 |
Description  |
|
|
BACKGROUND OF THE INVENTION
The present invention relates to ignition compositions, and more
particularly to ignition compositions for inflator gas generators utilized
in vehicle occupant restraint systems.
A steel canister is commonly utilized as the inflator pressure vessel for
an automobile occupant restraint system because of the relatively high
strength of steel at elevated temperatures. However, emphasis on vehicle
weight reduction has renewed interest in the use of aluminum in place of
steel in such pressure vessels.
One test that vehicle occupant restraint inflator systems must pass is
exposure to fire whereupon the gas generating material of the inflator is
expected to ignite and burn, but the inflator pressure vessel must not
rupture or throw fragments. Steel pressure vessels pass this test
relatively easily because steel retains most of its strength at ambient
temperatures well above the temperature of which the gas generant
autoignites. Aluminum, however, loses strength rapidly with increasing
temperature and may not be able to withstand the combination of high
ambient temperature and high internal temperature and pressure generated
upon ignition of the gas generant. If, however, the gas generant of the
inflator can be made to autoignite at relatively low temperatures, for
example, 1350.degree. C.(275.degree. F.) to 210.degree. C. (410.degree.
F.) the inflator canisters can be made of aluminum.
Providing autoignition compositions for use in aluminum pressure vessels
has heretofore been problematic. U.S. Pat. No. 4,561,675 granted to Adams
et al, which discloses the use of Dupont 3031 single base smokeless powder
as an autoignition gas generant, is exemplary of an unreliable
autoignition composition found in the prior art. While such smokeless
powder autoignites at approximately the desired temperature of 177.degree.
C. (.apprxeq.350.degree. F.), it is largely composed of nitrocellulose.
One of ordinary skill in the propellant field will appreciate that
nitrocellulose is not stable for long periods at high temperatures, which
is a specific requirement in automotive applications.
In addition, commonly assigned U.S. Pat. No. 5,084,118 to Poole, describes
other autoignition compositions, which comprise 5-aminotetrazole,
potassium or sodium chlorate, and 2,4-dinitrophenylhydrazine. While the
compositions disclosed in U.S. Pat. No. 5,084,118 autoignite and cause
ignition of the gas generant when heated to approximately 177.degree. C.
(.apprxeq.350.degree. F.), the compositions have not proven to be fully
satisfactory. The manufacture of these compositions is difficult and
hazardous because of the utilization of hexane and xylene in the
manufacturing process. Hexane has a low boiling temperature and thus
requires careful handling, while xylene is a suspected carcinogen. In
addition, the compositions disclosed in U.S. Pat. No. 5,084,118 are not
effective after long-term ageing. Vehicle occupant restraint inflator
systems must pass ageing requirements in order to ensure reliable ignition
despite exposure to a wide range of temperatures over the life of the
vehicle.
SUMMARY OF THE INVENTION
The present invention solves the aforesaid problems by providing ignition
compositions and processes comprising an oxidizer, such as potassium
chlorate, wet mixed with a fuel comprising one or more carbohydrates. The
ignition compositions are utilized in an automobile occupant restraint
system and autoignite and cause ignition of the gas generant when heated
to approximately 135.degree. C. (.apprxeq.275.degree. F.) to 210.degree.
C. (.apprxeq.410.degree. F.), thereby permitting the use of an aluminum
pressure vessel to contain the generant and gases produced by the
generant. The ignition compositions of the present invention are
relatively unaffected by long-term high temperature ageing, and do not
utilize hazardous or carcinogenic solvents during manufacture. Further,
the energy output of the ignition compositions of the present invention is
suitably high for use with gas generating compositions in vehicle occupant
restraint systems.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
The ignition compositions of the present invention comprise a mixture of an
oxidizer and a fuel. The oxidizer is selected from the group consisting of
alkali metal or alkaline earth metal chlorates or mixtures thereof,
preferably potassium or sodium chlorate. In accordance with the present
invention, potassium chlorate (KClO.sub.3) is rich in oxygen, containing
39.17% oxygen by weight, and is very reactive and receptive to propagative
burning. Potassium chlorate is preferred over less sensitive oxidizers,
such as potassium perchlorate, ammonium perchlorate, sodium nitrate, and
potassium nitrate, which are not reactive enough to result in a quick
autoignition.
In further accordance with the present invention, the ignition compositions
comprise the aforesaid oxidizers in mixtures with fuels to provide
autoignition temperatures of the ignition compositions which are
sufficiently low, i.e., approximately 135.degree. C. (275.degree. F.) to
210.degree. C. (410.degree. F.), for suitable use in an aluminum pressure
vessel. Mixtures of potassium chlorate with most organic fuels exhibit
undesirably high ignition temperatures and cannot be utilized in an
aluminum pressure vessel. However, low-melting, readily decomposed organic
fuels are more reactive with potassium chlorate, have much lower
autoignition temperatures, and are appropriate for use in aluminum
pressure vessels.
More specifically, the low-melting, readily decomposed organic fuels are
selected from the group consisting of one or more carbohydrates. Because
of the low decomposition temperatures exhibited by carbohydrates, mixtures
of potassium chlorate with one or more carbohydrates provide an
autoignition temperature between approximately 135.degree. C. (275.degree.
F.) and 210.degree. C. (410.degree. F.). For example, monosaccharides such
as D-glucose, D-galactose, D-ribose, pyruvic acid, or ascorbic acid are
effective fuels, but disaccharides and polysaccharides may also be
utilized. Preferably, potassium chlorate is selected as the oxidizer, and
is present in a concentration of from about 60% by weight to about 85% by
weight, while D-glucose or D-galactose is chosen as the fuel, and is
present in a concentration of from about 15% by weight to about 40% by
weight.
Exemplary of a combustion reaction of an oxidizer, such as potassium
chlorate, and a carbohydrate, such as D-ribose, is as follows:
3C.sub.5 H.sub.10 O.sub.5 +10KClO.sub.3 .fwdarw.10KCl+15H.sub.2
O+15CO.sub.2
Similarly, the combustion reaction of potassium chlorate with an
alternative fuel, such as ascorbic acid, is as follows:
3C.sub.6 H.sub.8 O.sub.6 +10KClO.sub.3 .fwdarw.10KCl+18CO.sub.2 +12H.sub.2
O
It is noted that whereas carbohydrates are effective fuels in mixtures with
the aforesaid oxidizers, sulfur is not a practical fuel for use in an
ignition composition, in accordance with the present invention. A mixture
of sulfur and potassium chlorate is an extremely unstable explosive, is
very dangerous, has a very low decomposition temperature of about
100.degree. C. (212.degree. F.) to 110.degree. C. (230.degree. F.), and is
thus ineffective as an ignition composition for inflator gas generators.
Further, despite the explosive dangers associated with even diluted
mixtures of potassium chlorate and organic fuels, the compositions of the
present invention are inherently safe while also achieving appropriate
autoignition temperatures. More specifically, in accordance with the
present invention, the ignition compositions are manufactured by a wet
process that utilizes water, ethyl alcohol, or mixtures thereof, as
described in the EXAMPLES hereinbelow. Thus, accidental ignitions are
eliminated while relatively low autoignition temperatures are produced.
The compositions of the present invention further increase manufacturing
safety by eliminating the use of toxic solvents such as hexane and xylene
during the manufacturing process.
In operation, the relatively low autoignition temperatures, i.e.,
approximately 135.degree. C. (.apprxeq.275.degree. F.) to 210.degree. C.
(410.degree. F.), produced by the compositions of the present invention
are maintained following long-term high temperature ageing, for example
after 400 hours at 107.degree. C. (.apprxeq.224.degree. F.). The ignition
compositions of the present invention therefore ensure ignition
reliability despite exposure to a wide range of temperatures over the life
of a vehicle, which may be 10 or more years.
In addition, an effective energy output is another advantageous feature of
the present invention. The ignition compositions have a calorific output
that is sufficient for use with a gas generating composition in a vehicle
occupant restraint system. In operation, the autoignition material must
produce enough heat to raise a portion of the gas generating composition
to the ignition temperature. The minimum energy output required varies
depending upon the type and configuration of gas generating composition,
but a calorific value of 800 calories per gram is typically effective and
is surpassed by the compositions of the present invention.
The present invention is illustrated by the following representative
examples. The following compositions are given in weight percent.
EXAMPLE 1
A mixture of D-glucose and potassium chlorate was prepared having the
following composition: 26.9% D-glucose and 73.1% KClO.sub.3.
Both of the raw materials were dried, and the potassium chlorate was ground
in a ball mill. The oxidizer and fuel were then wet blended with an 80/20
mixture of water and alcohol in a planetary mixer. Next, the wet blend was
granulated using a wide screen granulator, followed by drying the
granulated material. The dry product was then sieved.
The granulated powder was tested on a differential scanning calorimeter
(DSC), and the autoignition onset temperature was observed at
138.9.degree. C. (.apprxeq.282.degree. F.). The calorific value was 880
calories per gram.
Following long-term high temperature ageing at 107.degree. C.
(.apprxeq.225.degree. F.) for 400 hours, the DSC showed an onset
temperature of 145.degree. C. (293.degree. F.) with a weight loss of
0.1235%, and the calorific value was 902 calories per gram.
EXAMPLE 2
A mixture of D-glucose and potassium chlorate was prepared having the
following composition: 15% D-glucose and 85% KClO.sub.3.
The mixture was prepared as described in EXAMPLE 1. When the mixture was
tested in a DSC, the autoignition temperature was observed at
133.0.degree. C. (.apprxeq.271.degree. F.). Following long-term high
temperature ageing at 107.degree. C. for 400 hours, the mixture
autoignited at 144.0.degree. C. (.apprxeq.291.degree. F.), with a weight
loss of 0.1235%.
EXAMPLE 3
A mixture of D-glucose and potassium chlorate was prepared having the
following composition: 20% D-glucose and 80% potassium chlorate.
The mixture was prepared as described in EXAMPLE 1. When the mixture was
tested in a DSC, the autoignition temperature was observed at
133.5.degree. C. (.apprxeq.272.degree. F.). Following long-term high
temperature ageing at 107.degree. C. for 400 hours, the mixture
autoignited at 140.0.degree. C. (.apprxeq.284.degree. F.), with a weight
loss of 0.1205%.
EXAMPLE 4
A mixture of 30% D-glucose and 70% KClO.sub.3 was prepared and tested as
described in EXAMPLE 1. The mixture autoignited and burned at a
temperature of 135.0.degree. C. (.apprxeq.275.degree. F.). Following
long-term high temperature ageing for 400 hours at 107.degree. C., the
autoignition temperature was observed at 139.0.degree. C.
(.apprxeq.282.degree. F.), with a weight loss of 0.1078%.
EXAMPLE 5
A mixture of 40% D-glucose and 60% potassium chlorate was prepared and
tested as described in EXAMPLE 1. The autoignition temperature was
observed at 136.5.degree. C. (.apprxeq.278.degree. F.). Following
long-term high temperature ageing at 107.degree. C. for 400 hours, the
mixture autoignited at 136.5.degree. C. (.apprxeq.278.degree. F.), with a
weight loss of 0.1492%.
EXAMPLE 6
A mixture of 26.875% D-galactose and 73,125% potassium chlorate was
prepared as described in EXAMPLE 1. When the mixed powder was tested in a
DSC, the autoignition onset temperature was observed at 162.degree. C.
(.apprxeq.324.degree. F.), with a calorific value of 940 calories per
gram. Following long-term high temperature ageing at 107.degree. C. for
400 hours, the DSC showed an autoignition onset temperature of
149.0.degree. C., with a weight loss of 0.1532%.
The results of the foregoing examples are summarized in the following
tables.
TABLE I
__________________________________________________________________________
Autoignition
Potassium
Autoignition
Temperature (.degree.C.)
Example
D-Glucose
Chlorate
Temperature
After Ageing for
No. (weight %)
(weight %)
(.degree.C.)
400 Hrs at 107.degree. C.
Wt. Loss %
__________________________________________________________________________
1 26.9% 73.1% 138.9 145.0
2 15% 85% 133.0 144.0 0.1235
3 20% 80% 133.5 140.0 0.1205
4 30% 70% 135.0 139.0 0.1078
5 40% 60% 136.5 136.5 0.1492
__________________________________________________________________________
TABLE II
__________________________________________________________________________
Autoignition
Potassium
Autoignition
Temperature (.degree.C.)
Example
D-Galactose
Chlorate
Temperature
After Ageing for
No. (weight %)
(weight %)
(.degree.C.)
400 Hrs at 107.degree. C.
Wt. Loss %
__________________________________________________________________________
6 26.875%
73.125%
162.0 149.0 0.1532
__________________________________________________________________________
While the preferred embodiment of the invention has been disclosed, it
should be appreciated that the invention is susceptible of modification
without departing from the scope of the following claims.
* * * * *
|
|
 |
|
 |
Description |
|
