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BACKGROUND OF THE INVENTION 1. Field of the invention
This invention relates to a process for inflating a safety crash bag. 2.
Description of the prior art
In the prior art, the generation of nitrogen gas in order to fill an airbag
for use as an aircraft or automobile safety crash bag has involved the use
of azide compounds. Azide compounds such as sodium azide are highly toxic
materials prior to combustion. Such azide salts also readily react with
heavy metals such as copper, lead, etc. to form extremely sensitive solids
that are subject to unexpected iginition or detonation and therefore
require special handling in the manufacture, storage and disposal of such
compounds.
Methods of generating nitrogen gas to fill a safety crash bag using metal
salts of 5,5'-bitetrazole with oxidizers which contain no oxygen in the
molecule are disclosed in U.S. Pat. No. 4,370,181 to Lundstrom et al. The
prior art use of tetrazole compounds with oxygen containing oxidizers are
dismissed in view of the fact that such compositions do not meet the
present requirements for the generation of gases which are able to meet
the industrial standards for toxicity with respect to such gases as carbon
monoxide, carbon dioxide, etc. The disclosure of prior art non-azide
nitrogen gas generants by Lundstrom et al are to various hydroxamine acid
and hydroxylamine derivatives, various polymeric binders, hydrocarbons and
carbohydrates which are oxidized to produce non-corrosive and, often
termed, "non-toxic" gases. In addition, Lundstrom et al disclose as other
approaches to non-azide nitrogen gas generants utilizing tetrazole
compounds such as aminotetrazole, metal salts of aminotetrazole, or other
tetrazole salts which contain hydrogen in the molecule. These are used in
combination with oxygen containing oxidizers such as potassium
perchlorate. Upon combustion, these compositions tend to form various
toxic species such as hydrogen cyanide, nitrogen oxides, and carbon
monoxide in unacceptable proportions so as not to meet the present
toxicity requirements for the non toxicity of the gas generated.
In U.S. Pat. No. 4,369,079 to Shaw, solid, non-azide nitrogen gas generant
compositions for inflation of a safety crash bag are disclosed as
consisting essentially of a metal salt of a non-hydrogen containing a
tetrazole compound in admixture with an oxidizer containing nitrogen. The
specific tetrazole which is disclosed as useful is azobitetrazole.
In U.S. Pat. No. 3,910,595, an apparatus is described for aspirating air
into a gas mixture used to inflate a crash restraint device.
SUMMARY OF THE INVENTION
A process is disclosed for inflating an airbag suitable for a number of
purposes including use as a safety crash bag in aircraft or automobiles in
which a primary source of gas is generated by the ignition of pellets
prepared from a dry blend of a tetrazole or triazole compound and an
oxygen containing oxidizer. By the method of the invention it is possible
to obtain a substantially higher yield of gas for use in inflating the
crash bag by the provision of passing the gas mixture generated upon
combustion of said pellets through a venturi so as to aspirate outside air
to form a gas mixture which is then used to inflate the crash bag.
The method of the invention overcomes the disadvantages referred to above
in the discussion of the prior art relating to the use of certain
non-azide gas generant mixtures consisting of tetrazole compounds such as
aminotetrazole, triazole compounds such as 1,2,4-triazole-5-one, metal
salts of aminotetrazole, or other tetrazole salts which contain hydrogen
in the molecule in combination with oxygen containing oxidizers. While the
gases produced upon combustion of such generant mixtures may contain
higher amounts of toxic species of gases than are presently acceptable for
use in inflating airbags, by the novel provison of diluting the primary
source of gas (produced upon combustion) with a secondary source of gas
(air), acceptable levels of the toxic species are obtained thus making
such gas generants practical. Both tetrazole and triazole compounds are
useful in the process of the invention. The useful tetrazole compounds
include aminotetrazole, metal salts of tetrazole, other tetrazole salts
containing hydrogen in the molecule, and metal salts of such hydrogen
containing tetrazoles.
DETAILED DESCRIPTION OF THE INVENTION AND THE PREFERRED EMBODIMENTS
The present invention relates to a process for inflating a crash bag
utilizing a mixture of a triazole or a tetrazole compound and an oxygen
containing oxidizer. Upon combustion of this mixture a primary gas is
formed which when passed through an aspirating venturi draws in outside
air for use in cooling the primary source of gas and diluting the primary
source of gas, thus reducing the toxicity of the primary source of gas
below unacceptable levels and permitting the use of triazole or tetrazole
compounds which have safety and toxicity advantages over the azide
compounds used in the prior art.
A particularly unexpected aspect of the process of the invention is that
certain tetrazole compounds when admixed, for instance, with an oxygen and
chlorine containing oxidant compound, such as an alkali metal perchlorate,
have been found to be incapable of sustaining combustion at ambient
pressure, although these mixtures burn readily at elevated pressures.
Generally, a pressure of about 100 psi to about 3000 psi, preferably,
about 500 psi to about 2500 psi, and most preferably, about 750 psi to
about 2000 psi is used. Thus, there is a substantial safety advantage in
the use of such mixtures over the use of azide compounds, such as sodium
azide as a basis for the generation of a gas mixture for the inflation,
for instance of an automobile safety crash bag. In addition, the triazole
and tetrazole compounds are relatively nontoxic and therefore much more
suitable for use in this application than the azide compounds, which are
highly toxic. Thus, the triazole and tetrazole compounds in admixture with
said oxidizer compounds require less special handling in the manufacture,
storage and eventual disposal than is the case with the gas generants
prepared from azide compounds which are toxic.
An especially useful oxidizer compound is a mixture of ammonium perchlorate
and sodium nitrate in a 1 to 1 mole ratio so that the sodium and chlorine
combine during combustion to form the harmless sodium chloride. An excess
of chlorine must be avoided since toxic gases such as hydrogen chloride
would be formed. A small excess of sodium can be tolerated since it would
result in the formation of sodium carbonate. Other useful oxidizing
compounds are salts such as the ammonium, alkali metal, and alkaline earth
metal nitrates and perchlorates. The proportion of gas generant compound
utilized in admixture with an oxygen containing oxidizer is generally
about 20 to about 65 wt. % of tetrazole or triazole compound in
combination with about 35 to about 80% by wt. of said oxidizer.
In general, the ratio of oxidizer to the tetrazole or triazole compound
must be adjusted to provide an excess of oxygen after all carbon and
hydrogen have been oxidized to form carbon dioxide and water. The amount
of excess oxygen required is about 1 to 25% by volume in the gas formed
upon combustion.
By the method of the invention, the primary gas mixture formed upon
combustion at elevated pressure of the mixture of said tetrazole or
triazole compound and said oxygen containing oxidizer compound is
generally diluted with about 1 to about 4 volumes of air, usually about 1
to about 2.5 volumes of air. The amount of dilution of the gas mixture
formed upon combustion with air is dependent upon several factors
including the temperature of the primary gas mixture, the molecular weight
of the primary gas mixture and the design of the aspirator utilized. Any
toxic gases in the primary gas mixture upon dilution with air would be
decreased by a factor of about 2 to about 5 upon dilution with outside
air. The final diluted gas mixture generally contains less than about 16%
by volume carbon dioxide, less than about 4% by volume hydrogen, and less
than about 50%, preferably less than about 20% by volume of water.
The use of tetrazole compounds such as tetrazole, aminotetrazole, metal
salts of tetrazole or aminotetrazole, or other tetrazole salts which
contain hydrogen in the molecule in admixture with oxygen containing
oxidizer compounds such as potassium perchlorate have been dismissed as
unsuitable by workers in the prior art on the basis that such mixtures,
when burned, tend to form small amounts of various toxic species (in
addition to nitrogen) such as hydrogen cyanide, nitrogen oxides, and
carbon monoxide. Nevertheless, such mixtures have been found to be
eminently satisfactory, upon dilution with outside air, to inflate a crash
bag. Representative useful triazole compounds are 1,2,4-triazole;
1,2,4-triazole-5-one, and 3-nitro-4,5-dihydro-1,2,4,-triazole-5-one.
The use of a gas mixture comprising a primary gas mixture diluted with a
secondary gas mixture (air) provides several advantages, namely, the
primary gas mixture is cooled substantially by such dilution, thus
avoiding the potential for burning the occupants of the aircraft or
automobile in which the crash bag is utilized. In addition, the air
dilution of the primary gas mixture reduces the level of toxic species
present to much lower levels, which are acceptable. Thus the use of
tetrazole or triazole compounds containing hydrogen in the molecule is
practical, since the concentration of hydrogen in the gas produced can
generally be reduced by oxidation to very low levels, generally less than
4% by volume, by the formation of water. In addition, the level of water
in the gas mixture can be reduced preferably to about 2% to about 20% by
volume, depending upon the gas generant composition used.
Using hydrogen containing tetrazoles and triazoles as gas generant
compounds is particularly advantageous in conjunction with a system in
which outside air is aspirated so as to form a mixture with the gas
generated by combustion of the hydrogen containing tetrazoles and
triazoles in that water is formed in the combustion of the tetrazoles and
triazoles. Water has a low molecular weight and is non toxic. A low
molecular weight in the gases formed upon combustion is especially
desirable in a system in which aspiration of outside air is utilized. The
level of carbon dioxide can be reduced to less than about 1% to about 5%
by volume, an entirely acceptable level in this application. The hydrogen
cyanide, nitrogen oxides, and carbon monoxide levels obtained are also
acceptable.
In order to prepare the gas generating compositions utilized in the process
of the invention, the components, for instance, the sodium salt of
tetrazole, and the oxygen and chlorine containing oxidizer compound, for
instance, a mixture of ammonium perchlorate and sodium nitrate, can be dry
blended as powders by standard methods. The components can also be blended
with other additives for burning rate improvement or adjustment and for
improving the propellant grain processing properties. The blended powder
can, if desired, be compressed into granules, or pellets by conventional
techniques. Since the components of the gas generating composition used in
the process of the invention are not highly toxic or highly reactive and
ignite only at elevated pressure, special handling techniques, beyond
those required in the use of ordinary solid propellants (to minimize
exposure because of toxicity or contamination which might increase
reactivity) are not required in the fabrication of the gas generating
compositions used or in the pelletizing thereof.
One skilled in the art will recognize that at least one of any other alkali
metal, an alkaline earth metal or an ammonium salt of a tetrazole
containing hydrogen or an alkali metal, an alkaline earth metal, or an
ammonium salt of an aminotetrazole or a triazole can be substituted for
the metal salt in the below Examples or, alternatively, a hydrogen
containing tetrazole, aminotetrazole, or triazole compound can be utilized
per se in admixtures with an oxygen containing, preferably an oxygen and
chlorine containing oxidizing compound in the preparation of granules or
pellets by conventional techniques. The oxidizing compound is exemplified
by an alkali metal or alkaline earth metal nitrate or perchlorate or
mixtures of alkali metal or alkaline earth metal nitrates with ammonium
perchlorates. One skilled in the art will also recognize that catalysts or
combustion rate modifiers can be used or added in addition to the oxygen
containing oxidizing compound described above. Thus, additional catalyst
compounds such as vanadium pentoxide, copper oxide, and iron oxide may be
substituted or added to the mixture to be pelletized.
Preferably, an oxidizer compound or oxidizer compound mixture can be
selected which will result in complete conversion of any metal (whether
contained in the oxidizer or in the combustion compound used as fuel) to
the chloride salt. Thus, a metal salt such as the sodium salt of tetrazole
can be reacted with an equimolar quantity of ammonium perchlorate in order
to convert all of the sodium to sodium chloride. In order to provide the
additional oxygen required to oxidize the carbon to carbon dioxide and the
hydrogen to water, an oxidizer balanced to produce metal salt can be used.
For example, an equimolar mixture of ammonium perchlorate and sodium (or
potassium) nitrate can be used. Alternatively, a metal perchlorate or
chlorate can be used such as potassium perchlorate.
Ammonium perchlorate, although a good oxidizer, is not useful as the sole
oxidizer since it will produce hydrogen chloride or other toxic products
if not balanced by the presence of a metal such as sodium or potassium,
Alkali metal nitrates such as sodium or potassium nitrate can be used
without a chlorine containing oxidizer but the products obtained upon
combustion must be carefully evaluated in order to avoid disadvantageous
results. In general, a metal carbonate such as sodium carbonate can result
by the use of an alkali metal nitrate. The formation of such a salt is
disadvantageous because, in the formation of said salt, carbon dioxide is
removed as a component from the gases formed upon combustion. Carbon
dioxide is a useful gas for inflating a crash bag since it has a
relatively low toxicity. In addition, if not enough carbon dioxide is
available in the gas formed upon combustion, then other hazardous products
can be formed such as sodium or potassium oxide.
It is believed that the use of oxidizers which are balanced to produce a
metal chloride salt, particularly, a sodium or potassium chloride salt,
result in an additional safety advantage as compared with the use of
oxidizer compounds which do not produce a metal chloride salt because the
gas generants prepared from such oxidizers in combination with the
tetrazole compounds disclosed as useful in the process of the invention
burn with difficulty (if at all) at atmospheric pressure but burn
vigorously at elevated pressures. This unexpected result is not fully
understood but it is assumed that the salt vapor formed during combustion
quenches the flame at low pressures but not at higher pressures.
Although many satisfactory ignition mechanisms will occur to one skilled in
the art, a particularly convenient and preferred igniter composition
consists of a mixture of boron and potassium nitrate which is well known
to those skilled in the art as BKNO.sub.3. Other ignitor compositions such
as mixtures of potassium perchlorate, ammonium perchlorate, and aluminum
powder are also suitable. Firing of the ignitor composition may be
accomplished utilizing standard electrical means including any desired
safety devices in the circuitry, such as spark gaps and/or ferrite
resistors to prevent unwarranted initiation from strong radio frequency or
high voltage sources.
The process of the invention can utilize conventional gas generator
mechanisms of the prior art. These are referred to in U.S. Pat. No.
4,369,079, incorporated herein by reference. Other more suitable gas
generating devices are envisioned. Generally, the methods of the prior art
involve the use of a hermetically sealed metallic cartridge containing the
pyrotechnic material, the oxygen containing oxidizer, and an initiator.
Upon initiation of combustion by the firing of a squib, the sealing
mechanism ruptures. This allows gas to flow out of the combustion chamber
through several orifices and into an aspirating venturi through which
outside air is drawn into he gas formed upon combustion so that the gas
utilized to inflate the airbag is a mixture of outside air (secondary gas
source) and the gaseous mixture formed upon ignition (primary gas source)
which together constitute the total amount of inflation gas.
When utilizing the gas generating compositions described above, a less
efficient filter is required than when toxic solids are generated because
the solids formed upon combustion in the process of the invention are
generally considered to be nontoxic and consist, for example, of solids
such as sodium chloride, sodium carbonate, and potassium chloride. Such
solids of low toxicity are generally referred to as nuisance particulates.
The following Examples illustrate the various aspects of the invention but
are not intended to limit its scope. Where not otherwise specified
throughout this specification and claims, temperatures are given in
degrees centigrade and parts, percentages, and proportions are by weight
except for gases in which case percentages are by volume.
EXAMPLE 1
A mixture of the sodium salt of tetrazole, ammonium perchlorate, and sodium
nitrate was prepared having the following composition in percent by weight
34% sodium salt of tetrazole; 38.3% ammonium perchlorate; and 27.7% sodium
nitrate. These powders were dry blended and pellets were prepared by
compression molding. The pellets would not sustain combustion upon
repeated ignition at atmospheric pressure using a propane-oxygen torch
but, continued burning when ignited under a helium pressure of 300 psi.
Subsequent burning rate measurements at a pressure of 1000 psi. indicated
a burning rate of about 2 inches per second. The combustion temperature of
this mixture is theoretically 3345.degree. F. The primary gas composition
produced upon combustion contained 45.4% wt. of nitrogen, 9% by wt. of
carbon dioxide, 34.5% of water, and 11.1% by wt. of oxygen. The solid
residue formed upon combustion consisted of sodium chloride and sodium
carbonate. When this primary gas composition is diluted with 2.5 volumes
of air to each volume of the primary gas produced upon combustion, the
water content of the mixture is reduced to 9.9% by volume and the carbon
dioxide content of the mixture is reduced to 2.6% by volume.
EXAMPLE 2
A mixture of 5-aminotetrazole, ammonium perchlorate, and sodium nitrate was
made by dry blending and pellets were formed upon compression molding. The
percent by weight composition of the mixture was: 34% 5-aminotetrazole;
38.3% ammonium perchlorate; and 27.7% of sodium nitrate. The pellets would
not sustain combustion at atmospheric pressure but burn completely when
pressurized to 300 psi with helium. The burning rate measured at 1000 psi.
was 0.53 inches per second. The combustion temperature is theoretically
4300.degree. F. and the primary gas composition produced upon combustion
contains 42.9% by volume nitrogen, 12.9% by volume carbon dioxide, 40.3%
by volume water, and 3.7% by volume oxygen. The solid residue produced
upon combustion was sodium chloride. Upon dilution with air at a ratio of
2.5 to 1 a water content of 11.5% by volume is obtained and a carbon
dioxide content of 3.7% by volume is obtained.
EXAMPLE 3
The mixture described in Example 2 was modified by addition of 0.5% by
weight of iron oxide (Fe.sub.2 O.sub.3). The final composition used in
percent by weight was: 34% 5-aminotetrazole, 38.05% ammonium perchlorate,
27.45% sodium nitrate, and 0.5% iron oxide. This mixture was dry blended
and pellets were formed by compression molding. The pellets when ignited
at atmospheric pressure continued to burn slowly. The burning rate
measured at 1000 psi. was found to be 0.77 inches per second.
EXAMPLE 4
A mixture as described in Example 3 was prepared except that vanadium
pentoxide (V.sub.2 O.sub.5) was substituted for iron oxide. The mixture
was dry blended and pellets were formed by compression molding. The
pellets continued to burn slowly when ignited at atmospheric pressure. The
burning rate measured at 1000 psi. was found to be 0.56 inches per second.
EXAMPLE 5
A mixture of 40% by weight of the sodium salt of tetrazole, 49.7% by weight
of sodium nitrate, and 10.3% by weight of silicon dioxide was dry blended
and pellets were formed by compression molding. When ignited at
atmospheric pressure, the pellets burned completely and very rapidly. The
burning rate measured at 1000 psi. was found to be 1.5 inches per second.
The combustion temperature of this mixture is theoretically 3432.degree.
F. and the primary gas composition produced at combustion contains 72.2%
by volume nitrogen, 6% by volume carbon dioxide, 16.9% by volume water and
4.9% by volume oxygen. The solid products formed upon combustion consist
of sodium carbonate and sodium silicate. When the primary gas composition
is diluted with 2.5 volumes of air to each volume of primary gas formed,
the water content of the diluted mixture is 4.8% by volume and the carbon
dioxide content is 1.7% by volume.
EXAMPLE 6
A mixture of 30% by weight of 1,2,4-triazole-5-one, 40.4% by weight
ammonium perchlorate, 29% by weight sodium nitrate, and 0.5% by weight
vanadium pentoxide was dry blended and pellets were formed by compression
molding. When ignited at atmospheric pressure, the pellets continued to
burn slowly. The burning rate measured at 1000 psi. was found to be 0.37
inches per second. The theoretical combustion temperature of this mixture
is 4309.degree. F. and the primary gas composition produced at combustion
contains 30.5% by volume nitrogen, 24.6% by volume carbon dioxide, 42.5%
by volume water, and 2.4% by volume oxygen. The solid product formed by
combustion is sodium chloride. When the primary gas is diluted with 2.5
volumes of air to each volume of primary gas, the water content is reduced
to 12.2% by volume and the carbon dioxide is reduced to 7% by volume.
While this invention has been described with reference to certain specific
embodiments, it will be recognized by those skilled in the art that many
variations are possible without departing from the scope and spirit of the
invention, and it will be understood that it is intended to cover all
changes and modifications of the invention disclosed herein for the
purposes of illustration, which do not constitute departures from the
spirit and scope of the invention.
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