GB2312274A - Pyrotechnic device - Google Patents

Abstract

A pyrotechnic device comprising an igniferous explosive train, in a container 13 in association with appropriate ignition means 20. The explosive train comprises a charge 12 of incendiary material ignitable by the initiation means and a charge 11 of deflagrating explosive material ignitable by the charge of incendiary material, the deflagrating explosive material being a compressed mixture of particulate secondary explosive material and incendiary material. The deflagrating charge maximum dimension is less than the minimum distance for burning to detonation transition to occur in the deflagrating charge. Suitable deflagrating charge compositions comprise secondary explosive materials such as for example, PETN (pentaerythritoltetranitrate) HMX (cyclo-tetramethylene tetranitramine), RDX (trimethylene trinitramine), HNAB (2,2,4,4,6,6 hexanitroazobenzene) or HNS (hexanitrostilbene) intimately mixed with an incendiary material, for example potassium picrate, or barium styphnate. The charge 12 of incendiary material may comprise barium styphnate or lead styphnate and the initiation means is conveniently a low voltage bridgewire.

Description

PYROTECHNIC DEVICE This invention relates to a pyrotechnic device which reacts extremely fast and generates very high pressure and heat. The device is especially useful for igniting fast, high pressure gas-generating compositions and for rupturing closures, for example bursting discs, on containers such as gas bottles, the stored gas being optionally simultaneously heated and/or ignited. The invention also includes a method of opening a pressure vessel having a pressure-rupturable closure element such as a bursting disc using the pyrotechnic device, a pressure vessel closure assembly comprising the pyrotechnic device and a hybrid inflator for a vehicle occupant safety air-bag including the pyrotechnic device to heat stored gas and/or to release the stored gas.

Pyrotechnic devices used hitherto are relatively slowacting, generally taking about 2 milliseconds or more to generate useful heat and flame. The pressure developed is too low to rupture the usual bursting discs of gas bottles unless the device contains excessively large charges of pyrotechnic material and these are expensive and generally produce excessive amounts of unwanted solid products.

Explosively operated protractor devices (explosive actuators) or detonators are now generally employed to rupture bursting discs. However, detonators are unacceptable in many applications because of the risk of damage by the shock output of the detonator and protractor devices are more complicated and expensive.

In some gas-bottle opening applications it would be advantageous to open the bottle and simultaneously ignite combustible contents of the bottle but none of the conventional bottle opening devices will perform both of these operations. Moreover, the time required to release gas from a bottle using conventional devices is typically above a millisecond and many applications require much faster acting systems.

The pyrotechnic device of the present invention overcomes the disadvantages of the devices currently used.

In accordance with the invention a pyrotechnic device comprises an igniferous explosive train, means for containing and confining said explosive train and initiation means for igniting said explosive train, said explosive train comprising in ignition transmission relationship a charge of incendiary material ignitable by said initiation means and a charge of deflagrating explosive material ignitable by said charge of incendiary material, said deflagrating material comprising compressed particulate secondary explosive material in admixture with incendiary material and the deflagrating charge maximum dimension being less than the minimum distance required for burning to detonation transition to occur in the deflagrating charge. In the present context "secondary explosive material" is a material which can be made to detonate by a detonation wave or other shock front but which does not normally detonate when heated or ignited.

The charge of deflagrating explosive material, on ignition, rapidly generates products at very high pressure as required to open the closures of gas storage bottles.

As disclosed in United State Patent No. 4,316,412 a compressed charge of fine particulate secondary explosive such as PETN will undergo transition from deflagration to detonation when strongly confined and ignited by a deflagrating secondary explosive charge having higher density and diameter. In general a minimum charge length of 10mm is needed for transition to occur. Accordingly, since the possibility of detonation must be avoided in the pyrotechnic devices of the present invention, the charge of deflagrating material will generally have a maximum dimension of less than 10mm and be of uniform density throughout.

The charge of incendiary material ensures rapid and reliable ignition by the initiation means and the onward, substantially instantaneous transmission of a large area flame front to the deflagrating charge. The incendiary material incorporated in the charge of deflagrating explosive material ensures extremely rapid spread of flame throughout the charge.

A preferred deflagrating explosive material comprises particulate secondary explosive material, for example, PETN (pentaerythritoltetranitrate) HMX (cyclotetramethylene tetranitramine), RDX (trimethylene trinitramine), HNAB (2,2,4,4,6,6 hexanitroazobenzene) or HNS (hexanitrostilbene) intimately mixed with an incendiary material, for example potassium picrate, or barium styphnate. The secondary explosive material and the incendiary material should preferably be present in a weight ratio in the range from 90:10 to 50:50.

Both the secondary explosive material and the incendiary material in the deflagrating charge should consist of fine particles of which preferably at least 60% by weight have maximum dimension less than 100 m.

In order to burst a 1.8mm thick steel bursting disc the charge of deflagrating explosive material typically has a mass of 130 to 200mg.

The charge of incendiary material ignitable by the initiation means preferably comprises a material which is readily ignitable from a low voltage hot bridgewire.

Suitable materials include barium styphnate and lead styphnate . The charge should preferably consist of very finely divided, particulate material pressed at a pressure of at least 600psi, more preferably at least 1000psi.

Preferably all the particles should have a diameter less than 25ym in order to allow the material to be pressed around an initiator bridgewire (typically having a diameter of 25-40ym) without breaking the wire. A typical charge of incendiary material is 5 to 50mg.

The igniferous explosive train is preferably contained in a tubular container, preferably a metal container, typically 0.5 to lcm internal diameter and having a wall thickness of 0.4 to lrnm.

The initiation means may comprise any conventional igniferous initiator, for example a percussion or stab initiator, but an electrical initiator comprising a low voltage hot bridgewire such as is used in a conventional squib is preferred. A typical bridgewire is a 1.5-2mm length of a 0.025-0.04mm diameter Nichrome (80% Ni 20% Cr) wire. The bridgewire is conveniently attached to metal conductor wires passing through insulating sealing material in a metal header which can be inserted and crimped into a metal tube containing the explosive train.

The sealing material may conveniently comprise ceramic material or glass.

From another aspect the invention consists in a method of opening a pressure vessel closed with a pressurerupturable closure element, in which method a pyrotechnic device in accordance with the invention is disposed with the charge of deflagrating explosive material adjacent to said closure element and is ignited, thereby generating sufficient pressure to rupture the closure element.

From a further aspect the invention includes a pressure vessel closure assembly comprising a pressure-rupturable closure element adapted, in use, to seal an opening in a pressure vessel in association with a pyrotechnic device in accordance with the invention disposed with the charge of deflagrating explosive material sufficiently close to said closure element that, in use, the pressure generated by the burning deflagrating material ruptures the closure element.

The closure element may be in any appropriate form for sealing an aperture but will preferably be a flat element such as a conventional bursting disc. Usually the element will be a metal element for example, mild steel or stainless steel of a thickness appropriate to withstand the pressure of the contents of the pressure vessel.

The pyrotechnic devices of this invention produce gaseous products with an extremely low solid content (i.e. they are very "clean" burning). A device of the invention can therefore be used advantageously to heat and expand gases in systems, such as hybrid air-bag systems, where the gaseous products in admixture with stored gas heated thereby must pass through a nozzle without risk of blocking the nozzle aperture.

Accordingly the invention further includes a hybrid inflator for a vehicle safety air-bag wherein a pyrotechnic device in accordance with the invention is disposed to heat a stored supply of inflation gas and/or to release said supply of gas. This inflator is extremely fast-acting and is therefore advantageous for vehicle side-protecting air-bags where rapid deployment of the air-bag is required.

The invention is further described, by way of example only, with reference to the accompanying drawings wherein Fig 1 shows diagrammatically a longitudinal medial crosssection of a pyrotechnic device; Fig 2 shows diagrammatically a fragmentary longitudinal medial section of a pressure vessel having a device of Fig 1 positioned adjacent to the presure vessel closure element; and Fig 3 shows diagrammatically a fragmentary longitudinal medial section of a hybrid inflator having a pyrotechnic device of Fig 1 as a gas heating element.

In the pyrotechnic device 10 shown in Fig 1 a charge of deflagatory explosive material 11 and an overlying charge of incendiary material 12 are pressed into a thin cylindrical metal cup 13. A header assembly 14 comprising a metal header element 15 and a pair of conductor wires 16 and 17 extending through a central bore 18 in the header element 15 is firmly held in the cup 13 by a circumferential hermetic crimp seal formed by swaging the cup into a cannelure 19 formed around the header element 15. The ends of the conductor wires 16 and 17 are electrically connected by a bridgewire 20 which is immersed in the incendiary material 12. The conductor wires 16 and 17 are spatially separation and located in the bore of the header elements 15 by a glass plug 21 which forms a glass to metal seal round the wires 16 and 17 and with the header element 15.

The header element 15 has a thick end portion 22 which is stepped internally to provide an internal annular seal 23 and a terminal annulus 24. A locating element 25 moulded from insulating plastics material and having a curcumferential flange 26 is located within the annulus 24, with the flange 26 resting on the seat 23. The annulus 24, is swaged around the flange 26 of the locating element to retain the element 25 firmly in the header element 15. The locating element 25 has two longitudinal holes 27 and 28 through which the conductor wires 16 and 17 are trained, the wires fitting tightly in the holes 27 and 28 so that the element 25 provides a moisture proof seal between the wires 16 and 17 and the header element 15.

In use, the passage of electric current through the conductor wires 16 and 17 causes the bridgewire 20 to heat and ignite the igniferous explosive train of incendiary material 12 and deflagrating material 11.

The pressure vessel shown in Fig 2 comprises a pressure vessel 30 having welded to one end a closing ring 31 defining an aperture 32 which is sealed by a rupturable diaphragm 33. A pyrotechnic device 10 as shown in Fig 1, and shown in Fig 2 in outline only, is positioned close to the diaphragm 33 and firmly held in position by a tubular gas outlet manifold 35 which is welded to the ring 31.

The manifold 35 is formed without outlet ports 36.

In use the vessel 30 contains gas at a pressure which is insufficient to rupture the diaphragm 33. On firing the device 34 combustion products at extremely high heat and pressure impinge on the diaphragm 33, causing it to burst within a very short time interval, whereupon the pressurised gas escapes through the aperture 32 and passes through the outlet ports 36.

The hybrid inflator shown in Fig 3 comprises a cylindrical pressure vessel 40 which contains a stored supply of inflation gas under pressure. A pyrotechnic device 10 shown in Fig 1, and shown in Fig 3 in outline only, is sealed into one end of vessel 40. The other end of the vessel 40 comprises a closure ring 41 is welded to the end of the cylindrical wall of vessel 40. The ring 41 defines an aperture 42 which is sealed by a rupturable diaphragm 43. A tubular manifold 44 formed with outlet ports 45 is welded to the closure ring 41.

In use the vessel 40 contains gas at a pressure which is insufficient to rupture diaphragm 44. On firing the device 10 the stored gas is heated and augmented by the combustion products from the device so that the pressure of gas in the vessel 40 rapidly increases to a pressure exceeding the pressure required to rupture the diaphragm 44 and the diaphragm rupture allowing the heated gas to escape through the aperture 43 and the ports 42 The invention is further illustrated by the following Examples wherein all parts and percentages are given by weight.

Example 1 Pyrotechnic device for opening a bursting disc A deflagrating explosives material was prepared by intimately mixing 90 parts of PETN having mean particle diameter of 300yam and 16 parts of potassium picrate having mean particle diameter of 20pom.

A mass of 140 milligrams of this mixture was pressed with 50Kg force into a copper cup having internal diameter of 6mm and 0.6mm wall thickness. A mass of 10 milligrams of barium styphnate of which all the particles were less than 25m diameter was loaded into the cup on top of the deflagrating material to serve as incendiary material.

A header assembly comprising two metal conductor wires connected by a Nichrome bridgewire having a diameter of 35 pm and sealed by a glass metal seal to a metal header element was inserted into the cup and pressed to immerse the bridgewire into the barium styphnate and to compress the barium styphnate under a pressure of 1000psi. The cup was crimped tightly around the metal header element. A moulded locating plug of plastics material was fitted over the conductor wires and an annulus at the end of the metal header element was swaged around the circumference of the plug.

The device was firmly located against a peripherally supported stainless steel bursting disc having a diameter of 25mm and a thickness of 1.8mm, the cup being confined by a length of relatively thick walled tube (as shown in Fig 2).

On passing a one amp current through the bridgewire of the device the device exploded and the pressure output clearly sheared the disc around the periphery of the support position.

Example 2 Pyrotechnic device for opening a burstings disc The procedure described Example 1 was repeated using a steel bursting disc 0.89mm thick. The bursting disc was fragmented into several pieces.

Example 3 Pyrotechnic device for opening a bursting disc A device was prepared and tested as described in Example 1 except that the deflagrating composition contained 50 parts PETN, 25 parts potassium picrate and 25 parts potassium perchlorate.

On passing a one amp current through the bridgewire the 1.8mm thick bursting disc was bowed into a hemisphere but was not sheared from the peripheral support.

Example 4 Releasing and igniting inflammable gas The procedure described in Example 2 was repeated with the bursting disc attached as a closure for a gas bottle containing a mixture of argon, oxygen and hydrogen in the molar ratio 90: 10 : 20 at a pressure of 5000psi. The bursting disc was again fragmented and the gas inside the bottle was ignited.

Example 5 Gas release with oxidation of combustion products The procedure of Example 4 was repeated except that the gas bottle contained a mixture of argon and oxygen in the molar ratio 90 : 10. The bursting disc was again fragmented and the oxygen negative combustion products of the pyrotechnic device were oxidized to give heated, nontoxic gas.

Example 6 Ignition of gas generator The pyrotechnic device described in Example 1 was located as an igniter against the base of a 25g charge of ammonium nitrate-based gas generating composition in a gas generator designed to operate at 10,000 psi. On firing the device the generator reached its operating pressure within 2.5 milliseconds.

In a similar experiment using a conventional igniter the time to reach the operating pressure was 5 milliseconds.

Example 7 Fast acting protractor The pyrotechnic device described in Example 1 was used as the power unit of an explosive protractor device. The action time for a stoke of 5mm was 1 millisecond whereas with a conventional power unit containing black powder the action time is 10 milliseconds.

Example 8 Ignition of pyrotechnic film The pyrotechnic device described in Example 1 was used to ignite a 5g charge of a pyrotechnic film (Enerfoil registered trade mark) inside a pressurised gas bottle containing argon at 3000 psi. The film was rolled onto a 3mm diameter steel former, the formed charge being 15cm long with external diameter of 12mm. The charge was completely reacted in about 2 milliseconds whereas using conventional igniters only half the film reacts in 2 milliseconds, the remainder taking up to 40 milliseconds for complete reaction.

Claims (22)

1. A pyrotechnic device comprising an igniferous explosive train, means for containing and confining said explosive train and initiation means for igniting said explosive train, said explosive train comprising in ignition transmission relationship a charge of incendiary material ignitable by said initiation means and a charge of deflagrating explosive material ignitable by said charge of incendiary material, said deflagrating material comprising compressed particulate secondary explosive material in admixture with incendiary material and the deflagrating charge maximum dimension being less than the minimum distance required for burning to detonation transition to occur in the deflagrating charge.

2. A device as claimed in claim 1 wherein the said deflagrating explosive material comprises PETN, HMX, RDX, HNAB or HNS intimately mixed with an incendiary material comprising potassium picrate or barium styphnate.

3. A device as claimed in claim 1 or claim 2 wherein the secondary explosive material and the incendiary material are present in the deflagrating material in a weight ratio in the range from 90 : 10 to 50 : 50.

4. A device as claimed in any one of claims 1 to 3 wherein both the secondary explosive material and the incendiary material in the deflagrating charge consists of fine particles of which at least 60% by weight have maximum dimension less than 100cm.

5. A device as claimed in any one of claims 1 to 4 wherein the charge of deflagrating explosive material has a mass of 130 to 200mg.

6. A device as claimed in any one of claims 1 to 5 wherein the said charge of incendiary material comprises a material which is readily ignitable from a low voltage hot bridgewire.

7. A device as claimed in claim 6 wherein the charge of incendiary material comprises barium styphnate or lead styphnate.

8. A device as claimed in claim 6 or claim 7 wherein the said charge of incendiary material consists of finely divided particulate material pressed at a pressure of at least 600psi.

9. A device as claimed in any one of claims 6 to 8 wherein the said charge of incendiary material consists of particles all having a diameter less than 25yam.

10. A device as claimed in any one of claims 1 to 9 wherein the charge of incendiary material amounts to 5 to 50mg.

11. A device as claimed in any one of claims 1 to 10 wherein the igniferous explosive train is contained in a tubular container.

12. A device as claimed in claim 11 wherein the tubular container is a metal container, 0.5 to lcm internal diameter having a wall thickness of 0.4 to imam.

13. A device as claimed in any one of claims 1 to 12 wherein the said initiation means comprises a low voltage hot bridgewire.

14. A device as claimed in claim 13 wherein the bridgewire is a 1.5 - 2.0mm length of a 0.025 -0.04mm Nichrome wire.

15. A device as claimed in claim 13 or claim 14 wherein the bridgewire is attached to metal conductor wires passing through insulating sealing material in a metal header inserted and crimped into a metal tube containing the explosive train.

16. A device as claimed in claim 16 wherein the said sealing material comprises ceramic material or glass.

17. A pyrotechnic device comprising an igniferous explosive train and initiation means therefor substantially as described herein with reference to the accompanying drawings.

18. A method of opening a pressure vessel closed with a pressure-rupturable closure element wherein a pyrotechnic device as claimed in any one of claims 1 to 17 is disposed with the charge of deflagrating explosive material adjacent to said closure element and is ignited, thereby generating sufficient pressure to rupture the closure element.

19. A method of opening a pressure vessel substantially as described herein with reference to the accompanying drawings and Examples 1 to 4.

20. A pressure vessel closure assembly comprising a pressure-rupturable closure element adapted, in use, to seal an opening in a pressure vessel in association with a pyrotechnic device as claimed in any one of claims 1 to 17 disposed with the charge of deflagrating explosive material sufficiently close to said closure element that, in use, the pressure generated by the burning deflagrating material ruptures the closure element.

21. A pressure vessel closure assembly substantially as described herein with reference to the accompanying drawings.

22. A hybrid inflator for a vehicle occupant safety air bag wherein a pyrotechnic device as claimed in any one of claims 1 to 17 is disposed to heat a stored supply of inflation gas and/or to release said supply of gas.