[0001] The present invention relates to gas generant or propellant compositions which when
formed into cylindrical pellets, wafers or other appropriate physical shapes may be
combusted in a suitable gas generating device to generate cool nitrogen gas and easily
filterable condensed phase products. The resultant gas is then preferably used to
inflate an air bag which serves as an automobile occupant cushion during a collision.
More particularly this invention relates to azide-based gas generant compositions
including special additives, and additive amounts, to control the linear burning rate
of any such shapes produced therefrom and to control the viscosity or melting point
of the slag or clinker produced.
[0002] Even though the gas generant compositions of this invention are especially designed
and suited for creating nitrogen for inflating passive restraint vehicle crash bags,
it is to be understood that such compositions would function equally well in other
less severe inflation applications, e.g. aircraft slides, inflatable boats, and inflatable
lifesaving buoy devices as in U.S. Pat. No. 4,094,028, and would in a more general
sense find utility any place where a low temperature, non-toxic source of nitrogen
gas is needed.
[0003] Automobile air bag systems have been developed to protect the occupant of a vehicle,
in the event of a collision, by rapidly inflating a cushion or bag between the vehicle
occupant and the interior of the vehicle interior.
[0004] The use of protective gas-inflated bags to cushion vehicle occupants in crash situations
is now widely known and well documented. In early systems of this type, a quantity
of compressed, stored gas was employed to inflate a crash bag which, when inflated,
was positioned between the occupant and the windshield, steering wheel and dashboard
of the vehicle. The compressed gas was released by the action of actuators or sensors
which sensed a rapid change in velocity of the vehicle during a rapid impact, as would
normally occur during an accident. Because of the bulk and weight of such stored,
compressed gas systems, their generally slow reaction time and attendant maintenance
difficulties, these type systems are now largely obsolete, having been superseded
by air bag systems utilizing a gas generated by chemical gas-generating compositions.
These advanced systems involve the use of an ignitable propellant composition for
inflating the air cushion, wherein the inflating gas in generated by the exothermic
reaction or the reactants which form the propellant.
[0005] The most common air bag systems presently in use include an on-board collision sensor,
an inflator, and a collapsed, inflatable bag connected to the gas outlet of the inflator.
The inflator typically has a metal housing which contains an electrically initiated
igniter, a gas generant composition, for example, in pellet or tablet form, and a
gas filtering system. Before it is deployed, the collapsed bag is stored behind a
protective cover in the steering wheel (for a driver protection system) or in the
instrument panel (for a passenger system) of the vehicle. When the sensor determines
that the vehicle is involved in a collision, it sends an electrical signal to the
igniter, which ignites the gas generant composition. Then the gas generant composition
burns, generating a large volume of relatively cool gaseous combustion products in
a very short time. The combustion products are contained and directed through the
filtering system and into the bag by the inflator housing. The filtering system retains
all solid and liquid combustion products within the inflator and cools the generated
gas to a temperature tolerable to the vehicle passenger. The bag breaks out of its
protective cover and inflates when filled with the filtered combustion products emerging
from the gas outlet of the inflator. See, for example, U.S. Pat. Nos. 3,904,221 and
4,296,084.
[0006] The requirements of a gas generant suitable for use in an automobile air bag device
are very demanding. The gas generant must have a burning rate such that the air bags
are inflated rapidly (within approximately 30 to 100 milliseconds). The burning rate
must not vary with aging or as a result of shock and vibration during normal deployment.
The burning rate must also be relatively insensitive to changes in moisture content
and temperature. When pressed into pellets or other solid form, the hardness and mechanical
strength of the pellets must be adequate to withstand the mechanical environment to
which it may be exposed without any fragmentation or change of exposed surface area.
Any breakage of the pellets would potentially lead to an undesirable high pressure
condition within the generator device and possible explosion.
[0007] The gas generant must efficiently produce cool, non-toxic, non-corrosive gas which
is easily filtered to remove solid or liquid products, and thus preclude damage to
the inflatable bag(s) or to the occupant(s) of the automobile.
[0008] The requirements as discussed in the preceding paragraphs limit the applicability
of many otherwise suitable compositions from being used as air bag gas generants.
[0009] Mixtures of sodium azide and iron oxide are favored because a low reaction temperature
(approximately 1000 degrees centigrade) is produced, the reaction products are solids
or liquids which are easily filtered within a gas generator device, and the mixtures
produce a high volume of non-toxic gas. Without the use of other oxidizers and additives,
however, the burning rates are typically very low. Iron oxide is also a very hard
substance which causes machinery to wear with prolonged use, and can impart a hygroscopic
nature to the formulations if very fine ferric oxide is used. Some severe aging problems
have also been experienced particularly when certain additives have been used in conjunction
with sodium azide and ferric oxide. U.S. Pat. No. 4,203,787 discloses that ferric
oxide based gas generants with azide fuels have been less preferred than other oxidizers
because they burn unstably and slowly, and are difficult to compact into tablets.
[0010] The problems associated with the low burning rate of sodium azide and ferric oxide
compositions have largely been overcome by the use of co-oxidizers such as an alkali
metal nitrate or perchlorate (see, for example, U.S. Pat. Nos. 4,203,787; 4,547,235;
4,696,705; 4,698,107; 4,806,180 and 4,836,255. The inclusion of co-oxidizers has,
however, in addition to causing an increase in the burning rate of the compositions,
resulted in an increase in the flame temperature, with some consequent loss in the
ability to form good solid product clinkers.
[0011] The hygroscopic nature of the sodium azide and ferric oxide formulations has been
shown to be reduced by the addition of hydrophobic fumed silica (see aforementioned
U.S. Pat. No. 4,836,255). The use of the hydrophobic fumed silica reduces the moisture
sensitivity of sodium azide and ferric oxide compositions and also interacts with
the solid or liquid products to improve clinkering by the formation of alkali metal
silicates which have a higher melting point than the alkali metal oxide products.
The silicates also likely serve to increase the viscosity of the liquid products making
them easier to filter in a gas generator device.
[0012] The use of silicate additives for the purpose of improved clinkering and burning
rate control in compositions containing sodium azide, ferric oxide, and potassium
nitrate is described in aforementioned U.S. Pat. No. 4,547,235. While clinkering is
improved, the large amounts of silica used were actually effective in reducing the
burning rate of the formulations when the silica levels were increased at the expense
of the potassium nitrate.
[0013] Aforementioned U.S. Pat. Nos. 4,696,705; 4,698,107 and 4,806,180 describe formulations
comprised of sodium azide, ferric oxide, sodium nitrate, silica, bentonite (a mineral),
and graphite fibers. These patents disclose the burning rate enhancement qualities
of the graphite fibers, but does not expressly state the purpose and function of the
bentonite and fumed silica additives. The patents also imply an equivalence of the
fumed metal oxides (alumina, silica, and titania). Within these patent disclosures
bentonite is not considered to be equivalent to the fumed metal oxides.
[0014] Also of interest is the teachings regarding the use of various combustion catalyts
and/or slag/residue control and similar agents in azide-based propellants in general
found in U.S. Pat. Nos. 2,981,616; 3,883,373; 3,947,300; 4,376,002; 4,604,151; 4,834,818
and 4,981,536.
[0015] U.S. Pat. No. 4,533,416 is also of general interest in the Example 6 teaching of
adding 2% bentonite to a NaN₃-Fe₂O₃ based propellant, presumably for its binding properties
which proved ineffectual.
[0016] Throughout this specification all percentages of compositional ingredients are by
weight based on total composition weight unless otherwise indicated.
[0017] In accordance with the present invention there is provided an azide-iron oxide-metal
nitrate based generant composition which is made to burn at a controlled linear burn
rate of about 0.8 to 1.5 inches per second while providing excellent slag melting
point or viscosity control by the addition of optimum amounts of one or more of the
metal oxide additives: silica, alumina, titania and bentonite. Preferably a combination
of the oxide additives is provided. A small amount of molybdenum disulfide may also
be incorporated. The presence of fibrous mechanical additives, such as graphite fibers,
is excluded from the generant mixture or matrix.
[0018] The generant composition according to the invention contains from about 65-74% azide
fuel, preferably sodium azide; from about 17-29.5%, preferably about 17-25%, iron
oxide, preferably ferric oxide; from about 1.0-6%, preferably from about 2,5-6%, metal
nitrite or nitrate co-oxidizer, preferably sodium nitrate; a metal oxide comprising
about 0.5-8%, preferably 2.34-8% silica, alumina, titania or mixtures thereof, preferably
a combination of silica and alumina; together with up to about 6% bentonite and up
to about 4% molybdenum disulfide. One preferred additive mixture comprises about 2.34-5%
silica plus alumina, most preferably about 0.34% silica plus 2% alumina, together
with about 3-6%, preferably about 3%, bentonite for driver side air bag application.
Another preferred additive mixture comprises about 5-8% silica plus alumina, most
preferably 0.34% silica plus about 5% alumina, together with less than about 3%, preferably
0%, bentonite for passenger side application. The preferred amount of molybdenum disulfide
present in either application is about 1.0-1.75%.
- Fig. 1
- illustrates in graph form the effect on the burning rate of a stoichiometric propellant
formulation of sodium azide, ferric oxide and sodium nitrate (5%) of various additive
metal oxides.
- Fig. 2
- illustrates in graph form the effect on the slag melting point of the same stoichiometric
formulation shown in Fig. 1 of various additive metal oxides.
[0019] The gas generant according to the invention broadly includes the following ingredients:
(1) an azide, which is one or more alkali or alkaline earth metal azides, preferably
one or more alkali metal azides, most preferably sodium azide,
(2) iron oxide, which is one or more of the three iron oxides (FeO, Fe₂O₃ and Fe₃O₄),
preferably ferric oxide (alpha or gamma),
(3) a metal nitrite or nitrate, which is one or more alkali metal nitrites or nitrates,
preferably sodium nitrate,
(4) special additives selected from the group consisting of silica, alumina, titania,
bentonite and mixtures, thereof, and
(5) may include molybdenum disulfide.
[0020] The azide is the gas generant fuel which liberates nitrogen gas when oxidized by
the oxidizers. The iron oxide functions as an oxidizer. The iron oxide may be replaced
in whole or in part by one or more of the oxides of chromium, manganese, cobalt, copper
and vanadium. The metal nitrite or nitrate is a co-oxidizer which provides additional
heat to the azide and iron oxide formulation which in turn increases the linear burning
rate of the composition and also provides good low temperature ignition characteristics.
The silica additive provides increased linear burning rate control and, to a limited
degree, higher slag melting point or viscosity control, forming silicates as products.
The alumina additive primarily provides for increased slag melting point or viscosity
control and secondarily provides for increased linear burning rate control by the
formation of aluminates as products. The titania provides for higher linear burning
rate control, forming titanates as products, but does not increase the melting point
or viscosity of the slag. The metal oxide additives silica, alumina and titania may
or may not be fumed. The bentonite additive is a montmorillonite mineral which is
hydrous aluminum silicate of the approximate formula: Al, Fe
1.67, Mg
0.33) Si₄O₁₀(OH)₂ (Na,Ca
0.33). Bentonite provides for increased burning rate control, particularly when used at
low levels, presumably by the formation of silicates and aluminates as products. The
molybdenum disulfide functions as a binder and pressing aid for machine pressing (molding)
operations, and also has a limited effect on the composition burning rate, presumably
by making it opaque.
[0021] When considered as a group the metal oxides (silica, bentonite, titania, alumina
as well as excess iron oxide) all produce increased burning rates relative to a stochiometric
formulation comprised of sodium azide, ferric oxide and sodium nitrate as, for example,
shown in Fig. 1. Burning rate enhancement is shown to be greatest with silica, bentonite
and titania, and least for the excess ferric oxide. The effect of alumina is intermediate
between the above two groups. The burning rate enhancement is a maximum when the level
of the metal oxides is approximately 6% by weight. Fig. 1 illustrates that the burn
rate of the compositions are tailorable within the range of approximately 0.8-1.5
inches per second. Intermediate burning rates are also obtained with additive mixtures.
For example, using a composition including bentonite at a level of 3% and alumina
at 2% produces a burning rate intermediate between either ingredient at the 5% level.
The formulations of Fig. 1 all contain sodium nitrate at the 5% level.
[0022] The effect of the metal oxide levels on the slag melting point is shown in Fig. 2
for bentonite, alumina, and ferric oxide. (These are the same basic NaN₃-Fe₂O₃-NaNO₃
formulations for which the burning rate effects are shown in Fig. 1). Examination
of Fig. 2 reveals that alumina is more effective than than either bentonite or iron
oxide (excess) in the promotion of high slag melting points. The melting points of
comparable formulations containing silica show it to have about the same effect as
bentonite.
[0023] The preceding examples serve to illustrate that the metal oxides (SiO₂, Al₂O₃, TiO₂,
and bentonite) are not fully equivalent in their effects on both the burning rate
and slag melting points of a gas generant composition comprised of sodium azide, ferric
oxide, and sodium nitrate. The technology of using combinations of the metal oxides
(silica, bentonite, alumina, and titania) in sodium azide, ferric oxide and sodium
nitrate gas generant compositions is especially shown to meet the balanced formulation
objectives of producing high burning rate and high slag melting point (which allows
excellent clinkering and easy particulate filtering by the gas generator device).
[0024] In general the nitrogen gas generant composition according to the invention consists
essentially of the above named ingredients in the amounts shown as follows:
TABLE 1
INGREDIENT |
AMOUNT (%) |
azide fuel |
about 65-74 |
iron oxide |
about 17-29.5 |
nitrite/nitrate co-oxidizer |
about 1.0-6.0 |
metal oxide (silica, alumina, titania or mixtures) |
about 0.5-8.0 |
bentonite |
up to about 6.0 |
molybdenum disulfide |
up to about 4.0 |
[0025] A preferred general composition of the gas generant under the above genus consists
essentially as follows:
TABLE 2
INGREDIENT |
AMOUNT (%) |
sodium azide |
about 65-74 |
ferric oxide |
about 17-29.5 |
sodium nitrate |
about 1.0-6.0 |
metal oxide (silica, alumina, titania or mixtures) |
about 0.5-8.0 |
bentonite |
up to about 6.0 |
molybdenum disulfide |
up to about 4.0 |
[0026] Preferred sub-generic compositions under the Table 2 genus have been developed depending
on whether used for driver side or passenger side air bag applications. A composition
with a slightly higher burning rate, preferred for the driver side, is generally represented
as follows:
TABLE 3
INGREDIENT |
AMOUNT (%) |
sodium azide |
about 65-74 |
ferric oxide |
about 17-25 |
sodium nitrate |
about 2.5-6.0 |
metal oxide (silica, alumina, titania or mixtures) |
about 2.5-5.0 |
bentonite |
about 3.0-6.0 |
molybdenum disulfide |
about 0-4.0 |
[0027] Two specific compositons under the Table 3 genus preferred for the driver side are
as follows:
TABLE 4
INGREDIENT |
AMOUNT (%) |
sodium azide |
about 68.12 |
ferric oxide |
about 20.54 |
sodium nitrate |
about 5.0 |
*silica |
about 0.34 |
alumina |
about 2.0 |
bentonite |
about 3.0 |
molybdenum disulfide |
about 1.0 |
*flowing agent for the azide. |
[0028]
TABLE 5
INGREDIENT |
AMOUNT (%) |
sodium azide |
about 66.57 |
ferric oxide |
about 23.85 |
sodium nitrate |
about 2.5 |
*silica |
about 0.33 |
alumina |
about 5.00 |
molybdenum disulfide |
about 1.75 |
*flowing agent for the azide. |
[0029] A composition with a slightly lower burning rate and even better slag producing qualities,
preferred for the passenger side, is generally represented as follows:
TABLE 6
INGREDIENT |
AMOUNT (%) |
sodium azide |
about 65-74 |
ferric oxide |
about 17-25 |
sodium nitrate |
about 2.5-6.0 |
metal oxide (silica, alumina titania or mixtures) |
about 5.0-8.0 |
bentonite |
up to about 3.0 |
molybdenum disulfide |
up to about 4.0 |
[0030] A specific composition under the Table 6 genus preferred for the passenger side is
as follows:
TABLE 7
INGREDIENT |
AMOUNT (%) |
sodium azide |
about 66.81 |
ferric oxide |
about 23.35 |
sodium nitrate |
about 3.5 |
*silica |
about 0.34 |
alumina |
about 5.0 |
molybdenum disulfide |
about 1.0 |
*flowing agent for the azide. |
[0031] Another specific composition under the Table 3 genus for either the driver or passenger
side is as follows:
TABLE 8
INGREDIENT |
AMOUNT (%) |
sodium azide |
about 68.35 |
ferric oxide |
about 24.56 |
sodium nitrate |
about 2.5 |
*silica |
about 0.34 |
alumina |
about 2.5 |
molybdenum disulfide |
about 1.75 |
*flowing agent for the azide. |
[0032] As can be seen from the above disclosure the compositions of the invention have been
tailored for the express purpose of maximizing the burning rate and the viscosity
or melting point of the solid combustion products to provide a rapidly functioning
device with easily filterable products. In contrast to the formulations making up
the grain in aforementioned U.S. Pat. Nos. 4,696,705; 4,698,107 and 4,806,180, the
use of graphite fibers would not only be undesirable, but deleterious in the compositions
of this invention because the inclusion of such fibers within the formulation would
not increase the burning rate and would not increase the mechanical strength of the
consolidated material (i.e. when pressed into cylindrical pellets, wafers or other
physical forms). Moreover, such a mixture would not be amenable to a wide variety
of manufacturing methods such as spray drying to form prills or pellets of the materials
suitable for machine pressing into wafers or grains, and would further reduce the
gas yield of the composition.
[0033] The compositions of the present invention have been designed to provide high performance
(high burning rate and high gas output) relative to those of the above patents, and
these performance gains relative to the compositions of the patents are achieved by
avoiding the use of such graphite fibers and, in general, the inclusion of higher
levels of metal oxide additives. In accordance with the present invention it has been
shown that the metal oxides (silica and titania) and bentonite promote high burning
rate while alumina is most effective in producing combustion products of a higher
melting point producing easily filterable products.
[0034] In the compositions of this invention the addition of graphite fibers would not be
effective in enhancing the burning rate because the thermal conductivity of the fibers
would be slow compared to the burning rate and hence in-depth heating of the propellant
grains would not be achieved to any substantial degree. The mechanical effect of the
fibers to increase the burning rate would also be diminished by the fact that the
fiber orientation cannot be controlled and therefore higher levels of the randomly
distributed fibers would be required to achieve the same burning rate as could be
achieved with total fiber orientation parallel to the direction of burn. The addition
of the graphite fibers represents the addition of an inert ingredient which must be
used in large quantities to achieve the same overall effects of reduced quantities
of metal oxide ingredients. The increased burning rate and gas output of the compositions
of this invention allow simple grain configurations to be used within the gas generator,
such as cylindrical pellets or wafers rather than complex multiperforated grains,
and allows the use of smaller quantities of compositions within the inflator devices
due to the increased gas output of the compositions.
[0035] Similarly other known fibrous mechanical additives, such as glass fibers, and especially
those which have a fairly large thermal conductivity, such as iron, copper and nickel
fibers, are equally undesirable and deleterious in regard to the subject invention
and are avoided.
1. A composition for generating nitrogen gas consisting essentially of (all percentages
by weight):
A. between 65 and 74 percent of an aside fuel,
B. between 17 and 29.5 percent of an oxidizer which is a transition metal oxide selected
from iron oxide, chromium oxide, manganese oxide, cobalt oxide, copper oxide, vanadium
oxide and mixtures thereof,
C. at least 1.0 percent but less than 3.5 percent of a co-oxidizer selected from metal
nitrites, nitrates and mixtures thereof,
D. between 0.5 and 8.0 percent of a metal oxide additive selected from silica, alumina,
titania and mixtures thereof,
E. up to 6.0 percent bentonite, and
F. up to 4.0 percent molybdenum disulfide, said composition having a controllable
burning rate of from 0.8 to 1.5 inches (20 to 38 mm) per second.
2. A composition for generating nitrogen gas consisting essentially of (all percentages
by weight):
A. between 65 and 74 percent of an aside fuel,
B. between 17 and 29.5 percent of an oxidizer which is a transition metal oxide selected
from iron oxide, chromium oxide, manganese oxide, cobalt oxide, copper oxide, vanadium
oxide and mixtures thereof,
C. between 1.0 and 6.0 percent of a co-oxidizer selected from metal nitrites, nitrates
and mixtures thereof,
D. at least 0.5 percent but less than 2.5 percent of a metal oxide additive selected
from silica, alumina, titania and mixtures thereof,
E. up to 6.0 percent bentonite, and
F. up to 4.0 percent molybdenum disulfide, said composition having a controllable
burning rate of from 0.8 to 1.5 inches (20 to 38 mm) per second.
3. A composition for generating nitrogen gas consisting essentially of (all percentages
by weight):
A. between 65 and 74 percent of an azide fuel,
B. more than 25 and up to 29.5 percent of an oxidizer which is a transition metal
oxide selected from iron oxide, chromium oxide, manganese oxide, cobalt oxide, copper
oxide, vanadium oxide and mixtures thereof,
C. between 1.0 and 6.0 percent of a co-oxidizer selected from metal nitrites, nitrates
and mixtures thereof,
D. between 0.5 and 8.0 percent of a metal oxide additive selected from silica, alumina,
titania and mixtures thereof,
E. up to 6.0 percent bentonite, and
F. up to 4.0 percent molybdenum disulfide, said composition having a controllable
burning rate of from 0.8 to 1.5 inches (20 to 38 mm) per second.
4. A composition according to claim 3 comprising from 25.1 to 29.5 percent of iron oxide
as said oxidizer.
5. A composition according to any preceding claim which comprises between 2.5 and 6.0
percent of said metal nitrite or nitrate co-oxidizer and between 2.34 and 8.0 percent
of said metal oxide additive.
6. A composition according to any preceding claim wherein said fuel comprises at least
one alkali metal or alkaline earth metal azide.
7. A composition according to claim 6 wherein said fuel comprises sodium azide.
8. A composition according to any preceding claim wherein said co-oxidizer comprises
at least one alkali metal nitrate.
9. A composition according to claim 8 wherein said co-oxidizer is sodium nitrate.
10. A composition according to any preceding claim wherein said additive consists of a
mixture of silica and alumina.
11. A composition according to claim 10 wherein 2.34 to 5.0 percent of said additive is
present.
12. A composition according to claim 11 wherein 3.0 to 6.0 percent bentonite is present.
13. A composition according to claim 10 wherein 5.0 to 8.0 percent of said additive is
present.
14. A composition according to claim 13 wherein less than 3.0 percent bentonite is present.
15. A composition according to claim 12 or claim 13 wherein 1.0 to 1.75 percent molybdenum
disulfide is present.
16. A composition according to claim 2 or claim 3 consisting of (all percentages by weight):
A. about 68.12 percent sodium azide,
B. about 20.54 percent ferric oxide,
C. about 5.0 percent sodium nitrate,
D. about 0.34 percent silica,
E. about 2.0 percent alumina,
F. about 3.0 percent bentonite, and
G. about 1.0 percent molybdenum disulfide.
17. A composition according to claim 1 consisting of (all percentages by weight):
A. about 66.57 percent sodium azide,
B. about 23.85 percent ferric oxide,
C. about 2.50 percent sodium nitrate,
D. about 0.33 percent silica
E. about 5.00 alumina, and
F. about 1.75 molybdenum disulfide.
18. A composition according to claim 1 consisting of (all percentages by weight):
A. about 68.35 percent sodium azide,
B. about 24.56 percent ferric oxide,
C. about 2.50 percent sodium nitrate,
D. about 0.34 percent silica,
E. about 2.50 percent alumina, and
F. about 1.75 percent molybdenum disulfide.