Explosive and thermic compositions or charges – Structure or arrangement of component or product – Coated component
Reexamination Certificate
2000-08-31
2001-11-27
Nelson, Peter A. (Department: 3641)
Explosive and thermic compositions or charges
Structure or arrangement of component or product
Coated component
C149S097000, C102S288000, C102S289000
Reexamination Certificate
active
06322649
ABSTRACT:
The present invention relates to the field of protection, in the event of an impact, of the occupants of a motor vehicle by means of an airbag inflated by a pyrotechnic gas generator. More specifically, the invention relates to a process for ensuring gradual deployment of the airbag. The invention also relates to a pyrotechnic charge allowing implementation of the process according to the invention.
It is known, in the event of a collision of a motor vehicle, to protect the occupants of this vehicle by means of airbags which are inflated by the gases arising from a pyrotechnic generator. The gases serving to inflate the airbag may come entirely from the combustion of a pyrotechnic charge or result from mixing the gases arising from the combustion of a pyrotechnic charge with gases stored in a pressurized chamber, as is the case for so-called “hybrid” gas generators. As pyrotechnic gas-generating compositions, those skilled in the art may use inorganic compositions based on sodium azide such as those described, for example, in French Patent FR-B-2,649,478 or in its corresponding American Patent U.S. Pat. No. 5,139,280. Those skilled in the art may also use compositions similar to the propellent powders for weapons, having a single base comprising nitrocellulose or two bases comprising nitrocellulose and nitroglycerine, or else so-called “composite” powders based on an organic binder and an oxidizing charge, as described, for example in French Patent FR-B-262,322 or in its corresponding American Patent U.S. Pat. No. 5,160,163.
Airbags for protecting the occupants of a motor vehicle must be able to be deployed in an extremely short time, of the order of 30 to 40 ms, and require, in order to do this, the use of pyrotechnic charges having very high burning rates. However, the use of such charges poses a new problem since too sudden a deployment of the airbag in the first milliseconds of operation may cause serious injuries to the vehicle's occupant who the airbag is supposed to protect. It is therefore necessary to use a charge having an overall burning rate compatible with the requirements of motor-vehicle safety but having a gradual combustion law. Those skilled in the art say that, on firing in a closed chamber, the curve representing the pressure in the chamber as a function of time has, for such a charge, a general S-shaped appearance.
This type of consideration departs completely from the use of pyrotechnic charges completely or partially covered with an ignition enhancer coating. These charges, which are described, for example, in U.S. Pat. No. 4,246,051, have an initial pressure development which is incompatible with the desired gradual behaviour.
In order to obtain gradual deployment of the airbag, those skilled in the art have explored several types of solution.
Those skilled in the art have sought to manufacture charges comprising several layers having different thicknesses and different burning rates. U.S. Pat. No. 5,507,890 thus describes such sodium-azide-based charges produced by pressing. Nevertheless, the technique thus described is limited in its possibilities of application to compositions based on sodium azide, which compositions are, moreover, increasingly forsaken because of the toxicity and safety problems associated with compositions of this type.
Those skilled in the art have also sought to obtain gradual charges by coating charges based on compositions having a high burning rate with an inhibiting skin based on a resin or on inert inorganic substances, as described, for example, in Patent Applications EP-A-0,586,045 and EP-A-0,586,060. However, these solutions are not satisfactory from the standpoint of the non-toxicity and cleanness of the gases thus generated.
Those skilled in the art have therefore sought to obtain a gradual emission of gases by using generators having multiple coaxial combustion chambers, each containing a different pyrotechnic charge. This solution, described for example in U.S. Pat. No. 5,529,335 which relates more particularly to a generator for a seat-belt retractor, involves a complex structure of the generator and therefore a high cost of the latter.
Those skilled in the art do not currently have available a simple and economical solution for providing gradual inflation, under the time conditions imposed by motor-vehicle safety requirements, of an airbag.
The object of the present invention is specifically to provide a solution to this problem.
The invention therefore relates to a process allowing an airbag for protecting an occupant of a motor vehicle to be inflated in a gradual manner by means of a device consisting mainly of the said airbag connected to a pyrotechnic generator of gases arising at least partially from the combustion of a solid pyrotechnic charge placed inside the said generator, the combustion of which charge may be initiated by an igniter coupled to a collision detector, characterized in that:
i) a solid pyrotechnic double-composition charge is used which consists of a main pyrotechnic charge having a linear burning rate V
1
at least equal to 25 mm/s under a pressure of 20 MPa and the surface of which is at least partially covered with an external skin of pyrotechnic composition adhering to the said main charge by simple deposition and having a linear burning rate V
2
which satisfies the relationship:
0.05 V
1
≦V
2
≦0.5 V
1
;
ii) the said double-composition charge is placed inside the said generator so that the entire surface of the main charge thus covered serves as the combustion initiation surface of the latter when the said igniter is actuated by the collision detector.
The process according to the invention thus relates both to protection devices in which all of the gases serving to inflate the airbag arise from the combustion of a solid pyrotechnic charge and to those in which the gases which serve to inflate the airbag arise only partly from the combustion of a solid pyrotechnic charge, the other part coming either from a pressurized gas container, as is the case for so-called “hybrid” generators, or even from the outside, as is the case for siphon systems.
According to a first essential characteristic of the invention, a solid pyrotechnic double-composition charge is used which consists of a main gas-generating pyrotechnic charge having a linear burning rate V
1
at least equal to 25 mm/s and preferably close to 40 mm/s under an operating pressure of 20 MPa in order to be able to meet the motor-vehicle safety requirements. This main charge may be in the form of discs, but advantageously it will be in the form of an annular axisymmetric block having at least one central channel allowing it to be positioned around the igniter. This block will also preferably comprise at least one peripheral channel and, in general, a plurality of peripheral channels.
The surface of this main charge must be at least partially covered with an external skin of a pyrotechnic gas-generating composition whose burning rate will be less than the burning rate of the main charge but which will not be zero. The inhibiting compositions, which degrade with heat but which do not have a proper burning rate, are therefore expressly excluded from the field of the present invention. In practice, a pyrotechnic composition will be chosen, in order to form this skin, which has a linear burning rate V
2
which satisfies, under the same pressure conditions, the relationship:
0.05 V
1
≦V
2
≦0.5 V
1
and preferably:
0.1 V
1
≦V
2
≦0.5 V
1
.
This external skin of pyrotechnic composition must adhere perfectly to the main charge without any pyrotechnic discontinuity and especially without the use of adhesive. This skin must therefore adhere to the main charge by simple deposition. For this reason, the external skin and the main charge will usually be of the same nature.
According to a first embodiment of the invention, nitrocellulose-based pyrotechnic compositions will be is used to constitute the said main pyrotechnic charge and the said external skin. This is because these compositions lend themselves well to the preparati
Delwarde Jean-Charles
Duvacquier Daniel
Marsaud Benoit
Perotto Christian
Livbag SNC
Nelson Peter A.
Pillsbury & Winthrop LLP
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