GB2530295A

GB2530295A - Inflator propellant

Info

Publication number: GB2530295A
Application number: GB1416491.7A
Authority: GB
Inventors: Tim Scott; Robert Grzelak
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 2014-09-18
Filing date: 2014-09-18
Publication date: 2016-03-23
Also published as: DE102015114794A1; CN105439784A; GB201416491D0

Abstract

A propellant for an inflator, and airbag inflator and a method of producing a propellant are disclosed. The propellant comprises a plurality of granules which are adhered together such that cavities exist between the granules. Selecting the size of the granules and the volume of the cavities allows the rate of combustion of the propellant to be controlled. Propellant may be sodium azide, and the granules may be adhered together with wax.

Description

I

ftWLATOR PROPELLANT The present invention r&ates to a propellant for an inflator, in particular to a solid propellant for an airbag inflator. The present invention also relates to an airbag inflator comprising a solid propellant and to methods of making the solid propellant.

Background

Airbags are a commonly employed vehicle safety device. They are provided at one or more of various locations within the vicinity of a passenger seating position in order to protect a passenger from impact with interior parts of the vehicle in the event of a collision, Airbags typically comprise a flexible cushion and an inflator system which is designed to inflate the cushion with gas in the event that a requisite deployment condition is met.

Several types of airbag inflator are known. Pyrotechnic inflators derive a gas source from a combustible gas generating material which, upon ignition, generates a quantity of gas sufficient to inflate an air bag. Stored gas inflators utilise a quantity of stored pressurised gas which is selectively released to inflate an airbag. Hybrid inflators combine the use of a gas generating material and a quantity of stored pressurized gas to inflate an airbag.

Pyrotechnic inflators or hybrid inflators typically utilise a solid, ignitable, propellant formulation of fuel, oxidizer and binder. Sodium Azide is an example of a known solid chemical propellant which, when ignited, rapidly produces nitrogen gas which fills the air bag. More recently, alternative propeliants have been proposed which are more efficient, less toxic and less expensive and may incorporate, for example, a combination of nitroguanidine, ammonium nitrate or other non-metallic oxidizers and a nitrogen-rich fuel other than azide such as tetrazoles, triazoles and their salts.

The solid propeflant is typically formed into a pellet, for example by pressing or by extrusion. For a given propellant formulation, the amount of gas yield is determined by propellant mass and the efficiency of combustion. The rate of combustion and slope may be controlled by varying the surface area to mass ratio of the pellet(s) to be comprised in the inflator, e.g. by selecting the pellet size or by controlling the extrusion profile of the pellet. Whilst extruded propellants offer the possibility of a well controlled surface area to mass ratio, they tend to be less favoured since extruded prop&lant requires the use of a binder which enables the propeflant to undergo extrusion and this binder contributes to effluent levels. Furthermore, care needs to be taken to allow the extruded propellant to dry out without cracking, since any smafi cracks of fissures alter the surface area of the resultant propeflant and, therefore, the rate of combustion.

Accordingly, there is a need to improve the control of combustion rate and/or slope of solid propellant used in airbag inflator systems.

Statements of Invention

According to a first aspect of the present invention there is provided a propellant for an airbag inflator system, the propeuant comprising a plurality of granules which are adhered together to form a propellant body and wherein a plurality of cavities exist within the propellant body between the granules.

The propellant is therefore a solid body with a granulated structure which may be advantageously utilised as a propellant for a pyrotechnic or a hybrid inflator system.

Preferably the granules exhibit a curved surface and/or are substantially spherical in shape. Thus, due to the surface curvature of the granules, clefts and cavities are maintained between the granules when they are adhered to each other. Alternatively, the granules may be disk-shaped or generally cylindrical. The gap regions between the granules of the propellant body serve to increase the surface area to mass ratio of the propellant. Due to the internal gap regions created between the granules, the propellant can be considered to be a solid, yet air-permeable, body.

Advantageously, the combustion rate and/or slope of combustion can be more readily controlled by selecting the size of the granules and/or the volume of the gap regions within the propellant body. The increased surface area to mass ratio increases the combustion rate of the propellant and, as such, allows a chemical propellant formulation which exhibits a slower combustion rate to be selected. This has a number of consequential advantages for the inflator device since such formulations tend to demonstrate an improved combustion process, including reduced combustion pressure and/or lower effluent levels.

The propellant granules preferably comprise a fuel, an oxidiser and a binder.

Afternatively, the granules comprise a fuel and an oxidiser wherein an adhesive binder serves to adhere the granules together. As a further alternative, oxidizer granules may be provided wah an adhesive binder such as wax which also serves as a fuel.

The propellant may comprise Sodium Azide or may be formulated e.g. from Guanidine nitrite, 5-Amino Tetrazole, Potassium Amino Tetrazole or Potassium Nitrate. Other propellant families include Arnmonium Nitrate, Potassium Perchlorate, Nitro-cellulose double base, SINCO (Dynamite Nobel trade name). Numerous other propellant formulations are envisaged within the scope of the present disclosure.

Thus, the ability to achieve a slower combustion rate/slope means that more recently developed, non-azide, propellant formulations may be utilised, These alternative formulations may benefit from being lower cost, they tend to be easier to manufacture and/or they tend to exhibit lower effluent levels. A reduction in combustion pressure allows the provision of a simplified generator housing design. For example, it is envisaged that an inflator system according to the present disclosure may utilise a plastic housing for the propellant in circumstances where this is sufficient to withstand the lower combustion pressure.

The propellant body can advantageously be moulded to a specific, required shape and size giving good repeatability in terms of mass and, therefore, gas yield. It is therefore possible for the shape of the propellant body to be tailored such that the propellant may be mounted close to the ignitor of the airbag inflator, allowing for faster and/or more effective lighting of the propellant.

In a normal pellet type" inflator an ant-rattle pad is used to minimise movement of the pellets to avoid the generation of pellet dust or cracks which might adversely affect the combustion rate. Advantageously, the present propellant may not require the use of anti-rattle pads. Optionally, small deformable elements or pips may be added to the outer surface of the propellant body which further alleviates the need for anti-rattle pads to be employed as part of the inflator assembly.

The size/diameter of the granules can range between a few micrometers -e.g. a fine powder -to a few millimetres. Thus, the diameter or cross-sectional width of the granules may be between 2 micrometers and 10mm. Various methods for forming the propellant granules are envisaged within the present disclosure. In addition to convenflonal methods such as pressing, or extruding and chopping, other methods may be possible. For example, the granules may be formed by a method similar to the way in which lead shot is made by dropping molten lead from a great height into cold water or, alternatively, in the manner by which ball bearings are formed by rolling between two plates. Various other methods of forming the granules will be appreciated by the skilled reader.

The granules may be formed as a powder. Optionally, each of the granules comprises a propellant apelletl thus the propellant may be formed from a plurality of pellets which are adhered together to form the propellant body wherein the individual pellets can be considered to constitute the granules of the propellant. In this case the pellet granules can be made by pressing or extrusion. A propellant formulation which is pressed or extruded may be chopped to provide a pellet granule of the required size.

Optionally, the granules are adhered together by means of an adhesive or glue. The adhesive may be a solvent-based adhesive, a thermosetting adhesive, a pressure sensitive adhesive or a hot-melt adhesive which is applied in molten form and solidifies on cooling to form a bond between the granules. In the case of a thermoplastic, or a pressure-sensitive, adhesive, the granules may be coated in a thin film of the adhesive and then pressed together in a mould to form the propellant body. Alternatively, the granules may be adhered together by means of wax or a plastic material. The wax/plastic acts to bind the granules together and may also supplement the fuel in the propellant.

Alternatively, the propellant granules may exhibit magnetic properties such that it becomes possible to magnetise the granules in order that they adhere to each other.

For example, the granules may comprise ferric oxide.

The size of the gaps between the granules forming the solid propellant block depends on the size of the individual granules. The size of the gaps may also depend on the properties of the adhesive used to adhere the granules together. Thus, it is possible to accurately select the granule size and the gap volume (and thus the permeability of the propellant body) in order to control, or "fine tune", the rate of combustion.

According to a further aspect of the present invention there is provided an airbag inflator, the airbag inflator comprising a soild propeflant body, the propeHant body formed from a puraUty of granules which are adhered together such that a plurality of gaps exist within the propeflant body between the granules.

According to a further aspect of the present invention, there is provided a method of producing a propeflant according to the first aspect.

Brief Description of the Drawings

For a better understanding of the present invention and to show how the same may be carried into effect. reference will now be made, by way of example, to the accompanying drawings in which: Figure 1 is a schematic of an airbag inflator according to a first example; Figure 2 shows a propeflant according to a second example; and Figure 3 shows an inflator housing containing a propellant according to a third

example.

Detailed Description

Figure 1 shows an airbag inflator system comprising a crash sensor 3, an inflator comprising an ignitor and a housing 4 containing a chemical propellant 5 and a safety airbag 6. In this particular example, the airbag inflator system is provided in the steering wheel of a vehicle. However, it will be appreciated that the present disclosure extends to airbag inflators designed for deployment in a variety of vehicle locations.

In use, when a crash is sensed by the crash sensor 3 -e.g. by detecting a sudden deceleration -a signal is transmitted to the ignitor which ignites the propellant and initiates combustion of the propellant which generates gas to fill the airbag 6, causing it to deploy through the module cover 7. The filling of the airbag results in the airbag occupying the space directly in front of the chest and head of the driver, thereby preventing injurious forward motion.

According to the present example, the propeflant 5 is a solid, air permeable, body having a granulated form. The gap regions between the granules allow air to permeate through the propellant, thereby increasing the surface area to mass ratio and, thus, the rate of combustion as compared to extruded or pressed propellant blocks which wiH have had any gaps or cavities pressed out during manufacture.

Figure 2 illustrates in more detail the granulated structure of a semi-permeable propellant body 10 according to a second example. As shown in Figure 2, the granules 11 are substantially spherical in shape and are adhered together such that gaps 12 are maintained between the granules. A thin coating of adhesive material binds the granules together.

Figure 3 shows an inflator device comprising an ignitor 13 and a housing 15 which contains a propeflant 16. In this example the propellant comprises granules 17 having a generally rectangular or square-shaped cross-section. The granules have been formed from an extruded pellet of propellant, thereafter being chopped up to form the granules 17 of propellant. The chopped pellets or granules are coated with a wax which serves to adhere the granules together whilst still ensuring that gaps 12 are maintained between the granules.

Optionally, the method for making the propellant body may comprise a step of coating propellant granules in an adhesive e.g. by rolling the granules in adhesive. For example, the granules may be coated in a thin film of thermosetting adhesive and pressed together in a mould. This step should preferably ensure that the space between the granules is not filled.

Alternatively, the granules may be coated in a wax or thermoplastic. Advantageously the wax may supplement the fuel in the propellant granules. Preferably the was/plastic exhibits a melting point which is between 120 degrees Celsius and 400 degrees Celsius.

As a further alternative, the permeable propellant body may be made by a process of sintering the granules of propellant in order to fuse them together.

As a still further alternative, the propellant body may be made by aerating a mix, or slurry. of grain and binder by means of a foaming agent or by bubbling gas through the mix. A step of free drying (lyophiusafion) converts the aerated Uquid to a soUd propeflant body.

Examples of the present invenUon exhibit a number of technical advantages which are demonstrated by a variety of chemical propellant formulations. The selection of granule size and gap volume provides an effective mechanism for controlling or fine tuning the combustion rate of a given propellant formulation. The increased surface area to mass ratio resulting from the presence of gaps within the prop&lant body enables slower combustion rates to yield the same required gas volume to fill the airbag in an adequate time. This may allow the choice of alternative propellant families which might offer: * Lower combustion pressure * Higher gas yield with less residue. Increasing the surface area may lead to easier lighting of the propellant mass such that higher combustion pressures are not required to reduce residue, Typically pressures of l5obarto 300bar are required in inflators. More efficient combustion at lower temperature would also reduce the requirements and cost for the filters.

* Reduced variability in gas output These advantages potentially give rise to further, consequential, benefits in terms of simplifying inflator design, in particular the propellant housing or receptacle for the propellant. It has also been demonstrated that propellant according to the present disclosure advantageously exhibits improved linearity of combustion, particularly at the extreme ends of normal vehicle operating temperatures.

It will be appreciated by those skilled in the art that although the invention has been described by way of example with reference to one or more examples, it is not limited to the disclosed examples and that alternative examples could be constructed without departing from the scope of the invention as defined by the appended claims.

Claims

CLAIMS1. A propeUant for an airbag inliator system, the propellant comprising a plurality of granules which are adhered together to form a propellant body wherein cavities exist between the granules.
2. A propellant as claimed in claim 1, wherein the granules are adhered together by means of an adhesive.
3. A propellant as claimed in claim 2, wherein the adhesive comprises wax.
4. A propellant as claimed in claim 2, wherein the adhesive comprises a thermosetting adhesive or a pressure sensitive adhesive.
5. A propellant as claimed in any preceding claim, wherein the granules are substantially spherical in shape.
6. A propellant as claimed in any preceding claim, wherein the granules are formed as a powder.
7. A propellant as claimed in any preceding claim, wherein the granules have a diameter of between 2 micrometers and 10mm.
8. A propellant as claimed in any preceding claim! wherein the granules comprise Sodium Azide.
9. An airbag inflator comprising the propellant according to any one of claims I to & A method of producing a propellant according to any one of claims ito 8.11. A method as claimed in claim 10, comprising the step of selecting the size of the granules and the volume of the cavities within the propellant body in order to control the intended rate of combustion of the propellant.12. A propellant, airbag inflator and method of producing a propellant substantially as hereinbefore described with reference to the accompanying drawings.