GB2312492A - Hybrid inflatator for an air bag - Google Patents

Abstract

In a hybrid inflator suitable for use in a vehicle occupant restraint safety system. The inflation gases are heated by at least one coiled charge 14 of pyrotechnic sheet material ignited simultaneously over a curved surface area of the coil by igniferous booster means which may be spaced-apart plies 27 of the same pyrotechnic sheet material wound coaxially with the slow-burning coiled charge 28. Preferably the gases are heated by two or more coiled charges 14,15, the delay interval between the initiation of the charges varying in accordance with the level of the velocity of a vehicle crash and/or the position of a vehicle occupant at the time of the crash. The coiled charges are preferably located inside the gas storage vessel 10, each charge being advantageously confined in a perforated combustion tube 16,17. The spacing between the plies in portions the charge coils may be varied to effect different burning rates in the different portions of a charge. The charges comprise substrate layers of oxidizing polymeric film, for example, polytetrafluoroethylene coated with a metal, for example magnesium.

Description

EYBRID INFLATOR This invention relates to a hybrid inflator for an air-bag or cushion suitable for use in vehicle occupant restraint safety systems. The invention also includes a heating element for the inflator and a vehicle safety system employing said inflator.

In a hybrid inflator a stored supply of compressed inflation gas is heated by burning propellant material thereby increasing the effective temperature and pressure of the gas and possibly also augmenting the quantity of gas. Such inflators have been described in US Patent Nos.

5,290,060, 5,226,561, 5,131,680 and 5,351,989. The inflators described in these specifications employ a single heating charge of solid propellant material to provide a fast release of inflation gas with a fixed inflation rate characteristic. However it has been found during the development of air-bag safety systems that, if the initial inflation rate is too high, injury may be caused to vehicle occupants who may be out of the normal seated position (for example, a small child located close to an air-bag module) when the bag is inflated.

Consequently, it would be desirable to maintain the inflation rate below a value which could contribute to passenger injury. Such low inflation rates may be adequate for low velocity crashes but may not be fast enough to protect passengers effectively in the event of high crash velocities where the inflator may be required to function at its highest permitted rate, Hybrid inflators have therefore been designed to function for an initial period at relatively low inflation rate and thereafter at a relatively high inflation rate, Such inflators have been described in US Patent Nos 5,345,876 and 5,351,988. In the inflator of US Patent No 5,345,876, a first charge of pyrotechnic material releases the stored gas and a second charge of pyrotechnic material, ignited by the combustion products of the first charge, heats the released gas, fixed predetermined time delays being effected between the release of stored gas and the ignition of the second charge and between the ignition of the second charge and the heating of the released gas.

The inflator of US Patent 5,351,988 has two charges of propellant material both located outside the gas storage pressure vessel and both contributing to the heating of the gas. Ignition of the first charge causes the bursting of a gas retaining bursting disc and releases the gas and the second charge is ignited simultaneously, or after an appropriate time delay in response to a crash sensor signal characteristic of the level of the velocity or deceleration of the vehicle involved in the crash. The time delay and consequently the inflation rate characteristics will therefore vary appropriately in accordance with a programme which is pre-set to provide the required vehicle occupant protection for variable crash velocities.

For safety reasons solid heat-generating pyrotechnic material used in the hybrid inflators is generally contained in a thick-walled pressure-resisting vessel and arranged to deliver its combustion products to the stored gas in a mixing zone outside the vessel in which the gas is contained. This arrangement of inflator is therefore complicated as it requires accurately constructed and expensive gas flow-control devices. A simple form of hybrid inflator employing a heating charge of pyrotechnic sheet material consisting of one or more substrate layers of oxidizing polymeric film coated with oxidizable material exothermically reactable therewith has been described in US Patent No 5,411,290, which is incorporated herein by reference. In this inflator the charge of pyrotechnic sheet material can advantageously be located in the gas storage pressure vessel in direct contact with the stored gas. However, like the aforedescribed hybrid inflators using a single charge of pyrotechnic heating material, the inflator has fixed, designed, inflation rate characteristics and is designed for fast burning.

An object of the present invention is to provide an improved hybrid inflator of the kind employing a gas heating charge comprising polymeric film coated with oxidizable material, said inflator having inflation rate characteristics appropriate for safe deployment of the inflator over a wide range of levels of vehicle crash velocities and/or vehicle occupant positions when the vehicle is involved in a crash.

A further object is to provide a programmable hybrid inflator of said kind, whereby the inflation rate characteristics can be varied in accordance with the level of crash velocity and/or vehicle occupant position when the vehicle is involved in a crash.

We have found that a coiled charge of pyrotechnic sheet material comprising one or more substrate layers of oxidizing polymeric film coated with a layer of oxidizable material, said sheet being wound with the plies of the coil in close contact and ignited at one end, burns slowly in its axial direction at a rate which is too low to provide adequate heating for a hybrid inflator. However, when the plies are spaced apart the burning rate becomes extremely fast due to flame and combustion products travelling through the space and igniting material in advance of the burning front A closely wound coil of pyrotechnic sheet will, however, burn at an intermediate rate suitable for slow, safe inflation of an air-bag when the coiled charge is ignited simultaneously over an area of one or both of its curved surfaces. In this case the coiled charge burns in a radial direction over a large burning zone, thereby increasing the rate of consumption of the pyrotechnic material and the rate of heat production. Substantially simultaneous ignition over the inner or outer curved surface of the coil can be efficiently effected by burning igniferous booster material simultaneously over the surface to be ignited.

Thus in accordance with the invention, a hybrid inflator for an air-bag comprises a gas storage vessel having a sealed exit; a supply of gas stored in gaseous form under pressure in said vessel; pyrotechnic material effective to heat and expand said supply of gas; initiation means to ignite said pyrotechnic material and means to open said sealed exit to permit heated gas to flow through said exit to an air-bag when the bag is required to be inflated; said pyrotechnic material comprising at least one closely wound coiled charge of pyrotechnic sheet material consisting of one or more substrate layers of oxidizing polymeric film having a layer of oxidizable material on at least a portion of at least one surface of the, or each, substrate layer, the polymeric film and the oxidizable material being conjointly capable of reacting together exothermically on ignition; and igniferous booster means ignitable by said initiation means and adapted and located so that, burning of the booster ignites the said coiled charge substantially simultaneously over a curved surface of the coil.

A convenient booster charge comprises spaced-apart plies of pyrotechnic sheet material wound co-axially with the closely wound coil plies and in ignition transmission relationship therewith. The spaced-apart plies are preferably wound around the outside of the closely wound coil of sheet material as, with this arrangement, there is less tendency for the burning booster material to break the plies of the closely wound coil and consequently more uniform ignition and burning speed is obtained. The coiled charge and the igniferous booster charge of spacedapart plies may comprise the same pyrotechnic sheet material and conveniently may be wound from a single portion of pyrotechnic sheet material.

We have also found that two or more separate coiled charges of the aforedescribed pyrotechnic sheet material may be disposed inside a stored supply of gas under pressure in a pressure vessel and, if desired, ignited with a delay between the ignition of the charges to provide variable controlled heating of the stored gas, the heated gas consequently being effective to inflate at a variable rate an air-bag into which it is released. Once ignited the coil burns lengthwise along the coil and there is little lateral transmission of ignition between adjacent coiled charges and, therefore, the charges may be disposed closely without being in contact. We have also found that the burning rate of each individual coiled charge can be varied controllably by varying the spacing between adjacent plies of sheet material in the coil and that radially disposed discrete portions of a coiled charge having different ply spacings will burn at different rates, larger spacing resulting in higher burning rates.

Accordingly a hybrid inflator of the invention having a relatively low inflation rate initially and a faster inflation rate thereafter, includes a second coiled heating charge of pyrotechnic sheet material in addition to the aforedescribed slow-burning coiled heating charge, said coiled charges being sufficiently spaced apart to burn independently without transmission of ignition between the charges and each coiled charge being provided with initiation means whereby the charges may be ignited either simultaneously or with a controlled delay between successive ignitions. The second charge may be similar to the slow-burning charge but for fast inflation rate will generally be a faster burning coiled charge, the increased burning being conveniently achieved by winding the coil with at least part of the plies spaced apart.

The coiled charges of pyrotechnic sheet material may be located outside the vessel containing the stored gas and be effective to heat the stored gas after it flows through the vessel exit before it enters the air-bag. However, for simplicity of design and reduced production costs, it is preferred to dispose the, or all, pyrotechnic charges inside the pressure vessel, each charge being either in a thin container or in direct contact with the stored gas.

The sealed exit of the storage vessel is conveniently sealed by a frangible element which may, for example, be a bursting disc designed to burst when the stored gas attains a predetermined pressure after being heated by the burning pyrotechnic material. Alternatively the exit seal may be removed independently of the heating of the stored gas, for example, by a an electric squib or detonator fired in response to vehicle collision conditions. In this case the pressurised gas may be released to commence inflation of the air-bag before, simultaneously with or after the pyrotechnic charges are ignited.

The initiation means for igniting the, or all, the pyrotechnic charges may comprise any convenient flame source such as for example an electric squib, a shock tube, an exploding bridgewire, a semi-conductor bridge, a spark discharge, an electric current or an electrically heated wire. The conventional squib will generally be convenient. Preferably each coiled pyrotechnic charge has an initiation means positioned to ignite the charge at one end of the coil which may advantageously be fringed, for example with longitudinal cuts to facilitate ignition. In operation the actuation of the individual initiation means may advantageously be effected in response to a vehicle collision sensor capable of determining crash velocity, the delay between actuation of successive initiation means being determined in accordance with a pre-set programme relating the delay to crash velocity and/or vehicle occupant position.

The pyrotechnic sheet material in the slow-burning coiled charge will generally be in flat form, tightly wound to leave little, if any, space between adjacent plies. In the said second coiled charge at least part, and generally all, of the coiled plies may be wound to leave spaces between adjacent plies wherein flame and combustion products may travel in advance of the burning front to facilitate fast initiation of the charge. The spacing between the plies of the second charge and also the spacing of the aforedescribed preferred igniferous booster for the said slow-burning charge may be provided by the use of spacing elements or by providing protrusions on the sheet surface, for example by forming raised portions on the sheet or by folding, wrinkling, corrugating or otherwise deforming the sheet. A preferred method of providing spacers is by stamping dimples on the sheet as described in European patent publication no 645354. If desired, coiled charges wherein individual portions are designed to burn at different speeds may be formed by coiling portions of pyrotechnic sheet having defined areas provided with spacers such as the aforementioned dimples.

Thus a coiled charge designed to have a slow burning inner portion constituting the slow-burning charge, and a relatively fast burning outer portion constituting the igniferous booster for the slow-burning charge may be formed from a length of pyrotechnic sheet which is plain over a portion at one end and dimpled over a portion at the other end by winding the coil from the end having the plain portion.

A convenient spacing between the plies of the fast burning portions of the coiled charges is from 50-200 ym preferably about 100 ym and accordingly dimples having a height in this range are preferred.

Pyrotechnic sheet material suitable for use in the aforedescribed coiled charges are described in PCT International Publications Nos. WO 90/10611 and WO 90/10724. Preferably the ratio of oxidizable material to oxidizing polymer is substantially stoichiometric or there is a small excess of oxidizable material at the location of the film underlying the oxidizable material. The burning rate of this type of sheet is enhanced by forming protrusions on the surface, the protrusions preferably having flame-permeable apertures in either the substrate or the oxidizable material, which apertures expose the interface between the substrate and oxidizable material layer as described in European patent publication no 645345.

The preferred oxidizing substrate comprises polymeric film containing atoms chemically bound therein selected from the group consisting of hydrogen, halogens (especially fluorine), oxygen, sulphur, nitrogen and phosphorous. One preferred film substrate comprises fluoropolymer such as polytetrafluoroethylene (PTFE) which produces a high energy pyrotechnic sheet, but other suitable polymeric films include those comprising polychlorotrifluoroethylene, polyhexafluoropropylene, copolymers of trifluoroethylene and hexafluoropropylene, copolymers of trifluoroethylene and tetrafluoroethylene, copolymers of hexafluoroproylene and tetrafluoroethylene, copolymers of hexafluoropropylene and vinylidene fluoride, copolymers of chlorotrifluoroethylene and vinylidene fluoride, trichloroethylene homopolymers, copolymers of trichloroethylene and vinylidene fluoride, mixtures of two or more of such polymers or mixtures of any one of such polymers with PTFE.

The polymeric film may optionally be a porous film, the pores advantageously occupying 6-956 of the film volume (i.e. porosity of 6-95). Preferably the pores are vapour permeable pores containing at least part of the oxidizable material. Pyrotechnic sheet material comprising such porous film generally has faster burning rates than that containing only solid polymeric film.

The oxidizable material may advantageously comprise metal selected from the group consisting of lithium, sodium, magnesium, beryllium, calcium, strontium, barium, aluminium, titanium, zirconium and alloys thereof, which metal may advantageously be deposited on the substrate by vapour deposition or magnetron sputtering. A metal layer is especially advantageous as it significantly enhances the dimensional stability of the pyrotechnic sheet. A most preferred metal for high heat generation is magnesium or an alloy thereof preferably coated on to a substrate film comprising fluoropolymer. Preferably the ratio of metal to the substrate of oxidizing polymeric film is substantially stoichiometric or there is a small excess of metal at the location of the film underlying the metal. A typical pyrotechnic sheet consists of a substrate of PTFE film coated with magnesium. The reaction between PTFE and magnesium can be represented empirically as

This reaction releases 9.6 megajoules/kilogram of reactant pyrotechnic material.

The rate of energy release on ignition varies inversely with the thickness and directly with porosity of the pyrotechnic sheet material and, accordingly, the thickness and porosity will be chosen to attain the desired energy release. Thus the preferred polymeric film will generally have an areal mass of 10 to 150g/m2 (corresponding to thickness in the range from 4.5 to 682cm for PTFE having porosity of 0 to 90%), typically 25-75g/m2 (corresponding to thickness in the range from 11.4 to 114cm for PTFE having porosity of 0 to 90%) and the total amount of the oxidizable material will be equivalent to a laminar thickness of 2 to 30 ym typically 4 to 15 ym.

One or more of the coiled charges may advantageously be encased in a perforated combustion tube to provide confinement for the burning charge and increase its burning rate whilst still allowing the combustion products to escape into the stored gas. The combustion tube is conveniently a metal tube, for example a steel tube, preferably having perforations of 1.5 - 2.5mm diameter distributed along its length, the perforations defining an open area of tube corresponding to 2 to 30% of the total wall area.

Further in accordance with the invention a vehicle occupant restraint safety system comprises a hybrid inflator of the invention; an inflatable bag into which inflation gas is supplied from the inflator; sensor means to detect vehicle crash conditions and/or vehicle occupant position and initiation control means to actuate the initiation means of the closely wound coiled pyrotechnic charge of the inflator in response to a signal from said sensor means.

In an especially useful vehicle occupant restraint safety system designed to inflate the air-bag initially at a relatively slow rate and thereafter at a faster rate the hybrid inflator has an additional coiled charge of pyrotechnic sheet material and initiation means there for, and the initiation control means is operable to actuate the initiation of the additional coiled charge after a delay interval starting with the actuation of the initiation means of the first coiled charge, the control means being programmed to select the delay interval in accordance with the crash velocity and/or vehicle occupant position when a vehicle containing the safety system is involved in a crash.

The invention also includes a slow-burning heating charge suitable for a hybrid inflator for an air-bag, which charge comprises a closely wound coil of pyrotechnic sheet material consisting of one or more substrate layers of oxidizing polymeric film having a layer of oxidizable material, preferably metal, on at least a part of at least one surface of the, or each, substrate layer, the polymeric film and the oxidizable material being conjointly capable of reacting together on ignition, and igniferous booster means adapted and located to ignite said charge substantially simultaneously over at least one of its curved surfaces, preferably an outer curved surface. The preferred igniferous booster means comprises plies of pyrotechnic sheet wound co-axially with the closely wound coil but with spacing between the plies to allow rapid transmission of flame through the spaces, the igniferous booster being ignited at one end by ignition initiation means.

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 an inflator of the present invention.

Fig 2 shows diagrammatically a longitudinal medial crosssection of a slow-burning coiled charge of Fig 1.

Fig 3 shows a cross-section of the charge of Fig 2 Fig 4 shows diagrammatically a plan view of a partially embossed pyrotechnic sheet suitable for the charge of Fig 2.

Fig 5 shows diagrammatically a longitudinal medial crosssection of a faster-burning coiled charge of Fig 1..

Fig 6 shows a cross-section of the coiled charge of Fig 5.

Fig 7 is a pressure v time record obtained with a charge constructed as shown in Figs 2 and 3.

Fig 8 is a pressure v time record obtained with a charge constructed as shown in Figs 5 and 6.

Fig 9 is a pressure v time record obtained with an assembly of two charges constructed as shown respectively in Figs 2 and 5 fired with a controlled delay interval between initiation of the charges.

Fig 10 shows pressure v time records for a charge constructed as shown in Fig 2 and, for comparison, with two charges constructed and fired as those used for the record of Fig 9.

The inflator shown in Fig 1 comprises a high pressure vessel 10 having approximate internal dimensions of 53mm diameter x 229mm length, the internal volume being approximately 500ml. One end of the vessel 10 was fitted with a closure element 11 formed with two identical ports 12 into each of which were sealed two electrically actuable squibs 13. The squibs each contained an incendiary charge consisting of 60mg barium styphnate and 210mg of a balanced mixture of Zr, B and KC104 Two coiled charges of pyrotechnic sheet material 14 and 15 were encased respectively in perforated tubular containers 16 and 17 and one end of each tube was connected to a squib by screw adaptors 18 and 19 fitted in the ports 12, thereby presenting an end of each charge 14 and 15 to a squib 13. The perforated tubes 16 and 17 were both made of stainless steel and had dimensions of 11.3mm OD, lOmm ID and length 203mm. The perforations 34 were 2.2mm diameter holes providing an open area amounting to 22W of the total wall area of the tubes. The coiled charges 14 and 15 each had a coil length of 160mm.

The opposite second end of the vessel 10 was closed with a closure assembly 21 having an exit passage 22 sealed with a replaceable rupture disc 23 seated over the aperture.

The passage 22 was 12.7 mm diameter and the rupture disc was 0.127mm thick stainless steel (type 302) and had a bursting pressure of 37 s 0.7 MPa.

A sealed filling aperture 29 and a pressure transducer port 30 were provided in the walls of the vessel 10.

For test purposes the exit end of the pressure vessel was connected by a gas-tight flanged adaptor assembly 24 to a 60 litre tank 25. It will be understood however that when used in a vehicle safety system this end of the inflator would be connected to an air bag with the passage 22 leading to the air-bag interior. In the tests detailed in the following Examples the gas stored in the vessel 10 was a mixture of argon and oxygen at a pressure of 20.7 MPa at a temperature of 200C, the amount of oxygen being slightly in excess of that required to react completely with the carbon generated by the combustion of the coiled pyrotechnic charge or charges, a typical amount being 106 by volume of the total gas. The pressure in the pressure vessel (inflator pressure) and the 60 litre tank during the combustion of the pyrotechnic charges was measured by transducers.

Example 1 In this Example a coiled charge 14 (Figs 2 and 3) having a fast-burning outer portion 27 with spaced plies and a slower-burning inner portion 28 with tightly wound plies was burned in the pressure vessel 10.

A rectangular sample of pyrotechnic sheet 31 (ENERFOIL registered trade mark) as shown in Fig 4 160mm wide and 483mm long weighing 6.5g, was cut from pyrotechnic material prepared by coating a 25ym thick solid PTFE film on each side with an 8.5m thick vapour-deposited layer of magnesium (approximately stoichiometric proportions). An end portion 32 of the rectangular sheet extending over a length of 241mm from one end was embossed with regular rows of dimples 33 as shown in Fig 4 by passing the sheet between a patterned metal roll and a plain rubber roll.

The dimples were spaced at 3mm centres in each direction, each dimple being approximately 0.75mm square at the base, 0.5mm square at top and 0.3mm high.

The rectangular sheet was rolled around a 4.75mm solid steel supporting rod 26 under tension, starting from the end at the un-embossed portion. The resulting coiled charge had an outside diameter of just under 10mm and consisted of two distinct portions, viz. an inner portion 28 wherein the plies were in close contact and an outer portion 27 wherein the plies were spaced about 170ym apart as shown in the sectional view of Fig 2, the outer portion serving as igniferous booster to ignite the inner portion.

The squib-facing end of this roll was fringed by longitudinal cuts 4mm deep around the circumference at 3mm spacing. These cuts were found to facilitate the ignition of the coiled charge. The coiled charge was placed in a combustion tube as described above with the cut end facing the squib at approximately 3mm stand-off distance from the end of the squib. The spacing of the outer portion resulted in efficient and rapid ignition of the inner coiled charge.

The pressure vessel (inflator) and tank pressures are shown in Fig 7. The initial rate of pressurization inside the pressure vessel was relatively fast, reaching a maximum value of 41.90MPa at less than 3ms. However, the pressure decreased sharply indicating a slow energy release rate from the pyrotechnic charge. This pressure record indicated two distinct rates of energy release, very fast initial rate attributable to the fast burning of the embossed outer portion and a much reduced rate of energy release from the un-embossed closely wound inner portion. This is confirmed by the pressure record of the 60L tank. The initial rise in the tank pressure shows that the burst disc was ruptured within 3 milliseconds of passage of current to the squib. The pressure then rose steadily to a maximum value of 399kPa, beyond 100ms. The initial rate of pressurization was at 6.9kPa/ms.

Example 2 In this example a fast-burning pyrotechnic charge 15 (Figs 5 and 6) with spaces between the coils over the entire radius of the coil was burned in pressurised gas in the pressure vessel and the pressure v time records for the pressure vessel and the 60 litre tank were obtained.

A rectangular piece of pyrotechnic sheet was cut from the same pyrotechnic sheet material with the same dimensions as those used in Example 1. The sheet was embossed over its entire length with the same embossing pattern as detailed in Example 1.

The rectangular sheet was rolled around a 4.75mm diameter mandrel under tension. The mandrel was then withdrawn leaving a hollow coiled charge which had an outside diameter of just under 10mm and an inside diameter of 4.75mm wherein the plies were spaced about 100ym apart as shown in a sectional view in Figs 5 and 6. End cuts similar to those used in Example 1 were also made in this charge. The coiled charge was placed in a combustion tube described above with the cut end facing the squib at approximately 3mm stand-off distance from the end face of the squib. Only one squib was used in this test, the other squib port 12 being closed by a blank element. The pressure v time records obtained for the pressure vessel of the inflator and the 60 litre vessel are shown in Fig 8. The pressure inside the high pressure vessel showed a fast rise in pressure reaching a maximum value of 38.6MPa at about 5 milliseconds followed by a steady decrease due to venting of the gas into the tank. The rise in tank pressure shows that the bursting disc was ruptured within 3 milliseconds of passage of current to the squib. It then rose steadily to a maximum value of 455kPa at about 44ms. The average rate of pressurization rate was about 14.5kPa per millisecond, which was more than double that for the slower burning coiled charge of Example 1.

Example 3 In this Example two separate coiled charges 14 and 15 wound as described in Examples 1 and 2 respectively were burned in the pressure vessel. The first charge was prepared from a rectangular sample of pyrotechnic sheet 130mm wide and 483mm long, weighing approximately 5g, and cut from the same pyrotechnic material used in Examples 1 and 2. It was embossed over half its length with dimples, rolled onto a supporting steel rod 26 and the coil was fringed at one end as described in Example 1. The second charge was prepared from an identical piece of pyrotechnic material. It was fully embossed with dimples, rolled and prepared as in Example 2. The two coiled charges were placed into two separate combustion tubes each of which was initiated by a squib. In the test performed in this example, the first charge (partially embossed) was fired first and the second charge was fired after a delay of lOms using a programmable dual pulse firing circuit designed and built specially for this purpose. The inflator and tank pressures are shown in Fig 9. The inflator pressure shows the two distinct pulses from the two charges separated by the delay time of

These Examples show the effect of the pressurization rate of spacing the plies in the coiled charge and the inflation rates obtained in Example 3 show that two charges could be combined to provide the required twostage programmable hybrid inflator.

Claims (26)

1. A hybrid inflator for an air-bag comprising a gas storage vessel having a sealed exit; a supply of gas stored in gaseous form under pressure in said vessel; pyrotechnic material effective to heat and expand said supply of gas; initiation means to ignite said pyrotechnic material and means to open said sealed exit to permit heated gas to flow through said exit to an air-bag when the bag is required to be inflated; said pyrotechnic material comprising at least one closely wound coiled charge of pyrotechnic sheet material consisting of one or more substrate layers of oxidizing polymeric film having a layer of oxidizable material on at least a portion of at least one surface of the, or each, substrate layer, the polymeric film and the oxidizable material being conjointly capable of reacting together exothermically on ignition; and igniferous booster means ignitable by said initiation means and adapted and located so that1 burning of the booster ignited the said coiled charge substantially simultaneously over a curved surface of the coil.

2. An inflator as claimed in claim 1 wherein the igniferous booster means comprises a charge of spacedapart plies of pyrotechnic sheet material wound co-axially with the closely wound plies of the said coiled charge and in ignition transmission relationship therewith.

3. An inflator as claimed in claim 2 wherein the spacedapart plies are wound around the outside of the closely wound coil.

4. An inflator as claimed in claim 2 or claim 3 wherein the closely wound coiled charge and the igniferous booster comprise the same pyrotechnic sheet material.

5. An inflator as claimed in claim 4 wherein the closely wound coiled charge and the booster charge are formed by winding a single portion of pyrotechnic sheet material.

6. An inflator as claimed in claim 5 wherein the said booster means and the said coiled charge is formed by winding a length of pyrotechnic sheet which is flat over a portion at one end portion and formed with protrusions at the other end portion.

7. A hybrid inflator as claimed in any one of claims 1 to 6 having a relatively low inflation rate initially and a faster inflation rate thereafter, comprising a second coiled heating charge of pyrotechnic sheet material in addition to the aforedescribed slow-burning coiled heating charge, said coiled charges being sufficiently spaced apart to burn independently without transmission of ignition between the charges and each coiled charge being provided with initiation means whereby the charges may be ignited either simultaneously or with a controlled delay between successive ignitions.

8. An inflator as claimed in claim 7 wherein at least part of the plies of said second coiled charge are spacedapart.

9. An inflator as claimed in any one of claims 1 to 8 wherein the, or each, pyrotechnic charge is disposed inside the pressure vessel either in a thin container or in direct contact with the stored gas.

10. An inflator as claimed in any one of claims 1 to 9 wherein the sealed exit of the pressure vessel is sealed by a frangible element designed to break when the stored gas attains a predetermined pressure or by an electric squib or detonator fired in response to vehicle collision conditions.

11. An inflator as claimed in any one of claims 1 to 10 wherein the initiation means comprises an electric squib, or exploding bridgewire, a semi-conductor bridge, a spark discharge, an electric current or an electrically heated wire, or a mechanically actuated igniter.

12. An inflator as claimed in any one of claims 1 to 11 wherein the, or each, coiled pyrotechnic charge has an initiation means positioned to ignite the charge at an end of the coil.

13. An inflator as claimed in claim 12 wherein the said end portion of the coiled charge is fringed.

14. An inflator as claimed in claim 2 or claim 8 wherein the spacing between plies of the igniferous booster means and/or the said second coiled charge is provided by spacing elements, protrusions on the surface of the pyrotechnic sheet or by folding, wrinkling, corrugating, or embossing the pyrotechnic sheet.

15. An inflator as claimed in claim 15 wherein the spacing between adjacent spaced-apart plies of pyrotechnic sheet is from 50-400 ym.

16. An inflator as claimed in claim 12 wherein the said oxidizing polymeric film comprises polymer selected from the group consisting of polytetrafluoroethylene (PTFE), polychlorotrifluoroethylene, polyhexafluoropropylene, copolymers of trifluoroethylene and hexafluoropropylene, copolymers of trifluoroethylene and tetrafluoroethylene, copolymers of hexafluoropropylene and tetrafluoroethylene, copolymers of hexafluoropropylene and vinylidene fluoride, copolymers of chlorotrifluoroethylene and vinylidene fluoride, trichloroethylene, homopolymers, copolymers of trichloroethylene and vinylidene fluoride, mixtures of two or more of such polymers and mixtures of any one of such polymers with PTFE, and the said oxidizable material comprises metal selected from the group consisting of lithium, sodium, magnesium, beryllium, calcium, strontium, barium, aluminium, titanium, zirconium and alloys thereof.

17. An inflator as claimed in any one of claims 1 to 15 wherein the or each coiled pyrotechnic charge is encased in a perforated combustion tube.

18. A vehicle occupant restraint safety system comprising a hybrid inflator as claimed in any one of claims 1 to 17; an inflatable bag into which inflation gas is supplied from the inflator; sensor means to detect vehicle crash conditions and/or vehicle occupant position and initiation control means to actuate the initiation means of the closely wound coiled pyrotechnic charge of the inflator in response to a signal from said sensor means.

19. A safety system as claimed in claim 18 having an additional coiled charge of pyrotechnic sheet material and initiation means therefor, the initiation control means being operable to actuate the initiation of the additional coiled charge after a delay interval starting with the actuation of the initiation means of the first coiled charge, the control means being programmed to select the delay interval in accordance with the crash velocity and/or vehicle occupant position when a vehicle containing the safety system is involved in a crash.

20. A slow-burning heating charge suitable for a hybrid inflator for an air-bag, which charge comprises a closely wound coil of pyrotechnic sheet material consisting of one or more substrate layers of oxidizing polymeric film having a layer of oxidizable material, preferably metal, on at least a part of at least one surface of the, or each, substrate layer, the polymeric film and the oxidizable material being conjointly capable of reacting together on ignition, and igniferous booster means adapted and located to ignite said charge substantially simultaneously over at least one of its curved surfaces.

21. A heating charge as claimed in claim 20 wherein the igniferous booster means is adapted and located to ignite the charge substantially simultaneously over its outer curved surface.

22. A heating charge as claimed in claim 20 or claim 21 wherein the igniferous booster means comprises plies of pyrotechnic sheet wound co-axially with the closely wound coil with spaces between the plies.

23. A heating charge as claimed in any one of claims 20 to 22 wherein the closely wound coil of pyrotechnic sheet material are formed from a single portion of pyrotechnic sheet material.

24. A hybrid inflator for an air-bag substantially as described and illustrated herein with reference to the accompanying drawings.

25. A vehicle occupant restraint safety system substantially as described herein with reference to the accompanying drawings.

26. A slow-burning heating charge for a hybrid inflator for a vehicle air-bag substantially as described herein with reference to the accompanying drawings.