Note: Descriptions are shown in the official language in which they were submitted.
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8ELECTIVE NON-CATALYTIC REDUCT~ION (SNCR) OF TOXIC
GA8EOUS EFFLUENTS IN ~T~ INFLATOR8
BACKGROUND OF THE I~NTION
The present invention relates generally to inflatable
occupant safety restraints in motor vehicles, and more
particularly to reducing the toxi-ity of effluent gases
produced by nonazide gas generating (ompositions.
Inflatable occupant restraint devices for motor
vehicles have been under development ~orldwide for many years,
including the development of gas generating compositions for
inflating such occupant restraints. Because the inflating
gases produced by the gas generants must meet strict toxicity
ret~uirements, many gas generants now in use are based on alkali
or alkaline earth metal azides, pa]ticularly sodium azide.
When reacted with an oxidizing agent, sodium azide forms a
relatively nontoxic gas consisting plimarily of nitrogen.
However, azide-based gas generants are inherently
volatile to handle and entail relatively high risk in
manufacture and disposal. More s~ecifically, whereas the
inflating gases produced by azide-based gas generants are
relatively nontoxic, the metal azides themcelves are conversely
highly toxic, thereby resulting in e~tra expense and risk in
gas generant manufacture, storage, and disposal. In addition
to direct contamination of the environment, metal azides also
readily react with acids and heavy metals to form extremely
sensitive compounds that may spontane~usly ignite or detonate.
In contradistinction, nonaz de gas generants, such as
those disclosed in U.S. Patent No. 5,139,588 to Poole,
typically comprise a nonazide fuel selected from the group of
tetrazole compounds and metal salts thereof, and provide
_ significant advantages over azide-b~sed gas generants with
respect to toxicity related hazard~ during manufacture and
disposal. Moreover, most nonazide gas generant compositions
typically supply a higher yield of gas (moles of gas per gram
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of gas generant) than conventional azide-based occupant
restraint gas generants.
However, many nonazide gas generants heretofore known
and used produce high levels of toxic substances upon
combustion. The most difficult toxic gases to control are the
various oxides of nitrogen (N0X) and carbon monoxide (C0).
Because the gas generant of the passenger-side airbags is
generally four times greater than that of the driver-side, the
need for N0~ and C0 reduction is most keenly felt when
designing passenger-side airbags, although the concern exists
for other airbag systems within the vehicle as well.
~ eduction of the level of toxic N0x and C0 upon
combustion of nonazide gas generants has proven to be a
difficult problem. For instance, manipulation of the
oxidizer/fuel ratio only reduces either the N0X or C0. More
specifically, increasing the ratio of oxidizer to fuel
minimizes the C0 content upon combustion because the extra
oxygen oxidizes the C0 to carbon dioxide. Unfortunately,
however, this approach results in increased amounts of N0x.
Alternatively, if the oxidizer/fuel ratio is lowered to
eliminate excess oxygen and reduce the amount of N0x produced,
increased amounts of C0 are produced.
one way to improve the toxicity of the combustion
gases is to reduce the combustion temperature which would
reduce the initial concentrations of both C0 and N0X. Although
simple in theory, it is difficult in practice to reduce the
combustion temperature and to also retain a sufficiently high
gas generant burn rate for practical application in an
inflatable vehicle occupant restraint system. The burn rate of
the gas generant is important to insure that the inflator will
operate readily and properly. As a general rule, the burn rate
of the gas generant decreases as the combustion temperature
decreases. By using less energetic fuels, specifically fuels
which produce less heat upon combustion, the combustion
temperature may be reduced but the gas generant burn rate is
also reduced.
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Therefore, a need still ~xists for reducing the
toxicity of effluent gases produced by nonazide gas generants
without compromising the gas generanl: properties.
SUMMARY OF THE INVENTION
5The aforesaid problems are solved, in accordance with
the present invention, by a nonazide cas generating composition
which in and of itself is nontoxic, ald which upon combustion,
also produces inflating gases that hlve reduced levels Of NOI
and CO due to the use of a compound that generates NH2 radicals
in the gas phase. Selective non-catalytic reduction (SNCR)
employs an NH2 radical that selectiv~ly reacts with nitrogen
oxide (NO) in the gas phase to forrl non-toxic nitrogen gas
(N2). In an SNCR system, basic requi-ements for the reduction
of NO by an SNCR chemical include a well-mixed minimal 1:1
ratio of NH2 radical to NO, whereby th~ NH2 radical is generated
by the SNCR chemical and the NO is generated from the gas
generant combustion. Furthermore, tlhe NH2 radical must react
for a sufficient residence time at ~ temperature within the
range of 850-1150~C. The reduced content of toxic gases, such
as NO~ and CO, allows the use of nonazide gas generants in
vehicle occupant restraint systems while protecting the
occupants of the vehicle from exposure to toxic gases which
heretofore have been produced by nonazide gas generants.
More specifically, the present invention comprises a
nonazide gas generant composition, ard a separate NO~ reducing
agent (SNCR) chemical that liberates NH2 radical upon thermal
decomposition and/or reaction with ~2 The NO" gases generated
from the combustion of the gas genelant, such as NO and NO2,
selectively react with the NH2 radicaLs, or NH3 and ~2 ~ thereby
producing a harmless gas of N2. A corresponding reduction in
CO is an incidental benefit with the use of some of the
reducing agents, such as (NH4)2S04. Il addition, the chemistry
of the SNCR chemical is noninvasive and will not interfere with
the expected performance or stabi.ity of a gas generant
combustion.
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DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT~S)
In accordance with the present invention, a vehicle
occupant restraint device utilizing an SNCR system comprises a
gas generant and a de-NO~ agent. The de-NO~ agent is disposed
around the periphery of the gas generant within the gas
generant bed and is selected from a group including amides and
imides, ammonium compounds, amine compounds, or any compound
which produces an NH2 radical in the gas phase. Examples of
ammonium compounds include ammonium hydroxide (NH40H), ammonium
carbonate ((NH4)2C03), ammonium sulfate ((NH4)2SO4), ammonium
chloride (NH4Cl), ammonium carbamate (H2NC02NH4), and ammonium
fluoride (NH4F). Examples of amide and imide compounds,
respectively, compounds are urea (H2NCONH2) and cyanuric acid
((HNCO)3). Given the aforementioned benefits, the gas
lS generant is preferably nonazide, although other gas generants
such as an azide-based composition may be utilized in
conjunction with SNCR. The SNCR chemical is preferably
ammonium sulfate ((NH4)2S04) based on the optimum and unexpected
results given in Example 3 below. Not only does (NH4)2SO4
inhibit production of toxic NO2, it actually reduces NO2 over
time. In general, ammonium compounds will generate the highest
yield of NH2 radicals.
SNCR is well known and commonly used in industrial
boilers to decrease the levels of toxic nitrogen oxides. Until
now, SNCR technology has not been successfully implemented in
automotive airbag systems. NO is reduced to N2 by the
following gas phase reaction with an NH2 radical:
NH2 + NO ~ N2 + H20 (l)
Because NO2 is generated by NO, a reduction in NO necessarily
causes an overall NOX reduction within the inflator gas. The
critical parameters for the successful implementation of SNCR
in any system are the reaction temperature, NH2 radical/NO
ratio, mixing, residence time, and initial NO level. In
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addition, the presence of oxygen (~2) is critical when the SNCR
chemical is ammonia or an ammonium c~mpounds.
To obtain NH2 radical in th~ ! gas phase at the correct
level, the SNCR chemical must thermaLly decompose to generate
the NH2 radical or NH3 (which must sub;equently react with ~2 to
form the NH2 radical). The decompocition products determine
how much of the NH2 radical is generated in the gas phase
versus what is liberated directly frcm the SNCR chemical. The
minimum NH2 radical/N0 ratio in the ~las phase reaction should
be 1 mole of NH2 radical for each mcle of N0. In general, a
small excess of the NH2 radical will simply result in the
formation of small amounts of N~3 and provide minimal
additional N0 reduction. SNCR technclogy is most effective at
high initial levels of N0. When amm~nium compounds are used,
oxygen is necessary for the formation of NH2 radicals, and
should be present at levels of 0.1 t~ 11 volume percent.
The decomposition temperature, determinative of when
NH2 radicals are generated in the gas phase, is critical
because the NH2 radical must be "injl-cted" into the gas phase
at the correct temperature thereby enabling the selective
reduction reaction of NO~. For exampLe, (NH4)2SO4 decomposes at
about 235~C while (NH4)2C03 begins to decompose at room
temperature. During an inflator deployment, an SNCR chemical
that decomposes at a lower temperatu~e will be "injected" into
the system sooner and, as illustrate~ in Example 4, provide a
decreased reduction of nitrogen oxides. The importance of
temperature is demonstrated by the f~llowing reactions:
NO + NH3 + 1/4~2 - N2 + 3/2H20 (2)
NH3 + 5/402 - NO + 3/2H20 (3)
The desired reaction, (2 , will only occur at a
significant rate at temperatures abo~e 850-950~C. However, at
temperatures above 1050-1150~C, reaction (3) becomes dominant
and undesirable N0 is formed. In adcition to temperature, the
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importance of good mixing and a sufficient residence time are
obvious for the completion of any gas phase reaction. The gas
temperatures, degree of mixing, and residence time for a given
inflator are determined primarily by the gas generant
properties and the inflator configuration and operating
conditions.
The temperature of the gases in an inflator will
generally vary from the hottest at the generant burning surface
to the coolest at the inflator exit ports. Although
temperature is extremely difficult to measure, variables such
as the thermodynamic properties of the generant, the burning
rate of the generant, the cooling devices within the inflator,
- and the operating pressure of the inflator each contribute to
the overall operating temperature of the SNCR system. The
residence time of the gases in an inflator is dependent on the
presence of choked flow and the operating pressure. One
skilled in the art will readily realize that cognizance and
tailoring of these variables when choosing a gas generant will
enable the use of a wide variety of gas generant compositions
in conjunction with the SNCR system.
In accordance with the present invention, the SNCR
chemical is a noninvasive composition whereby the normal
combustion reaction of the gas generant is not interrupted or
significantly altered. The present invention is illustrated by
the following examples.
EXAMPLE 1
Two nonazide passenger inflators (NAPIs) with the
same gas generant and hardware were built. Ammonium carbonate
((NH4)2CO3) was added directly to the generant bed of one of the
inflators as a powder at 1.4 wt% of the generant mass. The
inflators were deployed in a 100 ft3 tank and the gaseous
effluents were measured over a 30 minute time period. Carbon
monoxide (CO) and ammonia (NH3) were measured by FTI~ while
nitrogen (II) oxide (NO), nitrogen lIV) oxide (NO2), and total
nitrogen oxides (NOX) were measured by Chemiluminescence. The
time weighted averages are reported below in ppm.
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Inflator C0 NO N~2 NOx NH3
Control 66585.7 29.6 117.6 14
1.4% (NH4) 2C~3 70552.8 0.9 53.6 96
Percent of Control 106%62% 3% 46% 686%
This example illustrates that the addition of this SNCR
ammonium salt significantly reduces the levels of toxic
nitrogen oxides while leaving the C0 essentially unchanged.
EXANPLE 2
s Two NAPIs with the same gls generant and hardware
were built and tested as described in Example 1. However, the
generant load and the cooling assembly differed from that used
in Example 1. ((NH4)2C03) was added directly to the generant
bed of one of the inflators as a powder at 2.6 wt% of the
generant mass. The time weighted averages are reported below
in ppm.
Inflator C0 NO N~2 NO~ NH3
Control 822106.1 50.5 162 16
2.6~(NH4)2C03 79882.0 30.7 116 147
Percent of Control 97% 77% 61% 72% 919%
This example demonstrates the imp~)rtance of choosing the
correct inflator configuration for su~:cessful implementation of
SNCR technology in an airbag inflator. In addition, this
example shows that an excess of an SNCR chemical does not
result in further N0X reduction, but only in higher levels of
NH3 production.
EXANPLE 3
Two NAPIs with the same gas generant and hardware
were built and tested as described ir Example 1. However, the
generant load and the cooling assembly differed from that of
Examples 1 and 2. (NH4) 2S04 was added directly to the generant
bed of one of the inflators as a pl~wder at 1.2 wt % of the
generant mass. The time weighted avl~rages are reported
below in ppm.
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Inflator CO N0 No2 N0x NH3
Control 43759.6 12.5 73.3 8
1.2% ~NH4)2S04 406 62.2 5.2 67.7 57
Percent of Control93% 104% 42% 92% 712
Two quite unexpected, yet beneficial results were
observed from these tests. First, the addition of ((NH4)2S04)
resulted in a reduction of both N0x and C0. Secondly, a
comparison of the N02 evolution in the control and in the SNCR
samples indicates a decline over time of the N02 species in the
SNCR sample and an increase in the N02 species in the control
sample. For the control inflator, the N02 was 9.4 ppm at 3
minutes and 16.4 ppm at 30 minutes. This is what is normally
seen since the N0 initially produced by the inflator slowly
converts to N02 in the presence ~f ~2- For the inflator with
the SNCR chemical, the N02 was 7.8 ppm at 3 minutes and
steadily decreased to 5.0 ppm at 30 minutes. This example
illustrates the effectiveness of this embodiment in retarding
the generation of toxic N02, despite the presence of increased
amounts of relatively nontoxic N0 and ~2
EXAMPLE 4
Four NAPIs with the same gas generant and hardware
were built and tested as described in Example 1. However, the
generant load and the cooling assembly differed from that used
in Examples 1,2, or 3. (NH4)2S04 (decomposes at 235DC) and
H2NC02NH4 (sublimes at 60~C) were each added directly to the
generant bed of one of the inflators as a powder at 2.7 wt % of
the generant mass. The time weighted averages are reported
below in ppm.
Inflator C0 N0 No2 N0x NH3
Control 552 82.2 30.2 115.2 ~0
2.7% (NH4)2S04 453 81.5 6.2 66.2 105
2.7% H2NC02NH4 715 79 31 112.9 196
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Again, the addition of (NH4)2S04 resulted in a reduction of NOx
and CO. Also, the NO2 level went fron 9.4 ppm at 3 minutes to
5.6 ppm at 30 minutes, verifying the data shown in Example 3.
The decomposition and sublimation ?oints of the different
compounds are provided to demonstrate that the decomposition
temperature must be considered as ~ critical factor to the
success of the SNCR chemical.
While the preferred embodirlent of the invention has
been disclosed, it should be appreciated that the invention is
susceptible of modification without d~parting from the scope of
the following claims.
