 |
Description  |
|
|
FIELD OF THE INVENTION
The present invention relates generally to gas generators used to inflate
devices such as vehicle occupant restraints (commonly known as airbags).
More particularly, the present invention relates to methods of providing
autoignition to gas generating materials in such gas generators.
BACKGROUND OF THE INVENTION
There are a variety of devices, such as thermostats, fuses and the like,
which respond to an increase in temperature beyond a specific point. Two
temperature responsive devices, which are employed in inflatable restraint
systems, (hereinafter referred to as "airbags"), are igniters and thermal
batteries. These temperature responsive devices are used to intentionally
activate the airbag system when it is exposed to an unusually high
temperature, such as in a fire.
The inflator for an airbag contains a gas generating material. The inflator
also includes a standard igniter which ignites the gas generating material
when the inflator is actuated. The inflator is actuated when a crash
sensor senses that the vehicle has been involved in a crash of a
predetermined magnitude.
The inflator may, on occasion, be subjected to an abnormally high
temperature, for example if the vehicle is involved in a fire. In such a
situation, the inflator housing may be weakened and/or the gas generating
material becomes much more reactive than normal. To avoid explosive
ignition of the gas generating material during a fire, the inflator should
have an autoignition means. The autoignition means may be mechanical,
electrical, or chemical and is typically located within the inflator. The
autoignition means are required for the safe use of airbags because
activation of the gas generates at high temperatures may result in the
fragmentation of the housing of the inflating system. Fragmentation of the
housing results from a combination of factors such as the development of
abnormally high pressure from the burning generant, weakening of the
housing at high temperatures and clogging of the vents where the gases are
normally channeled into the airbag. This fragmentation constitutes a
severe hazard and must be avoided.
As used herein and in the claims, the term "autoignition material" or
"autoignition composition" means a material which will spontaneously
ignite or combust at a temperature lower than that which would lead to the
catastrophic destruction (explosion, fragmentation or rupture) of the
airbag system. When the autoignition composition spontaneously ignites,
the generated heat ignites the gas generating material. Thus, the gas
generating material is ignited at a preselected temperature, which is
higher than normally encountered ambient temperatures, but lower than the
temperature at which the gas generating material itself would autoignite.
As used herein and in the claims, the term "autoignition system" means a
combination of elements or components that includes an autoignition
composition which ignites at a lower temperature than the temperature at
which the gas generating material ignites. As will be described below, the
system of the present invention, in one embodiment, uses an autoignition
composition that is based on lead thiocyanate as the fuel and chlorates as
the oxidizer. When an aluminum housing is used for the inflator, the lead
thiocyanate based composition must not come into direct contact with the
aluminum as undesired corrosion will occur. This is prevented through the
use of a barrier material. Also, the autoignition composition globule can
be coated with a protectant substance to reduce abrasion and absorption of
water by the autoignition composition.
The inclusion of an autoignition material in an inflator assembly incurs
increased expense as the autoignition material must be carefully prepared,
handled and installed. Also, the temperature sensitivity of the material
should not vary over the lifetime of the vehicle in which it is installed.
DISCUSSION OF THE PRIOR ART
U.S. Pat. No. 5,494,312 teaches an autoignition system for a fluid fueled
inflator. At a predetermined temperature, a storage element opens and the
fuel contacts an oxidant causing ignition. This patent teaches the use of
separate chambers for the autoignition system, thus incurring additional
cost and adding weight.
U.S. Pat No. 5,429,386 teaches a mechanical autoignition device for an
inflator wherein the autoignition device employees a bimetal disk which
deflects from concave to convex when the ambient temperature increases to
a predetermined level. When the bimetal disk deflects into a convex shape,
it moves a firing pin forcibly against a primer to actuate the prime,
which in turn ignites the gas generating material. This approach adds
additional weight to inflator assembly and considerable cost in the form
of materials and labor.
U.S. Pat. No. 5,100,170 and U.S. Pat. No. 5,167,426 teach electrical
autoignition devices for inflators wherein an autoignition sensing device
is located outside of the inflator housing. A thermoelectric battery is
adapted to initiate an electrical charge to set off the gas generating
material when the temperature outside the inflator reaches a predetermined
level of about 300.degree.-400.degree. F. (149.degree.-205.degree. C.).
Allegedly this autoignition device is not affected by the design criteria
and/or the thermal conductivity of the inflator housing, however,
substantial cost and weight penalties are incurred.
U.S. Pat. No. 4,561,675 teaches an autoignition device contained within an
aluminum inflator housing. This patent teaches that aluminum is too weak
at the temperature that the gas generating material autoignites to contain
the generated forces of such a reaction. The autoignition material
autoignites at a temperature where the inflator housing possesses
sufficient structural integrity to resist the forces generated when the
gas generating material is ignited. This patent teaches that the
autoignition material should be in a "container" which is in contact with
an exterior wall of the inflator housing.
U.S. Pat. No. 5,100,174 and U.S. Pat. No. 5,114,179 teach an autoignition
"packet" located within a hermetically sealed inflator housing. While the
housings employed are commonly metal, preferably aluminum, it is
understood that the present invention could be employed with a housing
made of plastic, ceramic or any other suitable material. The packet is
secured with a piece of adhesive tape inside a recess in the wall portion
of the housing. While avoiding additional weight to the inflator, such a
system would incur a substantial increase in manufacturing costs due to
increased labor requirements.
U.S. Pat. No. 5,409,259 and U.S. Pat. No. 5,443,286 teach an inflator made
of aluminum, with the autoignition material adjacent the igniter so that
if the inflator is subjected to extreme heat, as in a fire, the
autoignition material will autoignite and set off the gas generating
material. A thin foil seal is placed across the opening in which the
ignitor and the autoignition powder are mounted. The composition of the
autoignition material is not disclosed in this patent.
U.S. Pat. No. 5,468,017 teaches the use of a metal autoignition packet in
an inflator. The autoignition material is encased in metal, preferably
thin aluminum. The preferred autoignition material is a stabilized
nitrocellulosic composition, such as IMR 4895, which is available from E.
I. du Pont de Nemours & Co., Inc. of Wilmington, Del. The autoignition
material may also include an ignition enhancer such as BKNO.sub.3.
Encasing an autoignition material in a metal or fabric enclosure is costly
and could possibly impair the conduction of heat to the autoignition
material. Attempts have been made to overcome these limitations.
U.S. Pat. No. 4,858,951 teaches small grains of an autoignition material
physically mixed with the gas generating material, such that at a
predetermined temperature, the autoignition material will autoignite and
in turn ignite the gas generating material with which it is physically
mixed. The preferred autoignition material is nitrocellulosic and other
smokeless powders. The mixture may also contain BKNO.sub.3 (boron
potassium nitrate), TiH.sub.2 (titanium hydride) and KClO.sub.4 (potassium
perchlorate).
U.S. Pat. No. 5,299,828 teaches a cylindrical inflator housing made of
aluminum or aluminum alloy with an autoignition agent deposited
substantially over the entire inner surface of the housing. Smokeless
powder that ignites at about 150.degree.-200.degree. C. is disclosed as a
suitable autoignition agent. The autoignition agent is not protected and
is thus subject to abrasion and detachment from the inner surface of the
cylindrical vessel.
U.S. Pat. No. 4,944,528 teaches an autoignition device which is a cup
shaped member located in an aperture in the wall of the inflator housing.
An unspecified autoignition material is placed in the cup. The opening of
the cup faces the interior of the inflator housing and is sealed with an
elastic material such as, for example, rubber, plastic or silicone rubber.
U.S. Pat. No. 5,186,491 discloses an inflation device wherein an
autoignition material is located in a recess in the wall of the inflator
housing and the recess is covered by a sealing member. The autoignition
material ignites another ignitable material or the gas generating material
inside the inflator housing.
Providing autoignition compositions for use in aluminum inflator housings
has heretofore been problematic. U.S. Pat. No. 5,380,380 discloses
autoigniting compositions containing a hydrazine salt of
3-nitro-1,2,4-triazole-5-one. This reference claims rapid autoignition at
temperatures of approximately 150.degree. C. thereby allowing the use of
aluminum canisters or housings. The autoignition compositions of the
patent are disclosed to be insensitive to shock or impact, safe to
manufacture and handle, and are classified as class B materials.
Smokeless powders, such as du Pont 3031, are known autoignition materials.
While such smokeless powders autoignite at a temperature of about
180.degree. C., they are largely composed of nitrocellulose. One skilled
in this art appreciates that nitrocellulose is not stable for long periods
of time at high ambient temperatures and is thus unreliable as an
autoignition composition component.
Autoignition compositions are disclosed in U.S. Pat. No. 5,084,118 which
comprise 5-aminotetrazole, potassium or sodium chlorate and 2,
4-dinitrophenylhydrazine. While the compositions disclosed autoignite at
approximately 177.degree. C. they are also oversensitive to shock or
impact. These compositions are also difficult and hazardous to
manufacture, as they are classified as explosives and thus require special
facilities for manufacturing and storage.
U.S. Pat. No. 5,460,671 discloses an autoignition composition that is
prepared by wet mixing an oxidizer selected from the chlorates with a
carbohydrate fuel. The autoignition composition is dried and then placed
near the gas generating composition. This autoignition composition is
taught to be useful in aluminum inflator housings.
U.S. Pat. No. 5,501,152 discloses an autoignition composition which is a
mixture of nitrocellulose, carbon and an oxidizing agent. This composition
is then pressed into tablets, pellets, or similar other lumpy bodies.
The prior art fails to suggest or disclose a method for providing
autoignition for a gas generating device wherein the autoignition
composition comprises lead thiocyanate Pb(SCN).sub.2 as the fuel, a
chlorate such as potassium chlorate as the oxidizer, and optionally a
binder and a flow agent/thickener. The prior art also fails to suggest or
disclose a method wherein the autoignition composition is applied to the
interior of an inflator housing as a paste or paint. Further, the prior
art does not suggest use of a barrier substance for application to
aluminum housings or the use of coatings over the autoignition material to
prevent mechanical abrasion and the absorption of water.
BRIEF DESCRIPTION OF THE DRAWINGS
The features of the invention which are believed to be novel are set forth
with particularity in the appended claims. The present invention, both as
to its structure and manner of operation, may best be understood by
referring to the following detailed description, taken in accordance with
the accompanying drawings in which:
FIG. 1 is a diagrammatic representation of an exemplary fluid dispensing
apparatus which may be used in the method of the present invention;
FIG. 2 is a side view, partially in section, of an airbag inflating device
to which the method of the present invention may be applied;
FIG. 3 is an enlarged fragmentary view of an alternative embodiment of the
autoignition system resulting from the method of the present invention;
FIG. 4 is an enlarged fragmentary view of another alternative embodiment of
the autoignition system provided as a result of the method of the present
invention; and
FIG. 5 is an enlarged fragmentary view of another alternative embodiment of
the autoignition system as provided by the method of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
Basic requirements for autoignition of a gas generator used in a vehicle
occupant restraint system are that the autoignition composition be 1)
thermally stable up to 110.degree. C.; 2) not autoignite below a
150.degree. C.; 3) autoignite rapidly at approximately
190.degree.-220.degree. C.; and 4) possess physical integrity to withstand
abrasion and environmental changes. Many compositions presently known as
autoignition compositions, such as nitrocellulose, are not effective after
long-term aging. Vehicle occupant restraint inflator systems must pass
aging requirements in order to assure reliable ignition despite exposure
to a wide range of temperatures over the life of a vehicle.
One important aspect of this invention is the method in which the
autoignition material is applied to the inside of the housing of the gas
generating device. The autoignition material is deposited directly onto
the surface of the housing or may be placed over a protective layer of
material if the housing is made of aluminum and the autoignition material
contains a corrosive agent. As will be described below, the preferred
autoignition composition of the present invention should not be in direct
contact with aluminum housings and therefore a protective coating is
desired to separate the corrosive autoignition material from the aluminum.
In another embodiment, the autoignition material is coated with a
protective coating layer that reduces abrasion of the autoignition
material by pellets of the gas generating composition and also prevents
the absorption of water.
An advantage of the present method over the prior art resides in the ease
and low cost of providing a gas generating device with an autoignition
means. A further advantage of the present method resides in the use of an
autoignition composition in the form of a paste or paint, that can be
robotically deposited within the inflator housing which provides reliable
and accurate autoignition of the gas generating composition.
Thus, the present invention relates to a method for providing autoignition
to an apparatus for inflating an airbag, said method comprising: (1)
providing a housing; (2) providing a gas generating material within said
housing which, when ignited, generates gas for inflating the airbag; and
(3) providing at least one autoignition globule adhering to the interior
wall of said housing, said autoignition globule having an autoignition
temperature below the autoignition temperature of said gas generating
material and said autoignition globule comprising lead thiocyanate and a
chlorate.
In one embodiment of the inventive method, the autoignition globule is
applied to the interior wall of the inflator housing as a "dot" or
"globule" of a paste or paint which may be water based, solvent based or
based on a mixture of water and solvent. Further, the autoignition globule
may comprise a binder and a flow agent/thickener. The autoignition
composition uses chlorates as the oxidizer for the Pb(SCN).sub.2 fuel. The
chlorates useful in the present invention include the known salts of
chloric acid such as sodium chlorate, potassium chlorate, barium chlorate,
calcium chlorate and the like.
There is also disclosed a method of ma | | |
