Note: Descriptions are shown in the official language in which they were submitted.
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PATENT
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GAS GENERANT COMPOSITIONS WITH ALKALI OXIDE SCAVENGERS
The present invention is directed to gas generant
compositions suitable for automotive air bag restraint systems,
particularly restraint systems in which the gas generant is
encased in aluminum housing and/or generates gases which come
into contact with aluminum components, such as filters.
Background of the Invention:
Most automotive air bag restraint systems, presently in use,
use gas generant compositions in which sodium azide is the
principal fuel. Because of disadvantages with sodium azide,
particularly instability in the presence of metallic impurities
and toxicity, which presents a disposal problem for unfired gas
generators, there is a desire to develop non-azide gas generant
systems and a number of non-azide formulations have been
proposed. However, to date, non-azide gas generants have not
made significant commercial inroads.
U.S. Patent No. 5,015,309 to Wardle et al., the teachings of
which are incorporated herein by reference, teaches gas generant
compositions in which fuels are salts of 5-nitrobarbituric acid,
salts of 5-nitroorotic acid and nitrouracil (which doesn't form a
salt), oxidizers include alkali (except sodium) and alkaline
earth metal salts of nitrates, nitrites and hexanitrocolbatate,
and binders, when needed for pelletization.
This patent teaches that minimal amounts of non-gas generating
components should be added, except up to 1 wt% fibers, such as
graphite or iron fibers, which increase the burning rate and
transfer heat during combustion.
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The 5,015,309 patent teaches that the compositions are
useful for inflators as taught in U.S. Patent No. 4,547,342 to
Adams et al., the teachings of which are incorporated herein by
reference. The inflators taught in U.S. Patent No. 4,547,342
have aluminum housings and other aluminum components. A
disadvantage with compositions such as taught in the 5,015,309
patent is that combustion products include K2O which is highly
corrosive to aluminum. Improved formulations are needed to
reduce degradation of aluminum components during combustion of
the gas generant composition. Also, it is highly desirable that
the combustion products include a slag which is filterable. The
compositions taught in the 5,015,309 patent are not filterable.
Summary of the Invention:
A gas generant composition using as the fuel component a
compound selected from 5-nitrobarbituric acid, 5-nitroorotic
acid, alkali and alkaline earth metal salts thereof; an oxidizer
for the fuel selected from ammonium, alkali metal and alkaline
earth metal salts of chlorates, perchlorates and nitrates and
transition metal oxides; and a binder, also contains alumina
(Al2O3) and/or silica (SiO2) as a scavenger of alkali metal
oxides.
Detailed Description of Certain Preferred Embodiments:
The fuel comprises between about 20 and about 55 wt% of the
gas generant composition. Preferred fuels are the potassium
salts of 5-nitrobarbituric acid (also called dilituric acid) and
5-nitroorotic acid.
The oxidizer, which is used at a level of between about 50
and about 75 wt% is selected from ammonium, alkali metal and
alkaline earth metal chlorates, perchlorates, nitrates,
transition metal oxides, and mixture thereof.
Optionally, a portion of the oxidizer may be a transition
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metal oxide, such as cupric oxide. Such a transition metal oxide
preferably comprises between about 5 to about 35 wt~ of the
composition. In addition to their oxidizing function, these
oxides provide hard particles, facilitating compaction of the
composition into pellets or other consolidated solid shapes.
As is taught in U.S. Patent No. 5,139,588, the teachings of
which are incorporated herein by reference,
the cations of the fuel salts and oxidizers are preferably
mixture of alkali metal cations, i.e., lithium, sodium and
potassium, and alkaline earth metal cations, i.e., magnesium,
strontium, barium and cerium. Upon combustion, the alkali
cations form liquid combustion products and the alkaline earth
metal cations form solid combustion products, the mixture of
liquid and solid combustion products forming clinkers which can
be readily removed from the gas stream by filtration. The ratio
of solid to liquid combustion products may be adjusted by the
ratio of alkaline earth metal cations to alkali metal cations.
Of alkali metal cations, sodium is preferred over potassium as
sodium oxide is more readily scavenged by alumina or silica than
potassium oxide.
In accordance with the present invention, alumina, silica or
mixtures thereof are added to scavenge corrosive alkali metal
oxides, such as sodium oxide and potassium oxide. Accordingly,
the composition of the present invention contains alumina and/or
silica at a level of between about 0.5 and about 10 wt%,
preferably between about 1 and about 8 wt%. The alumina and/or
silica may be in the form of particulates or as fibers, such as
fibers of various silica/alumina content. Alumina is generally
preferred over silica, being a more efficient scavenger.
A binder is added at a level of between about 1 and about 5
wt%. Suitable binder materials include but are not limited to
molybdenum disulfide, graphite, polytetrafluroethylene, VitonR (a
copolymer of vinylidene fluoride and hexafluoropropylene),
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nitrocellulose, polysaccharides, polyvinylpyrrolidones,
polycarbonates, sodium silicate, calcium stearate, magnesium
stearate and mixtures thereof. Preferred binder materials are
molybdenum disulfite and polycarbonates.
Alkali metal and alkaline earth metal carbonates and/or
oxalates may optionally be added up to about 40 wt%. These act
as coolants, lowering the combustion temperature. Lower
combustion temperatures minimize production of toxic gases, such
as CO and NOx. Generally, if used, these coolants are used at a
lo level of at least about 1 wt%.
As noted above, the alumina and/or silica may be in the form
of fibers. Fibers help to mechanically reinforce the
consolidated unburned material and subsequently consolidate slag
material formed by burning the composition. Graphite fibers,
e.g., at between about 1 and about 10 wt%, may be also be used,
either as the sole fibrous material or in conjunction with
alumina-containing fibers to perform this reinforcing function.
The invention will now be described in greater detail by way
of specific example.
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Examples 1-8
Gas generant formulations according to the invention and
their combustion properties are shown in the table below.
Component levels are in weight percent.
1 2 3 4
KDL* 42.41 41.11
KNOA** 39.59 38.54
Sr(N03) 2 35.84 34.89
PPC (emulsion)*** 5.00 5.00
KC104 13.57 13.57
A1203 2.00 5.00 6.00 8.00
NH4C104 23.62 22.90
CuO 31.97 30.99
100% 100% 100% 100%
~lOOOpsi 0.696 0.627 0.407 0.398
(in/sec)
Combustion Temp. (K) 2000 1954 1772 1756
Moles of gas per lOOg 1.854 1.789 1.615 1.565
of generant
*KDL = potassium dilituric acid (5-nitro barbituric acid)
**KNOA = potassium nitroorotic acid
***PCC = poly propylene carbonate
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6 7 8
KDL* 34 3933 09
KNOA** 40.54 39.46
NH4C104 19.1318.41 18.52 18.03
S KC104 13.5713.57 29.94 29.51
CuO 25.9124.93
PPC (emulsion)*** 5.00 5.00 5.00 5.00
A1203 2.00 5.00 6.00 8.00
100% 100% 100% 100%
~lOOOpsi 0.4720.315 0.674 0.577
(in/sec)
Combustion Temp. (K) 2013 2000 2000 2001
Moles of gas per lOOg 2.001 1.940 2.343 2.083
of generant
*KDL = potassium dilituric acid (5-nitro barbituric acid)
**KNOA = potassium nitroorotic acid
***PCC = poly propylene carbonate
