistive element by means of a suitable technique such as ultrasonic welding. Several variants of PTMS have been created by applying these general principles. The nature of the plastic material constituting the base of the PTMS plays an important role with regard to the performance of the initiator, which must be brought into conformity with the requirements of the automotive industry. Plastic materials can contain fibers, which give them good mechanical resistance, especially when hot. Generally, they contain, independently of their nature, a mass content of glass or carbon fibers ranging from 20 to 50%. The desired properties determine the amount of fiber required, which varies depending on the nature of the plastic and the nature of the fibers used. Glass fibers are generally chosen for their low cost, but carbon fibers provide better mechanical performance for the same content.

The use of plastics to compose the base of initiators with compressed pyrotechnic layers intended for automobile safety has been envisaged in French patent FR 2698687. It recommends the use of polyamide or polycarbonate for a PTMS with welding by ultrasound. US patents US 5647924, US 5763814, US 5648634, and US 5711531 recommend the use of PBT (polybutylene terephthalate) or LCP (liquid crystal polymers) for the manufacture of bases of PTMS initiators with ultrasonic welding, intended to the automotive industry. Patent WO 03/058154 recommends the use of polyamide PA6-12, polyketone or PBT for PTMS with ultrasonic welding. German patent DE 10106366 describes an electrical initiator intended for igniting a gas generator. As the material constituting its base is not specified, it is not possible to determine whether this electrical initiator is of the PTMS or GTMS type. This electropyrotechnical initiator comprises a base which makes it possible to fix it to the gas generator, of which part of this base, referred to as a segment, provides the junction between the base of the initiator and the metal body of the gas generator. This patent recommends manufacturing said segment of the base in a material which is a good thermal conductor, in order to improve the thermal conductivity between the housing of the gas generator and a pyrotechnic charge contained in the cap of the initiator, thereby facilitating its Firing. Optionally, the initiator cap can be made of the same plastic material to further improve heat transfer. The materials recommended for use are plastics: PPS (polyphenylene sulfide), PPA (polyphthalamide) or an epoxy resin.

The manufacturing cost of initiators of the PTMS type is admittedly lower than that of initiators of the GTMS type, but the PTMS have some potential drawbacks. It has in fact been observed that abnormally long operating times could appear on PTMS initiators with a plastic matrix based on polyamide PA 6-12 or on a polyketone such as Carilon, (1) during initiation under high temperatures. high (> _ 90 C); (2) during initiation occurring after subjecting the initiator to an environmental test in damp heat. PA 6-12 and Carilon are two plastic materials used in the prior art for the manufacture of PTMS initiator bases. The term "operating time" of an electrical initiator or TTFP (Time To First Pressure) is understood to mean the time elapsing between the emission of the electrical signal and the release of energy by the pyrotechnic charge outside the chamber. the initiator, which results in a rise in pressure.

For the purposes of the present invention, a TTFP will be said to be abnormally long if it is greater than 2 milliseconds under a current of 1.2 A and under a temperature which may vary from 40 C to + 90 C. It must remain less than 2 ms even after the electrical initiator has been subjected to various environmental tests, such as for example a 5-day test in an enclosure where a temperature of 85 C and a relative humidity of 95 0/0 prevail (conditions of the tests of most severe damp heat environment among those required by vehicle manufacturers), a 408 hour test at 107 C. An abnormally long TTFP is only one of the possible operating time anomalies. The presence of an eccentric, in the usual statistical sense, also constitutes an anomaly, regardless of compliance with the specification "TTFP <2 ms". In addition, an average of TTFPs under 90 C higher than that obtained under 40 C reveals a weakness of the initiator structure under 90 C, regardless of the presence of an eccentric or compliance with the specification "TTFP <2 ms ".

The plastic materials used to constitute the structure of PTMS are polybutylene terephthalate (PBT), polyamides such as PA612, polycarbonates, a liquid crystal polymer such as Vectra A515 (polyester reinforced not by fibers but by 15% d 'a mineral filler) or polyketones such as Carilon. The characteristics of these materials are not entirely satisfactory and can lead to TTFP anomalies under certain conditions of temperature and humidity, i.e. hot, and especially under a humid heat environment. Another defect of PTMS known heretofore is the occurrence of filament breakage after environmental tests, in particular after exposure to moist heat. Repeated cycles of expansion / contraction over several years can also lead to filament breakage. A judicious choice of the geometry of the structural constituents of the initiator associated with good control of the loading of the compositions into the cap (volume and compression ratio) makes it possible to reduce the amplitude of the defects, however the risk remains. There is therefore a need for electrical initiators free from operating delay anomalies, which are at the same time perfectly stable under the temperature and humidity conditions required by automobile manufacturers. In the present invention, an electrical initiator or a load are said to be stable at a given temperature for a given time when a stay in an enclosure at said temperature during said time does not cause any degradation detrimental to their operation or to that of the other components. of the gas generator.

On the other hand, certain uses of electrical initiators, carried out in an environment which is not necessarily hot and humid, require very short TTFPs to be obtained. This is particularly the case with passive protection systems relating to automobile safety. The specification "TTFP <2 ms" was introduced for airbags intended for the protection of passengers in the event of a frontal impact. Other airbags, such as those intended for passenger protection in the event of a side impact, require the provision of even shorter TTFPs.

There is therefore also a need for electrical initiators whose operating times are very short, of the order of one ms or sometimes less under limited current intensities. In a simplified way, the operating time of an electrical initiator can be broken down into two delays: 1) the time necessary for a sufficient quantity of substance or pyrotechnic composition to reach its decomposition or reaction temperature, for example its temperature self-ignition. This delay corresponds to the heating delay of the resistive element and of the layer of pyrotechnic substance immediately on contact.

2) the time required for the exothermic reactions to propagate in the pyrotechnic compositions until the container bursts and energy is released outside the initiator, allowing the TTFP to be measured.

It is well known that the second delay can be modulated by adjusting the liveliness of the pyrotechnic compositions, the liveliness of the composition in contact with the resistive element being particularly important. The speed of propagation of the reaction front depends on the nature of the pyrotechnic substances but also on the confinement provided by the metallic or plastic materials enveloping these compositions. An original solution envisaged for reducing this second delay consists in placing a primary explosive such as KDNBF (potassium 4,6-dinitrobenzofuroxanate) or DDNP (diazodinitrophenol) in contact with the resistive element. This solution does reduce TTFPs, but KDNBF and other lead-free primary explosives such as DDNP have reduced stability for 408 hours at 107 C and high containment. However, reducing the second delay only brings a partial improvement, since it represents on average only 20% of the TTFP of the most conventional electrical initiators. This second delay can be calculated by plotting the graph TTFP = f (Intensity of the current) relating to the initiator PTMS considered, the TTFP tending asymptotically towards the second delay when the intensity increases, that is to say when the heating delay of the resistive element tends towards 0. It is therefore more interesting to try to reduce the first delay, which is the largely majority component of the TTFP.

On the other hand, since primary explosives such as KDNBF or DDNP have self-initiation temperatures much lower than those of compositions such as those of the ZPP type, to maintain the same level of electrical sensitivity it is necessary to increasing the thermal inertia of the resistive element, for example by increasing the diameter of the cylindrical filament, which tends to increase the operating time and greatly reduces the overall gain.

In general, the heating inertia of the resistive element can be reduced by reducing the mass of the resistive element. The most classic initiators are provided with a cylindrical metallic filament. A reduction in their mass therefore implies a reduction in their diameter. However, there is a minimum diameter that a cylindrical filament intended for use in an initiator of the PTMS or GTMS type can hardly achieve. Thus, a diameter less than 16 m is difficult to implement industrially. It is essentially for this reason that a category of filaments was created which will be grouped together here under the name of "flat filament".

By flat filament is meant in the present invention a resistive element comprising at least one conductive metal layer (pure metal or metal alloy) or at least one semiconductor layer, with a thickness less than or equal to 10 m. Such a small thickness advantageously makes it possible to reduce the mass of metal or semiconductor of the resistive element per unit of contact area with the primary load. For reasons of mechanical strength, a flat filament is necessarily fixed on a support, the contact with the primary load taking place on the opposite face. On the other hand, the implementation of resistive elements generating a low energy per unit of contact area with the primary load requires excellent contact between the surface heated by the Joule effect and said load, associated with a great homogeneity of the composition. pyrotechnic, the initiation temperature of which must be fairly low.

Under the name of flat filament are grouped together, independently of their embodiment, on the one hand the resistive elements comprising at least one so-called thin layer, generally metallic, and on the other hand the resistive elements comprising at least one so-called thick layer, metallic or semiconductor. The resistive elements of the SCB (semiconductor bridge) type, made up of a semiconductor bridge, can be considered as flat filaments of the thick film type. The limit between thin layer and thick layer is generally located, according to the literature, at a value of filament thickness which varies from 1 to 2 m. It is easier to distinguish the two categories according to their embodiment.

The expression flat filament of thin film type, as used in the present application, thus designates the flat filaments produced by vapor deposition, vacuum evaporation or cathode sputtering. Their thickness is less than or equal to 2 m and generally varies between 0.1 and 1 m. The expression thick-film type flat filament designates, in the present application, the SCBs and the flat filaments obtained by techniques for producing printed circuits, etching or screen printing. Their thickness generally varies from 1 to 10 m. Examples of thin film type flat filaments are given in patents US 3669022, DE 2020016, US 4409898, US 5847309, US 6129976, US 6316752, EP 1030159 and DE 1011619. Examples of thick film type flat filaments (except SCB ) are given in patents FR 2513751, FR 2538207, FR 2704944, US 5337674, WO 00/37395, FR 2800865, FR 2827377, WO 02/057705, US 6680668 and EP 1443298. Examples of flat filaments of SCB type are given in US Patents 3366055 (invention of the operating principle), US 4708060, US 4976200, US 5085146 and US 6772692. The conductive bridge of a resistive element of SCB type comprises at least one layer of a semiconductor material, of which the resistivity, in the field of use, decreases when the temperature increases. This semiconductor material is generally silicon. In its simplest version, it is heavily n doped. It can also form pn junctions constituting two diodes in opposition.

In another configuration, it can be made of undoped (polycrystalline or amorphous) or lightly doped silicon, coated with a thin layer of a conductive metal such as tungsten, which, when traversed by a current, must heat up. the silicon layer until it becomes conductive. The structure of an SCB can also include layers of insulation (SiO2), intermediate layers ensuring adhesion (Ti), and reactive layers of reducing agents (Zr) and oxidants (Fe2O3, CuO). SCBs can operate at significantly lower feed energy than conventional cylindrical filaments. When an electrical impulse passes through the SCB, it heats up, its resistivity decreases, it vaporizes, a plasma is generated, which initiates the primary charge. The processes for initiating pyrotechnic charges by means of conductive or semiconductor bridges are, among others, described in French patent FR 2732455.

The electrical initiators described so far, which use flat filaments, regardless of the production technology, are characterized by the deposition on the filament and adhering to it, of a pyrotechnic substance generally based on explosives. primary, and by the integration of the resistive element on a glass-metal base. The full initiator therefore constitutes a variant of a GTMS initiator. On the other hand, an initiator of the PTMS type provided with a flat filament has never been produced industrially, although certain patents such as US 5647924 envisage the possible possibility of using a flat filament, under different names, in particular a semi-rigid device. -conductor of SCB type, as alternatives to the implementation of a cylindrical filament. These patents do not provide the means necessary for the production of such initiators nor those necessary to comply with the usual specifications of initiators intended for automobile safety. In addition, the devices disclosed in these patents do not combine the two characteristics described above (initiator with a plastic structure + compressed pyrotechnic layers).

The objective of the present invention is to provide an electrical initiator with compressed pyrotechnic layers of the PTMS type satisfying the aforementioned criteria and overcoming the drawbacks of the PTMS initiators of the prior art, intended in particular for use in airbags and airbags. seat belt pre-tensioners. The present invention also relates to a gas generator comprising such an electric initiator.

The objects described above are achieved according to the invention by an electrical initiator with compressed pyrotechnic layers of the PTMS type, comprising a cap containing at least one pyrotechnic charge, a resistive element, and a base made of a composite material comprising a formed matrix. of at least one plastic material, in which fibers are dispersed, said composite material having a melting point ranging from 260 to 320 C and a coefficient of thermal expansion less than 30.10-6 K-1 in the axial direction according to the test ASTM E831. The cap of the electrical initiator according to the invention can, optionally, be made of a composite material comprising a matrix formed of at least one plastic material, in which fibers are dispersed, said composite material having a melting point. ranging from 260 to 320 C and a coefficient of thermal expansion less than 30.10-6 K-1 in the axial direction according to the ASTM E831 test. The materials constituting the base and the cap of the electrical initiator according to the invention may be identical or different. According to a preferred embodiment, the base and the cap are both formed from the same composite material according to the invention. More preferably, the matrix of composite material in which the base and / or the cap are made is formed from a single plastic material.

The use of a composite material as defined above greatly reduces the frequency and amplitude of operating delay anomalies, without modifying the process for producing the initiators, in a reliable and reproducible manner, without adversely affecting stability. fireworks, without generating additional danger and for a low additional cost of raw material. It makes it possible to produce electrical initiators that are more efficient than the PTMS initiators known previously. The electrical initiator of the invention can be characterized by high stability. It is stable within the meaning of the invention during the conventional test consisting of a stay at 107 ° C. for 408 hours. It is stable under 85 C and 95% relative humidity for 5 days.

The PTMS initiators according to the invention are distinguished by the use of a plastic matrix composite having on the one hand excellent thermal properties, and on the other hand excellent dimensional stability properties under the influence of humidity and temperature variations.

The composite materials constituting the base and optionally the cap of the initiator according to the invention comprise a matrix formed of at least one plastic material, reinforced by fibers. By "plastic" is meant a polymeric material having the property of being moldable or moldable. Said plastic material is preferably thermoplastic. The melting point of the composite material, generally close to the melting point of the plastic matrix, must not be too high to allow easy processing by injection into molds. It must not be too low either, to avoid the ejection of the initiator structure from the gas generator, during operation caused by a fire or by an environmental test of the "bonfire test" type. example. Preferably, the composite material used in the present invention has a melting temperature ranging from 260 to 320 C, better still from 275 to 300 C and even better still from 280 to 300 C. Preferably, its matrix contains 20 to 45%. by mass of fibers, this percentage being expressed relative to the total mass of the composite material. The fibers, long or short, inorganic or organic, are preferably chosen from carbon fibers, glass fibers and their mixtures. The presence of fibers provides the composite material with sufficient mechanical strength and reduces its coefficient of thermal expansion. The lower mechanical resistance of the composite material, when hot, can play an unfavorable role in the confinement at the time of the propagation of the combustion. The coefficient of thermal expansion is all the lower the higher the content of glass or carbon fiber. Carbon fibers have an even more beneficial effect on dimensional stability than glass fibers, and offer better mechanical strength for the same content. Glass fibers, however, offer satisfactory solutions at a lower cost. According to a first preferred embodiment, said matrix is reinforced with 20 to 35% by mass of carbon fibers. According to a second preferred embodiment, said matrix is reinforced with 30 to 45% by mass of glass fibers. A fiber mass content of less than 20% adversely affects the strength properties of the composite material and results in lower deflection temperatures.

Deflection temperature is defined as the temperature at which a normalized test bar flexes by a specified distance value under a given load. It can be increased by reinforcing agents such as fibers and depends largely on the melting point of the matrix. It can be measured according to the tests ASTM D648 (deflection of 0.25 mm) or ISO 75 (deflection of 0.32 mm flat or 0.34 mm on the edge). It is used to determine the resistance to heat distortion of a material in the short term. The most used loads (or pressures) are 0.46 MPa and 1.82 MPa. In the present invention, the deflection temperatures are expressed according to the ASTM D648 test under a load of 1.82 MPa. The composite material according to the invention has a high deflection temperature, that is to say greater than 230 C, and better still, greater than 240 C.

The composite material according to the invention has a low rate of moisture uptake. The absorption of humidity effectively causes an increase in the dimensions of the objects made of these materials, which is unfavorable to the operation of the initiator. The moisture uptake rate of plastic-based composites generally varies little with the fiberglass content. Preferably, the moisture uptake rate of the composite material of the invention is less than 0.05% in 24 hours, preferably less than 0.03% in 24 hours, according to the ASTM D570 test which consists of a immersion in a water bath at 23 C. These two rates correspond respectively to humidity uptake rates of less than 0.1% and 0.05% for a 5-day test in an enclosure where a temperature of 85 prevails. C and a relative humidity of 95%.

The composite material according to the invention has a low coefficient of thermal expansion. By thermal expansion coefficient of a solid is meant, in the present invention, the linear expansion coefficient of said solid a, expressed in K-1 and defined by equation (I) in which AL represents the linear expansion of the solid , Lo its initial length and AT the temperature variation: AL = aLoAT (I) It can in particular be measured according to the tests ASTM E831 (thermomechanical analysis over the temperature range of use) or ASTM D696 (analysis by means of a dilatometer between 30 and + 30 C) performed on standardized test specimens. It is used to determine the rate of expansion of a material as a function of temperature and the possible occurrence of a defect due to thermal stress. Preferably, the composite material according to the invention has a coefficient of thermal expansion less than 30.10-6 K-1 in the axial direction according to the ASTM E831 test, and better still less than 25.10-6 K-1 in the direction. axial according to the ASTM E831 test.

In addition to its two basic constituents, which are plastic and fibers, the composite material according to the invention can comprise a certain number of additives or adjuvants aimed, for example, at improving certain properties. Those skilled in the art will take care to choose the fibers and any additives as well as their amounts so that they do not harm the properties of the composite material of the present invention, in particular its thermal expansion coefficient, its recovery rate. humidity and its melting temperature. He will at least ensure that the various parameters remain within the specified ranges, so that a certain degree of harm from the additives and adjuvants to certain properties can be tolerated. Among these, it is possible to cite, without limitation, PTFE [Poly (tetrafluoroethylene)], lubricating agents to facilitate injection operations, ceramics to increase thermal conductivity, and nanotubes of carbon (electrical conductors) if they do not excessively increase the electrical conductivity, the composite material according to the invention having to be a good electrical insulator.

A composite having a polysulfide (PPS) matrix, the basic unit of which corresponds to the general formula (I) shown below, is preferred for the invention, since this plastic material is particularly suitable for solving the assembly. of the aforementioned technical problems.

(1) 20 PPS effectively has a low moisture uptake rate (0.02% moisture uptake in 24 hours according to the ASTM D 570 test), a low coefficient of thermal expansion (18.10-6 K-1 in the axial direction when reinforced with 40% glass fiber, according to ASTM E831), an adequate melting temperature (in the range of 285-290 C). This makes it possible to achieve suitably high deflection temperatures (greater than 240 ° C. under 1.82 MPa) after incorporation of fibers. By way of example, data from the technical literature relating to PPS are collated in Table 1 below.

Table 1

Material T of deflection '(C) T fusion Coefficient of thermal expansion (C )2 (10-6 K-1) PPS unloaded 100-135 285-290 27-49 PPS + 10-20% glass fibers 227-249 275-285 16-20 PPS + 30% glass fibers 264 275 285 PPS + 30% carbon fibers 260-263 275-285 6-16 PPS + 40% glass fibers 252-269 275-290 13-22 PPS + 40 % carbon fibers 263 275-285 8 1 Test conditions: ASTM D648 under a load of 1.82 MPa. Test conditions: ASTM D696, in axial direction.

PPS is also a high performance thermoplastic which is very resistant chemically, mechanically and thermally. Its good resistance to high temperatures constitutes a definite advantage for the resistance to the bonfire test. It also has inherent flame retardant properties. It is notably marketed under the names Ryton or Fortron.

Thus, the two composite materials preferred for the invention are (1) a material comprising a PPS matrix containing 20 to 35% carbon fibers; and (2) a material comprising a PPS matrix containing 30 and 45% glass fibers, the latter being the most economically attractive.

Composites having a liquid crystal polymer matrix of suitable melting temperature and coefficient of thermal expansion can also be employed in the present invention, but, due to their cost, PPS is preferred. Liquid crystal polymers are generally highly crystalline, thermally resistant, and thermotropic thermoplastic polymers.

The fact that an electrical initiator whose base and possibly the cap are made from composite materials according to the invention can overcome the problems of TTFP anomalies will become clear on reading the following paragraphs, which highlight the influence of the characteristics of the plastic material constituting the structure of the PTMS on the operating delay anomalies.

TTFP abnormalities can be seen in a hot environment, for example in a fire, especially in a hot and humid environment. Without wishing to be limited by a particular theory, the inventors believe that the fact that TTFPs can be abnormally long is due to strong differences between the thermal expansion coefficients of the plastics used in the PTMS initiators on the one hand and those of the substances. compressed pyrotechnics on the other hand.

The higher expansion of the plastic matrix composite material compared to the primary pyrotechnic compositions causes less containment, which tends to reduce the rate of propagation of combustion. The effect is even more noticeable when a vacuum forms between the resistive element and the pyrotechnic substance. The expansion of the cap and the base of the electrical initiator both penalize operating times, the most unfavorable expansion being that of the cap. An improvement in the geometry of the initiator consists in placing the resistive element on a protuberance of the base formed by the pins coated with the composite material with a plastic matrix. However, this protuberance can only be of low height. Table I shows the results of measurement of the coefficients of thermal expansion of cylinders of the primary pyrotechnic compositions (that is to say in contact with the resistive element) most used in initiators of the PTMS and GTMS type, compressed under the same efforts as in said initiators, namely the compositions ZPP (zirconium / potassium perchlorate), THPP (titanium hydride / potassium perchlorate) and boron / potassium nitrate. These measurements were carried out by thermomechanical analysis, under conditions close to those of the ASTM E 831 test. Table I also presents the results of the measurement of thermal expansion coefficients of different composite materials with a plastic matrix, having substantially the same rate. of glass fibers (about 35%), which can be used to compose the structure of electrical initiators. The measurements were carried out not on standardized test pieces, but on caps, and under conditions close to those of the ASTM E 831 test (thermomechanical analysis). In general, a compound does not expand in the same way in the three directions of space: the coefficient of thermal expansion is anisotropic. Here it was measured in the axial direction.

The coefficient of thermal expansion can vary with the geometry of the part and the injection conditions of the plastic material. The hood thermal expansion coefficients indicated in Table 2 are thus slightly higher than those which would have been measured on standard test specimens.

Table 2

Material Coefficients of thermal expansion (K-1) Polyketone Carilon 87. 10-6 Polyamide PA6-12 60.10-6 PPS 26.10-6 THPP * 11.10-6 Boron / KNO3 * 10.106 ZPP * 6.10-6 * Under 200 bars.

The most common compressed pyrotechnic compositions having lower thermal expansion coefficients than the plastic matrix composites constituting the initiators, a solution to this problem provided by the present invention is the use of a plastic material having a thermal expansion coefficient. as low as possible, this having a great influence on the thermal expansion coefficients of the corresponding composite materials. The difference between the thermal expansion coefficients of the most common compressed pyrotechnic compositions and the plastic matrix composite materials constituting the base and possibly the cap of the PTMS can be reduced by a factor of 3 to 4 thanks to the replacement of traditional plastic materials. by the plastic materials according to the invention.

Thus, with a PTMS initiator comprising a pyrotechnic charge 3 mm thick comprising a ZPP, THPP or boron / potassium nitrate composition, and a cylindrical filament with a diameter of about 20 m, a temperature rise of 70 ° C. (20 to 90 C) can generate a vacuum of 16 m when said PTMS initiator uses Carilon as plastic, and 10 m when said PTMS initiator uses polyamide PA6-12 as plastic, both of which are plastic materials used in the prior art. The diameter of the filament used being only 20 m, the importance of the phenomenon at the level of the interface filament / pyrotechnic composition appears clearly. With a PTMS initiator according to the invention using PPS as plastic material, the size of the vacuum generated by such a temperature rise is reduced to only 4 m.

It is important to consider that other means could have been implemented to reduce the importance of the fault relating to the TTFPs and its probability of occurrence. The solution consisting in developing pyrotechnic compositions having higher thermal expansion coefficients is not satisfactory. The improvement provided is only partial and long TTFPs could appear when cold (-40 C). Thus, an experimental composition compressed under 200 bars, consisting by mass of 70% of potassium chlorate, of 20% of guanidinium nitrate, of 10% of boron (with as additive 5% by mass of Viton binder, relative to the total mass of the composition), has a coefficient of thermal expansion of 52.10-6 K-1. Using this composition, disclosed in the French patent registered under the number FR 05 50821, the potential vacuum between the resistive element and the pyrotechnic substance with Carilon is still 10 m, it is reduced to 4 m with polyamide PA6- 12, while only a slight vacuum could appear when cold (-40 C) with the PPS. The solution consisting in using compressed pyrotechnic compositions having a greater liveliness is not satisfactory either. It only partially reduces the amplitude of the effects reducing the speed of combustion propagation, and leads to making the operations for manufacturing pyrotechnic compositions and fireworks more dangerous. Another possible solution for increasing the liveliness of compressed pyrotechnic compositions would consist in depositing a small amount of a primary explosive such as KDNBF (potassium dinitrobenzofuroxanate) so that it adheres to the resistive element. However, KDNBF poorly withstands storage tests when it is confined (408 hour environmental test at 107 C), and this solution, which cannot be directly integrated into a production line, increases costs.

Another possible cause of TTFP abnormality is the expansion of the plastic by moisture absorption, which can also lead to the formation of a vacuum between the resistive element and the pyrotechnic substance. One solution to this problem provided by the present invention is the use of a plastic material having a low moisture uptake rate. In addition, better dimensional stability of the composite material, and especially less moisture uptake, tend to reduce the probability of filament breakage, in particular after exposure to moist heat.

The other polymeric materials commonly used in the prior art to compose the structure of electrical initiators of the PTMS type are polybutylene terephthalate, polyamides such as PA6-12, polycarbonates, liquid crystal polymers or polyketones such as Carilon. . The characteristics of these materials are not satisfactory for solving all of the aforementioned technical problems.

Several of them lead, for example, to abnormally long TTFPs under certain temperature and humidity conditions.

Polyamides and polyketones are too hygroscopic, and / or have too low melting temperatures and / or too high coefficients of thermal expansion. Carilon polyketone and polyamide PA6-12 have a melting point that is too low, of the order of 220 C. Thus, a classic defect of PTMS (as well as GTMS) coated with a polyamide such as PA6-12 is l ejection of the initiator structure from the gas generator where it is introduced, during environmental tests called bonfire test intended to verify the behavior of gas generators during a fire. Polyamide PA612 and Carilon polyketone undergo moisture uptake of the order of 0.23% and 0.3% respectively, after 5 days in an enclosure where a temperature of 85 C and a relative humidity of 95 prevail. % (which corresponds to a rate greater than 0.1% in 24 hours according to ASTM D570). Polyamide PPA has a much too high moisture uptake rate and a slightly too high coefficient of thermal expansion. Polycarbonates have too high thermal expansion coefficients and too low deflection temperatures even with a heavy load of glass fibers. A polybutylene terephthalate (PBT) such as Valox DR48 has an insufficient melting temperature, of the order of 245-260 C. In addition, polyesters of the PBT type have a much too high coefficient of thermal expansion and a deflection temperature. too low (182 C at 1.82 MPa according to ASTM D648). Their use leads to frequent hot TTFP abnormalities. Liquid crystal polymers generally have a low coefficient of thermal expansion and a low rate of moisture uptake. However, they are expensive and some of them have too high melting temperatures to be used industrially. The name “epoxy resin” includes various compounds which are not suitable either, because of their thermosetting properties which are poorly suited to the production of bases and caps for a PTMS initiator by injection into a mold.

In a first configuration of the electrical initiator according to the invention, the resistive element is a conductive bridge with a cylindrical filament. Such an initiator is commonly referred to as a hot wire initiator. The cylindrical filament is metallic and is made of a pure metal or preferably a metal alloy. In a second configuration of the initiator according to the invention, the resistive element is a conductive bridge with a flat filament as defined above. It comprises at least one thin layer, preferably metallic, or at least one thick, metallic or semiconducting layer. Thus, the resistive element can be a resistive element of SCB (flat filament) type, comprising at least one layer of semiconductor material. When the resistive element of the electrical initiator according to the invention is a flat filament, its thickness is between 1 and 10 m, terminals included. Preferably, its thickness is 2 to 8 m, more preferably 2 to 6 m.

The electrical initiator according to the invention comprises at least one pyrotechnic charge. It necessarily comprises a primary load, most of the time in contact with the resistive element, and optionally a main load (called a secondary load), an additional load which can be a self-initiating load.

The fillers used in the present invention generally comprise, in appropriate amounts, a mixture of oxidants, a mixture of reducing agents and optionally additives. The oxidants suitable for the present invention can be chosen, without limitation, from: chlorates, perchlorates and nitrates, of potassium, sodium, guanidinium, silver or ammonium, 2,4-dinitrophenylhydrazine, peroxides, nitrocellulose, oxides of metals, preferably not very electropositive (Pb, Bi, Sn, Cu), and their mixtures. The oxidizing mixture can optionally be obtained by co-melting at least two oxidants as described in US Pat. No. 6101947. The proportions of the oxidants can be chosen so as to constitute a eutectic mixture. The reducing agents suitable for the present invention can be chosen, without limitation, from: metals (Mg, Al), metalloids (B, Si), transition metals (Mo, Ti, Zr, Fe, Mn), hydrides metals (TiH2 and ZrH2), sugars or carbohydrates (lactose) and their mixtures.

Metal or transition metal oxides, as well as transition metals and certain metalloids, are generally used in the field of pyrotechnics as catalysts for the decomposition of oxidants. Yunchang Zhang's work highlights probable mechanisms: "Catalytic effects of metal oxides on the thermal decomposition of sodium chlorate", Zhang, Y., Kshirsagar, G., Ellison, JE, Cannon JC Thermochimica Acta 1993, 228 and publications subsequent. As metal oxides, transition metals or metalloid oxides which can be used in the present invention, mention may be made of MoO3, CuO, Fe2O3, S102, CeO2, Al2O3, V2O5, CrO3, Cr2O3, MnO2, Co3O4, NiO, Cu2O, ZnO, ZrO2 , Nb2O5 and Ag2O, or mixtures thereof. Preferably MoO3, CuO, Fe2O3 or mixtures thereof are used. It is advantageous to use the above oxides in the form of extremely fine powders so as to benefit from a high specific surface. Preferably, they are used in the form of nanometric powders, an expression by which is meant, in the present invention, a powder with a particle size ranging from 1 to 100 nm, preferably from 3 to 30 nm.

Generally, the fillers used in the present invention comprise a binder, and generally contain 1 to 10% by mass of this binder, preferably 2 to 10% by mass of this binder, better 2 to 8%, this percentage being expressed relative to to the mass of the main formulation of the composition. By main formulation of the composition is meant all of its constituents except the additives intended to improve its use, such as the binder or ultra-fine silica. The preferred binder is a fluoroelastomer binder such as Viton, a hexafluoropropylene / vinylidene fluoride / tetrafluoroethylene copolymer sold by El Dupont de Nemours & Co. A fluoroelastomer binder such as Viton improves the stability of the fillers. Like other binders, it reduces the sensitivity to friction, to shock and to static electricity of the compositions and makes it possible to obtain a particle size distribution facilitating the use of the powders. In addition, the binder, by ensuring cohesion between the grains of the compositions, contributes to controlling the reproducibility of the stoichiometry within the compositions. These are in fact manufactured and handled in bulk in relatively large quantities, under conditions liable to promote the migration and aggregation of certain constituents, and therefore, a detrimental variation in the sensitivity characteristics, such as the auto-temperature. inflammation. The coating with an energetic and hydrophobic binder such as Viton also provides excellent resistance to humidity. Its overall role is largely beneficial.

It is also possible to include an amount by mass of up to 1%, preferably from 0.2 to 1% (relative to the mass of the main formulation), of an ultra-fine silica powder, called "fumed". of silica ", in the fillers which can be used in the invention to facilitate their processing and to improve their stability. By ultra-fine silica is meant, in the present invention, an extremely fine silica powder having a specific surface area ranging from 100 to 200 m2.g-1. In addition, it is possible to use talc, graphite or boron nitride to facilitate the implementation of the fillers.

The overall content of these additives remains less than 1% by mass (relative to the mass of the main formulation).

Preferably, the electrical initiator according to the invention comprises a primary load in contact with the resistive element. It is preferably in compressed form, its compression ratio is preferably greater than 30 bars and its self-ignition temperature generally greater than 350 ° C. by slow heating. It generally comprises a mixture of oxidants, reducing agents, and various additives well known to those skilled in the art. The charge in contact with the resistive element is preferably a composition of ZPP type, that is to say it comprises potassium perchlorate as oxidant and zirconium as reducing agent, the latter being a very electropositive metal, therefore very reactive, and quite stable. It can of course contain other oxidants and reducing agents than these. According to a preferred embodiment, the charge in contact with the resistive element comprises potassium perchlorate as sole oxidant and zirconium as sole reducing agent. US Pat. No. 5,337,674 indicates, without claiming it, the possibility of using a ZPP composition in contact with a resistive element which is not a flat filament. The use of a ZPP type composition in contact with a flat filament is unusual. The charge in contact with the resistive element may be the sole charge of the initiator according to the invention. It preferably comprises at least one additive chosen from a binder, a metal oxide, a transition metal oxide, a metalloid oxide, an ultra-fine silica powder called "silica fume", talc, graphite and nitride. boron. Preferably, the charge in contact with the resistive element comprises a binder and optionally other additives. The preferred binder is a fluoroelastomeric binder such as Viton. Good contact intimacy between the resistive element and a primary explosive-free primary load is essential for reliability of operation and, more so, for speed of operation. Those skilled in the art know, from experience, that the operating time suffers before the electrical sensitivity, from poor contact between the resistive element and the load. However, a pyrotechnic substance consisting of a chemical compound operating by decomposition of the molecule, such as primary explosives (lead mono, di or trinitroresorcinates, KDNBF, DDNP) does not require such intimate and controlled contact with the resistive element. .

Main charges used in electrical initiators intended for motor vehicle safety and in particular in airbags are described in US patents US 5847310 and US 5672843. The main charges of the initiators according to the invention are preferably boron compositions. / potassium nitrate (B / KNO3), THPP (titanium hydride / potassium perchlorate), TPP (titanium / potassium perchlorate) or ZPP (zirconium / potassium perchlorate), well known to those skilled in the art. Preferably, the main load, making it possible to obtain an ignition effect, is less intense than the load in contact with the resistive element. The main charges preferably used in the initiator according to the invention are of the THPP or B / KNO3 type.

The pyrotechnic system used in the initiator according to the invention can comprise an additional charge as defined above. These additional loads are generally placed between the load in contact with the resistive element and the main load or, preferably at the bottom of the cap after the main load in functional order.

The pyrotechnic system used in the initiator according to the invention can comprise a self-initiating charge. The self-initiating charge may be one of the charges constituting the normal pyrotechnic chain in the initiator, in particular the main charge or an additional charge. By an adequate choice of its constituents and their proportions, the self-initiating filler capable of being used in the present invention has a self-initiation temperature ranging from 130 to 220 C, preferably ranging from 150 to 180 C, and is perfectly stable at 107 C for 408 hours.

The electrical initiator according to the invention is an initiator with compressed pyrotechnic layers, which means that its primary charge, at least, is in the form of a compressed layer. Advantageously, its layers have been pre-compressed. The pre-compression is carried out with a pressure less than 120 bars for the primary charge and with a pressure greater than 150 bars for the secondary charge, if this is present. The final compressive force is ensured during the hermetic assembly of the cap with the base. The self-initiating filler, if used in the present invention, may be in compressed form, for example as layers or pellets, or in uncompressed form loose in electrical initiators or compartmentalized in cases preferably. metallic.

The present invention is not content only to greatly reduce the frequency and magnitude of operating delay anomalies. It also solves the problem of obtaining very short TTFPs, which will become clear from reading the following paragraphs.

As stated previously, the very fast running initiators employ rather than a cylindrical filament a flat filament, the thickness of which is very small. As a result, the energy density per unit of contact area of the resistive element is very low. Excellent contact between the resistive element and the primary load is therefore required. The contact must be all the more intimate as the thermal flywheel of the resistive element is weak: control of the interface therefore becomes crucial. This is the reason why it becomes necessary, for PTMS initiators, to use a composite material having very strict dimensional stability properties. Composite materials with a plastic matrix traditionally used to compose the base of initiators of the PTMS type (polyamides, polyketones, etc.) do not make it possible to use flat filaments. Thanks to the excellent dimensional stability of the initiators with compressed pyrotechnic layers PTMS whose base and possibly the cap are made with a composite material with a plastic matrix according to the invention, it now becomes possible to incorporate resistive elements of the filament type. flat, whether metallic layers or semiconductor layers, their thickness preferably varying from 1 to 10 m. The use of plastic materials having very low thermal expansion coefficients, very low moisture uptake rates, and high melting temperatures, such as the composite materials used in the present invention, effectively makes it possible to produce initiators of PTMS type provided with flat filaments exhibiting greatly reduced operating times, so as to meet the needs of the automotive safety industry, which cannot be satisfied with PTMS provided with cylindrical filaments due to the speed of temperature rise insufficient. The preferred material for obtaining very short TTFPs is the same as that defined above for the reduction in the occurrence of operating delay anomalies, namely a polyphenylene sulphide containing from 20 to 45% of fibers, chosen from among glass fibers, carbon fibers and their mixtures. According to a first preferred embodiment, the PPS matrix is reinforced with 20 to 35% by mass of carbon fibers. According to a second preferred embodiment, the PPS matrix is reinforced with 30 to 45% by weight of glass fibers. The PTMS structure based on composite materials according to the invention is more reliable than a GTMS structure for producing very fast operating initiators, comprising a flat filament / compressed pyrotechnic primary charge interface, in particular if said primary charge comprises a mixture of oxidants and reducing agents, such as mixtures of zirconium and potassium perchlorate. The dimensional stability of the plastic matrix composite is in fact greater than that of the metallic case of GTMS under the effect of variations in temperature and humidity.

Thus, the electrical initiator of the invention is preferably characterized by very fast operation. Preferably, the operating time of the electrical initiator according to the invention is less than 0.7 milliseconds under a current of 1.2 A and under a temperature which can vary from 40 C to + 90 C. Better still, it remains lower. at 0.7 ms even after the electrical initiator has been subjected to the various environmental tests mentioned above. It is not necessary to be limited to thin film flat filaments for PTMS needs having TTFPs meeting today's most stringent automotive industry specifications: thick film flat filaments are also suitable. The most recent safety specifications require manufacturers to use TTFPs of less than 0.7 ms at 1.2 A without modifying the electrical sensitivity specifications. However, a flat filament within the meaning of the invention, incorporated in an electrical initiator according to the invention can make it possible to achieve TTFPs of the order of 0.5 ms, and this without needing to have a primary explosive such as KDNBF or DDNP in contact with the resistive element.

The invention will be described in more detail with reference to the accompanying drawing, FIG. 1 being a schematic representation in longitudinal section of an electrical initiator according to the invention. The invention cannot be limited to the description of FIG. 1, which represents only one of the simplest embodiments that a person skilled in the art can make of it. The electropyrotechnic initiator, according to the invention, comprises a housing 1. This housing 1 comprises a cap 2, which constitutes the first sub-assembly and contains the products intended for firing, and a base 3 made of the composite material of the invention, which constitutes the second sub-assembly, pyrotechnically inert. The cap 2 comprises a wall 4 integral with a bottom 5, the cap 2 being made in one piece. The cap 2 contains at least one pyrotechnic charge. It receives all the pyrotechnic charges 6, generally by a dry loading process and contributes to the isolation of the pyrotechnic part. In the embodiment corresponding to FIG. 1, the cap 2 successively receives a secondary composition 7 and a primary composition 8 which is intended to be in contact with the resistive element 13. Each of these two pyrotechnic compositions is then charged in mass and then is pre-compressed.

The assembly of the cap 2 with the base 3 is preferably carried out by ultrasonic welding, ensuring excellent hermeticity. The cap 2 and the base 3 are therefore advantageously made of the same composite material according to the invention in order to allow better assembly, by ultrasonic welding. They are both made of an electrically insulating material. Compositions 7 and 8 are preferably free of heavy metals for the sake of respecting the environment. Said compositions are also resistant to the heating caused by friction of the grains of composition during ultrasonic welding. The base 3 comprises the electrical part of the initiator. It has a main axis 10 and is crossed by at least two electric current supply pins 11, 12 in the direction of said axis 10. The pins 11, 12 are electrically connected to each other by a resistive element 13 (electrically conductive bridge). on a face 14 of the base 3. Those skilled in the art will take care to ensure sufficient dielectric strength for the insulation of the conductive internal parts 11, 12 of the initiator. Base 3 is molded onto pins 11,12. In a first embodiment, the pins 11, 12 inserted into the base 3 comprise electrodes. Advantageously, the electrodes are striated. These ridges 9 not only provide a hermetic barrier for the initiator along the pins 11, 12 but they also facilitate the attachment of the composite material with a plastic matrix of the invention during overmolding. In a second embodiment, the pins 11, 12 are wires.

The electric initiator according to the invention can be used in motor vehicles to activate various passenger safety systems, such as the inflation of airbags or "airbags", the pre-tensioning of the seat belts, the locking. and unlocking doors. However, it is also suitable for any other pyrotechnic device requiring rapid and well-controlled triggering of a mechanism. By way of example, it can be used in offensive or defensive military systems and in protection systems against fires or floods. In the case where the initiator of the invention comprises a PPS-based matrix, it can without fear be brought into contact with or placed near vapors of chemical products.

The present invention finally relates to a gas generator comprising an electrical initiator according to the invention. Said initiator is suitable for use in gas generators using gas-generating compositions based in particular on 5-aminotetrazole, 1nitroguanidine, guanidinium azotetrazolate or triaminoguanidinium nitrate, but also based on sodium azide, all of which generate sodium azide. nitrogen during their decomposition. The gas generator generally contains an initiation reinforcing charge as defined above, placed between the initiator and the gas-generating composition. Such charges can have an auto-initiation function. The initiation reinforcing fillers frequently include a mixture of boron and potassium nitrate.

The gas generators of the invention are intended for use, in particular, in airbags.

Claims (29)

1. Electrical initiator with compressed pyrotechnic layers of PTMS type, comprising a cap (2) containing at least one pyrotechnic charge, a base (3) and a resistive element (13), characterized in that the base (3) is produced made of a composite material comprising a matrix formed of at least one plastic material, in which fibers are dispersed, said composite material having a melting point ranging from 260 to 320 C and a thermal expansion coefficient of less than 30.10-6 K- 1 in the axial direction according to the ASTM E831 test. 2. Electrical initiator according to claim 1, characterized in that the cap is made of a composite material comprising a matrix formed of at least one plastic material, in which fibers are dispersed, said composite material having a melting temperature ranging from 260 to 320 C and a coefficient of thermal expansion less than 30.10-6 K-1 in the axial direction according to the ASTM E831 test. 3. Electrical initiator according to any one of claims 1 and 2, characterized in that the plastic material of the matrix is poly (phenylene sulfide) (PPS). 4. Electrical initiator according to any one of claims 1 and 2, characterized in that the plastic material of the matrix is a liquid crystal polymer. 5. Electrical initiator according to any one of the preceding claims, characterized in that the composite material has a melting point ranging from 275 to 300 C. 6. Electrical initiator according to any one of the preceding claims, characterized in that the moisture uptake rate of the composite material is less than 0.05% in 24 hours, preferably less than 0.03% in 24 hours, according to ASTM D570 test. 31 2885684 7. Electrical initiator according to any one of the preceding claims, characterized in that the composite material has a coefficient of thermal expansion of less than 25.10.6 K "1 in the axial direction according to the ASTM E831 test. 8. Electrical initiator according to any one of the preceding claims, characterized in that the matrix is reinforced by 20 to 45% by mass of fibers. 9. Electrical initiator according to any one of the preceding claims, characterized in that the fibers are chosen from carbon fibers, glass fibers and mixtures thereof. 10. Electrical initiator according to any one of the preceding claims, characterized in that the matrix is reinforced with 20 to 35% by mass of carbon fibers. 11. Electrical initiator according to any one of the preceding claims, characterized in that the matrix is reinforced with 30 to 45% by weight of glass fibers. 12. Electrical initiator according to any one of the preceding claims, characterized in that the composite material has a deflection temperature under 1.82 MPa greater than 230 C according to the ASTM D648 test. 13. Electrical initiator according to any one of the preceding claims, characterized in that the resistive element is a cylindrical metal filament. 14. An electrical initiator according to any one of claims 1 to 12, characterized in that the resistive element (13) is a flat filament fixed on a support. 15. Electrical initiator according to claim 14 characterized in that the flat filament comprises at least one thin layer, preferably metallic. 16. An electrical initiator according to claim 14 characterized in that the flat filament comprises at least one thick, metallic or semiconducting layer. 17. Electrical initiator according to claim 14 characterized in that the resistive element is a resistive element of SCB type. 18. An electrical initiator according to any one of claims 14 to 17, characterized in that the flat filament has a thickness ranging from 1 to 10 m. 19. An electrical initiator according to any one of claims 14 to 17, characterized in that the flat filament has a thickness ranging from 2 to 8 m. 20. Electrical initiator according to any one of the preceding claims, characterized by the presence of a load (8) in contact with the resistive element in compressed form and having a compression ratio greater than 30 bars. 21. An electrical initiator according to claim 20, characterized in that the charge in contact with the resistive element comprises a mixture of oxidants and reducers. 22. An electrical initiator according to claim 20, characterized in that the charge in contact with the resistive element comprises a mixture of zirconium and potassium perchlorate. 23. Electrical initiator according to any one of claims 20 to 22, characterized in that the charge in contact with the resistive element 35 comprises at least one additive chosen from a binder, a metal oxide, a transition metal oxide, metalloid oxide, ultra fine silica powder, talc, graphite and boron nitride. 24. Electrical initiator according to claim 23, characterized in that the binder is a fluoroelastomer binder. 25. Electrical initiator according to any one of the preceding claims, characterized in that it comprises a self-initiating pyrotechnic composition. 26. An electrical initiator according to any one of the preceding claims, characterized in that it is stable at 107 C for 408 hours. 27. Electrical initiator according to any one of the preceding claims, characterized in that it is stable at 85 C and 95% relative humidity for 5 days. 28. Electrical initiator according to any one of the preceding claims, characterized by a TTFP of less than 0.7 milliseconds under a current of 1.2 A and under a temperature which can vary from 40 C to + 90 C. 29. Gas generator characterized in that it comprises an electric initiator according to any one of claims 1 to 28.