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Description  |
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BACKGROUND OF THE INVENTION
A. Field of the Invention
This invention relates to solid gas generants particularly those suitable
for the production of substantially pure nitrogen gas, more particularly
to the use of alkali metal azides as a base for such systems, processes
for their preparation and use as well as to systems employing such gas
generants in their operation.
B. Description of Prior Art
With the realization that in private automotive vehicles crash restraint
systems requiring positive effort on the part of the user for
effectiveness were ignored by a substantial portion of the population
intended to be protected, professional and governmental safety programs
have tended to emphasize automatic restraint systems, such as crash bags.
The details of crash bag systems have been widely discussed, as have the
reasons for selection of pyrotechnic devices when rapid dependable gas
supplies therefore are required. The operational constraints of crash bags
are also well known. The system must supply absolutely non-toxic gas to
inflate the bag because some bag systems vent into the passenger
compartment on deflation and because of the very real probability of bag
rupture in an actual crash situation. Naturally the gas must inflate the
bag at a temperature which the beneficiaries of the protection can
tolerate. The time period for attainment of maximum inflation has been
determined to be from 20 to 100 milliseconds, preferably 20 to 60
milliseconds. The device must be safe to handle and store prior to
production. It must be adaptable to mass production line installation
techniques and not introduce an unreasonable hazard then or during the
life of the vehicle. It must assure reliable operation during the life of
the vehicle containing it, which may be 10 years or longer.
The objectives of rapid generation of cool non-toxic inflation gas and
long-term operability depend to a large extent on the gas generant
selected and the physical form into which it is initially compounded.
If a suitable propellant can be designed, then the design of a complete
passive restraint system undertaken with consideration of the
characteristics of a particular propellant stands a better chance of
practical success.
Naturally, from every point of view, the most desirable atmosphere inside
an inflated crash bag would correspond in composition to the air outside
it. This has thus far proven impractical of attainment. The next best
solution is inflation with a physiologically inert or at least innocuous
gas. The most practical of these gases has proven to be nitrogen. The most
popular means of generating nitrogen has been the decomposition of alkali
metal, alkaline earth metal and aluminum derivatives of hydrazoic acid,
especially sodium azide.
Decomposition of these azides in the absence of other ingredients can only
be accomplished by means of a high heat source and the decompositions are
not self-sustaining. So as to provide favorable kinetic conditions for the
decomposition, various co-reactants have been suggested. It is evident
that such co-reactants must be selected to provide that the non-gaseous
decomposition products are capable of containment or easily converted to
manageable form. Non-gaseous decomposition products being those which may
be solid or liquid at 25.degree. to 40.degree. C. Among the by products of
the decomposition which must be contained or converted to a containable
form is the free metal derived from the counter-ion in the azide salt. A
number of co-reactants to provide cool sustained combustion together with
formation of little or no free counter-ion metal have been proposed. The
most notable of these are ferric oxide (U.S. Pat. Nos. 4,062,708;
3,931,040; 3,996,079 and 3,895,098) and a mixture of molybdenum sulfide
and sulfur (U.S. Pat. No. 3,741,585), both of which systems, while usable
in crash restraints, have proven to have inherent disadvantages to their
commercial practice.
The molybdenum disulfide sulfur systems burn with the requisite speed, give
cool gas when employed in a container of proper design and can be easily
pelletized to stable pellets which are abrasion and vibration resistant
for the expected life of the inflators. They do present an odor problem
apparently because of the presence of trace sulfur compounds in the gas,
the solid combustion residue is finely divided making containment thereof
a difficult engineering problem, and the gas is generated at high
pressures, over 2,000 pounds per square inch absolute (psia), requiring
heavy walled vessels for its containment. Iron oxide-azide systems on the
other hand are extremely difficult to compact to stable pellets in actual
practice, they are cool burning, but also generate their nitrogen at
pressures over 2,000 psia and tend to be unstable and slow burners. Their
residue on the other hand is described in the prior art as being in the
form of large particles of "clinkers" that are simpler to contain, and
having no sulfur, they are naturally free of any sulfur odor problem.
Containment of the hot combustion residues within the gas generator is
necessary to prevent them from damaging the fabric of the gas bag itself
and to prevent them from coming in contact with and injuring the occupants
of the vehicle.
Those gas generants of the prior art which provide a finely divided
combustion residue have been indicated in that art as requiring elaborate
and for the most part expensive filtering devices. None of these devices
has proven sufficiently attractive for commercial practice. The force of
the combustion gases, particularly in the initial stages of combustion,
results in a substantial pass through of high temperature particulates to
the exterior of the gas generator. The U.S. Pat. No. 4,062,708 discloses
that the compositions contained therein leave fused coherent combustion
residues which are relatively easy to contain with relatively simple
filtration means. The U.S. Pat. No. 3,996,079 also indicates that
combinations of azides and iron or nickel oxides will form a sintered
residue on combustion, thereby simplifying containment of said residue.
Unfortunately, for the reasons discussed herein, simple azide-oxide
systems have not as yet demonstrated commercial value as gas generants.
Azide-metal oxide systems are somewhat slower burning than azide-molybdenum
disulfide-sulfur systems although combinations within at least the upper
limits of the acceptable combustion rate range can apparently be
formulated. The incorporation of metal oxides into azide-molybdenum
disulfide-sulfur systems would appear on its face as a means to obtain
more rapid burning and an easily trapped sintered residue. Surprisingly,
while such a combination is rapid burning, it is also substantially cooler
burning, has no substantial objectionable odor in the gas generated and in
the absence of a properly designed container package, does not form a
sintered residue. In the absence of such a sintered residue, the elaborate
filtration and neutralization devices of the prior art are required for
use of this type propellant as a crash bag inflator. However, when used in
the specially designed gas generator of the hereinafter identified
Adams-Schneiter application, the azide-metal oxide-molybdenum
disulfide-sulfur generant compositions can be caused to form a sintered
residue enabling simplified neutralization and filtration of the gas
produced by their combustion.
The importance of pelletization will be immediately apparent to anyone who
understands that, all else being equal, the surface area of a gas generant
determines its gas production rate. This surface area depends on pellet
size and for reproducibility must be uniform. In the absence of pelleting,
routine handling will cause abrasion of individual particles changing the
burn characteristics of a generant batch unpredictably. Long term
vibration stresses will cause a generant which is a mixture of ground
components to separate according to the density of individual components,
again making performance unpredictable. The use of organic binders for
pelleting, while convenient, is not acceptable to automobile manufacturers
because the presence of carbon containing compounds immediately introduces
carbon monoxide into the combustion gas and frequently the presence of
compounds containing carbon, nitrogen and hydrogen will lead to formation
of HCN.
The use of an inorganic lubricant and binder for pelleting is therefore
indicated. A proven lubricant and binder is molybdenum disulfide. Simple
replacement of a portion of the iron oxide of iron oxide azide systems
with sufficient molybdenum disulfide for satisfactory pellet manufacture
of even greater quantities is not a direct solution because the resultant
compositions are either slow burning, difficult to ignite, or just
marginally acceptable (burn rates on soda straw size strands of 0.7 in.
per sec. at 1000 psia pressure). A number of sources, including U.S. Pat.
No. 4,062,708, suggest that the inclusion of perchlorate accelerators in
azide-iron oxide systems will increase the burn rate. Unfortunately, the
formation of hydrogen chloride and chlorine is a known problem of
perchlorates.
The present invention provides a gas generant consisting of a mixture of
non-explosive azides, iron oxide, molybdenum disulfide and sulfur which is
easily pelletizable, surprisingly cool burning, generates nitrogen at much
lower pressure than any prior art gas generant, has burn rates well within
acceptable limits, sustains burning reliably after ignition, has very
little or no odor in the nitrogen generated, and in a properly designed
container leaves a combustion residue which is principally an easily
contained "clinker" or fused mass.
SUMMARY OF THE INVENTION
The invention provides in a composition aspect a solid nitrogen gas
generant consisting of 60 to 80 weight percent alkali metal azide, 2 to 35
weight percent oxide selected from iron oxide, nickel oxide, palladium
oxide, cobalt oxide, silicon oxide or mixtures thereof, 2 to 26 weight
percent molybdenum disulfide and up to 6 weight percent sulfur.
The tangible embodiments of this composition aspect of the invention
possess the inherent physical property of being readily mechanically
compactable into stable tablets or pellets by conventional tableting or
pelletizing techniques.
The tangible embodiments of this composition aspect of the invention
possess the inherent applied use characteristics when in tablet or pellet
form of being of uniform composition, of being physically and chemically
stable at ordinary automotive operating temperatures including extremes
thereof which the remainder of the vehicle may survive without substantial
damage, of being insensitive to shock and vibration normally encountered
during ordinary use; when exposed to an appropriate high temperature
ignition source of burning rapidly and at relatively low pressure and
temperature to generate nitrogen gas substantially free of odor and
substantially free of noxious or toxic contaminants thereby evidencing
usefulness in automotive vehicle passive restraint systems employing gas
inflated cushions as restraining devices. The tangible embodiments of this
composition aspect of the invention also possess the inherent applied use
characteristic when employed as a tableted or pelleted gas generant in the
gas generator described in copending application, Ser. No. 970,687, of
Gary Adams and Fred Schneiter filed concurrently with this application of
providing a "clinker" or sintered combustion residue which permits
simplified trapping of the combustion residue and pH adjustment of
effluent material from the combustion chamber.
Special mention is made of those embodiments of this composition aspect of
the invention wherein the metal azide is sodium azide, and those wherein
the oxide is iron oxide, preferably ferric oxide.
The invention also provides an improved nitrogen gas generator based on
alkali metal azides containing reactants for combination with the free
alkali metal liberated by decomposition of alkali metal azides to produce
nitrogen wherein the improvement comprises the reactants for combination
with said free alkali metals being a mixture of oxides selected from iron
oxide, cobalt oxide, nickel oxide, palladium oxide, silicon oxide or
mixtures thereof; molybdenum disulfide and sulfur.
The invention also provides a method for the generation of substantially
pure and substantially particle free nitrogen gas at pressures below 1500
psia, where generation is initiated at normal room temperature, which
comprises:
(a) treating a nitrogen gas generant composition consisting of 60 to 80
weight percent alkali metal azide, 2 to 35 weight percent oxide selected
from iron oxide, cobalt oxide, nickel oxide, palladium oxide, silicon
oxide or mixtures thereof; 2 to 26 weight percent molybdenum disulfide and
up to 6 weight percent sulfur with hot combustion products of an igniter
combustion mixture of 5 to 25 weight percent boron and 75 to 95 weight
percent potassium nitrate, to which mixture is added 3 to 10 weight
percent lead azide said hot composition products being of sufficient
quantity to induce sustained combustion of said nitrogen gas generant
composition; and
(b) passing the products of combustion of said nitrogen gas generant
composition through cooling, filtration and pH adjustment means.
Brief Description of the Drawings
FIG. 1 is a representation of the pressure vs. time relationship obtained
in the interior of a gas generator by burning the composition of Example 5
and expelling the gases into a static tank.
FIG. 2 is a representation of the time pressure relationship of a burn
similar to FIG. 1 with expulsion of gases into crash bags.
FIG. 3 represents a pressure time relationship developed during inflation
of a passenger knee area crash bag by a burn similar to that of FIG. 2.
FIG. 4 represents a pressure time relationship developed during inflation
of a passenger torso area crash bag by a burn similar to that of FIG. 2.
FIG. 5 represents the variation of burn rate in a closed bomb with pressure
and temperature at ignition of gas generant of Example 5.
Description of the Preferred Embodiment
The manner of making and using the nitrogen gas generant compositions (I)
of the invention will now be described with reference to a specific
embodiment thereof, namely a nitrogen gas generant composition (Ia)
consisting of sodium azide, ferric oxide, molybdenum disulfide and sulfur.
To prepare Ia, sodium azide, ferric oxide, molybdenum disulfide and sulfur,
all of which are commercially available may be dry blended as powders by
standard methods. The blended powder Ia, may, if desired for use where
rapid, controlled, repeatable, and long term reliably accurate performance
is intended, be compacted into tablets, granules or pellets by
conventional techniques. For safety considerations as with most, if not
all, pyrotechnic substances, remote handling is preferred. Conventional
remote controlled tableting presses are convenient devices which may be
employed for compression to tablets.
One skilled in the art will recognize that one may substitute other alkali
metal azides for the sodium azide illustrated herein above, particularly
lithium azide or potassium azide and that one may substitute other oxides
of iron such as ferrous oxide (FeO) or magnetite (Fe.sub.3 O.sub.4) as
well as the common oxides of cobalt, nickel, and palladium, as well as,
silicon dioxide or mixtures of any two or more oxides for the ferric oxide
(Fe.sub.2 O.sub.3) illustrated to prepare other compositions I equivalent
to Ia.
The particle sizes of the azide, molybdenum disulfide and sulfur are not
particularly critical and the commercially available materials sized as
powders or small crystals are suitable. When rapid combustion rates are
essential, the oxide particle size must be more closely controlled.
Submicron size particles may be employed in preparing pelletized gas
generant compositions. Particle sizes of 0.7 to 0.9.mu. are particularly
preferred in obtaining embodiments of the invention with burning rates
within the desired range.
One skilled in the art will recognize that as the compositions of the
instant invention are cooler burning than those of the prior art, giving
nitrogen gas at combustion temperatures as much as 200.degree. C. lower
than earlier compositions, they require a hotter initiator to start the
combustion process reliably. Although many equivalent initiators will
occur to one skilled in the art, and the use of such equivalents is
comprehended in the process of the invention both in the specification and
appended claims, a particularly convenient and preferred initiator
composition is one consisting of 5 to 25 weight percent, preferably about
10 weight percent boron; 75 to 95 weight percent, preferably about 85
weight percent potassium nitrate to which mixture is added 3 to 10 weight
percent, preferably about 5 weight percent lead azide. Firing of the
initiator composition may be by standard electrical means including any
desired safety devices in the circuitry, such as spark gaps and/or ferrite
resistors to prevent unwanted initiation from strong radio frequency or
low voltage sources, at the option of the designer of the system.
While the gas generant compositions of this invention may be employed as
the charge in conventional gas generants of the prior art, they are most
advantageously employed in the particular gas generator construction
described in the copending application of Gary Adams and Fred Schneiter
referenced hereinabove.
This gas generator, which has a concentric configuration with the initiator
at the center of a suitable reaction chamber surrounded by the gas
generant compositions in suitable pelletized form which is in turn
surrounded by wire screen, specially selected woven fiber glass cloth, and
a second layer of wire screen covering radially arranged exit ports to a
concentric diffusion chamber, the radially arranged exit ports of which
are filtered by wire screen supporting an aluminum silicate fiber mat as a
secondary filter, enables the advantageous characteristics of the
inventive embodiments to be fully utilized.
Specifically, the pyrotechniqc material of the initiator, the gas generant
composition and the primary filter are all contained in a hermetically
sealed aluminum cartridge. This insures reliability of the generator over
long periods. The aluminum cartridge is positioned in the combustion
chamber of the generator. Upon initiation of combustion by the firing of
the squib, the rising gas pressure ruptures the side wall areas of the
cartridge adjacent the orifices of the combustion chamber. This allows gas
to flow through the primary filter and out of the combustion chamber
through the several orifices. The combustion chamber filter consists of
one to three layers of a coarse screen adjacent to the wall of the
chamber. This serves as a collecting area for gas to flow along the
chamber wall to the chamber orifices and permits gas to flow evenly
through the primary filter regardless of the proximity of a combustion
chamber orifice. Inboard of the coarse screen are one or more layers of
fiberglass woven fabric. The fiberglass fabric is selected for
compatibility with the temperature in the combustion chamber during
burning of the selected gas generant composition thereby to provide a
tacky surface for particle entrapment that does not melt or erode away
under the effects of the high temperature gas. An effect accompanying the
production of the tacky surface appears to be a swelling of the fibers of
the fiberglass fabric that reduces the porosity of the primary filter. It
is believed that this swelling causes the primary filter to restrict the
flow of gas and combustion residue out of the combustion chamber. This
effect is believed to continue for only a short interval, up to about 3
milliseconds, but long enough to allow cooling and condensation of hot and
molten particulate residue within the voids of the filter. Inside the
multiple layers of the fiberglass cloth are multiple layers of fine mesh
carbon steel screen. The layers of the fine mesh carbon steel provide a
large relatively cool surface for condensation of combustion solids prior
to encountering the multiple layers of fiberglass woven fabric.
Approximately 95 percent of all solid products of combustion are trapped
in the combustion chamber filter. It is noted that outside of the
combustion chamber, the velocity of the gases that are generated becomes
so high that trapping of the products of combustion in that region becomes
exceedingly difficult.
An added benefit of the fiberglass cloth is that under the high temperature
environment, the glass reacts with caustic sodium oxide, Na.sub.2 O,
by-product of the combustion process, to form innocuous sodium silicate.
The secondary filter is comprised of multiple wraps of wire mesh which
serves to cool the gas and provide surface for condensation of solid
particles. Surrounding the wire mesh filter pack are one or more wraps of
the aluminum silicate blanket. The aluminum silicate blanket serves two
distinct functions. One of these functions is to react with particles of
sodium oxide which come into intimate contact with the second filter to
form sodium silicate.
Surrounding the aluminum silicate blanket are several wraps of fine mesh
screen which provide structural support for the aluminum silicate blanket.
It is noted that aluminum silicate blanket is porous, has very little
strength, and tends to disintegrate under the effects of the high velocity
gas stream. The filter elements, however, retain the solids entrapped. The
fine mesh outer screen is used to trap these aluminum silicate filter
particles and prevent them from being carried out of the exit orifices of
the housing with the clean combustion gases.
One skilled in the art will recognize that the successful initiation of
combustion of any gas generant requires the use of an adequate quantity of
initiator to insure that sufficient hot combustion products of the
initiator contact enough of the exposed generant surface to kindle a self
sustaining flame front. The selection of such amounts by a number of
simple graduated experiments for any initiator-gas generant combination is
well within the skill of a journeyman in the art. In the case of the
compositions of the instant invention from 0.02 g to 0.03 g, preferably
from 0.024 g to 0.026 g of the boron, potassium nitrate, lead azide
initiator described herein per gram of gas generant composition may be
employed.
One skilled in the art will also recognize that although the combustion
temperature of the instant compositions is significantly lower than those
of the prior art, in order to reduce the gas temperature in the crash bag
to a level tolerable by the vehicle occupants, additional cooling means
must be provided. In addition to the cooling method of the aforementioned
Schneiter and Adams copending application, the standard cooling means,
normally layers of woven metal mesh which additionally may serve as
conventional filtration means may be employed. One skilled in the art will
also recognize that the effluent gases from combustion of the instant
composition may contain sufficient alkaline material to cause burns or
discomfort to someone coming in contact therewith. In addition to the
fiberglass of the aforementioned Schneiter and Adams application, the
conventional neutralizers of the prior art, conveniently carbonate salts,
may be employed to adjust the pH of the effluent gases from combustion of
the compositions of this invention to levels tolerable by humans,
conveniently pH levels below 10.0.
The following examples further illustrate the best mode contemplated by the
inventors for the practice of their invention.
EXAMPLES 1 through 7
Gas generant compositions are prepared by dry mixing and remote tableting
of the ingredients tabulated in the quantities indicated. Burn rate data
on pressed pellets and other pertinent information are recorded.
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Burn Rate
Example
Formulation (Wt. %)
in/sec at
No. NaN.sub.3
Fe.sub.2 O.sub.3
MoS.sub.2
S psia Remarks
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1 71 29 -- -- No data Poor pel-
letizing (no
lubrication)
2 70 26* 2 2 .421 at 1060
Trace
NaN.sub.3
in residue
3 70 26 2 2 .938 at 1068
Trace
NaN.sub.3
in residue
4 70 28 2 -- .728 at 1057
Trace
NaN.sub.3
in residue
5 66 30 2 2 .955 at 1056
Good
pressing-No
NaN.sub.3
in residue
6 66 32 2 -- .630 at 1052
Good
pressing-No
NaN.sub.3
in residue
7 66 31 2 1 .801 at 1060
Good
pressing-No
NaN.sub.3 - in
______________________________________
residue
*normal size iron oxide pigment .about.5.5.mu. all other tests used
"transparent iron oxide (0.7 to 0.9.mu. particles).?
EXAMPLE 8
The pelletized gas generant composition of Example 5 is inserted into a
passenger air bag style inflator of the type described in the Adams and
Schneiter copending application referenced hereinabove, ignited and the
gases so generated collected in an evacuated collection chamber with a
volume of approximately 300 liters. The pressure of the gases within the
gas generator is measured as a function of time after ignition. FIG. 1
represents the data so obtained graphically.
EXAMPLE 9
A passenger air bag inflator similar to that of Example 8 is charged with
the pelletized gas generant composition of Example 5, ignited and the gas
expelled into standard passenger knee and torso bags. The gas pressure in
the generator at various times is set forth in FIG. 2, the gas pressure in
the knee bag at various times is set forth in FIG. 3 and the gas pressure
in the torso bag at various time intervals is set forth in FIG. 4.
EXAMPLE 10
Strands of the compressed gas composition of Example 5 are ignited in a
closed vessel at various pressures and initial temperatures. The variation
of the burn rate of the strands with pressure and with initial temperature
at ignition is illustrated in FIG. 5.
The strands used in this test are cylindrical, approximately 0.5 in. in
diameter and 0.6 to 0.8 in. long. They are ignited on one end. The sides
of the strand are inhibited by wrapping with plastic tape to prevent side
burning.
EXAMPLE 11
Comparison of combustion characteristic data is determined by burning
compressed strands of the gas generant compositions of Example 5(A) and a
conventional sodium azide-molybdenum-sulfide-sulfur gas generant as
described in U.S. Pat. No. 3,741,585(B).
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Composition A B
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Burn rate (in/sec at 1000 psia)
0.97 1.3
Burn rate exponent 0.27 0.41
Temperature sensitivity (.pi. .sub.K)
0.33 0.33
Combustion temperature (.degree. K.)
1,298 1,470
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