Pyrotechnic method of generating a particulate-free, non-toxic odorless and colorless gas

An essentially particulate-free, non-toxic, odorless and colorless gas is generated in a pyrotechnic inflator device by using a eutectic solution of ammonium nitrate, guanidine nitrate and/or aminoguanidine nitrate, and minor amounts of polyvinyl alcohol and either potassium nitrate or potassium perchlorate. Ballistic modifiers such as triaminoguanidine nitrate (TAGN) or nitroguanidine (NQ) may be used as needed.

FIELD OF THE INVENTION 
The instant invention involves an improved method of using a eutectic 
solution of ammonium nitrate (AN), guanidine nitrate (GN) and/or 
aminoguanidine nitrate (AGN), and with minor amounts of polyvinyl alcohol 
(PVA) and either potassium nitrate (KN) or potassium perchlorate (KP) in a 
pyrotechnic inflator to generate an essentially particulate-free, 
non-toxic, odorless and colorless gas, for various purposes, such as 
inflating a vehicle occupant restraint, i.e., an air bag for an automotive 
vehicle. 
The propellants have also been found feasible for use in hybrid inflators 
for air bags. 
BACKGROUND OF THE INVENTION 
The present invention relates generally to solid composite propellant 
compositions and more particularly to solid composite propellant 
compositions useful as gas generators as described in related concurrently 
filed application Ser. No. 08/508,350. 
Recently, there has been a great demand for new gas generating propellants 
which are cool burning, non-corrosive and yield a high volume of gas and 
low particulates because attempts to improve existing gas generative 
compositions have been unsuccessful for various reasons. For example, 
while the addition of certain modifiers, such as metal carbonates, has 
lowered the flame temperature and yielded acceptable gas production, these 
same modifiers have contributed to the production of undesirable 
particulates. In turn, other modifiers utilized in the past, such as 
alkali metal chlorates, while not producing corrosive materials, have not 
succeeded in lowering the flame temperature significantly or increasing 
gas evolution, and also produce particulates. 
Gas generator compositions of interest here contain ammonium nitrate (AN) 
as the oxidizer and rubbery binders as the fuel. Ammonium nitrate is the 
most commonly used oxidizer since it yields no particulates and 
non-corrosive combustion products. Further, its use results in lower flame 
temperatures than do other oxidizers. Ammonium nitrate is cheap, readily 
available and safe to handle. 
The main objection to ammonium nitrate is that it undergoes certain phase 
changes during temperature variations, causing cracks and voids if any 
associated binder is not sufficiently strong and flexible enough to hold 
the composition together. Also, ammonium nitrate compositions are 
hygroscopic and difficult to ignite, particularly if small amounts of 
moisture have been absorbed. Since said compositions do not sustain 
combustion at low pressures, various combustion catalysts are added to 
promote ignition and low pressure combustion as well as to achieve smooth, 
stable burning. Gas generator compositions used for air bags should 
contain no metallic additives or even oxidizers such as ammonium 
perchlorate, because these give particulates and corrosive exhaust gases 
respectively. Commonly used additives, such as ammonium dichromate, copper 
chromite, etc., are also disadvantageous since they all produce toxic 
solids in the exhaust gases. 
THE PRIOR ART 
The art is replete with instances of compositions containing a 
guanidine-type compound with an oxidizer, such as ammonium nitrate. For 
example, in U.S. Pat. No. 3,031,347, guanidine nitrate and ammonium 
nitrate are listed together at column 2, as well as in Examples 3 and 5. 
However, compared with the present invention, not only does the guanidine 
compound lack an amino group, as in the aminoguanidine nitrate embodiment, 
but the composition disclosed in the patent is not a eutectic 
solution-forming mixture. Likewise, see U.S. Pat. No. 3,739,574, col. 2, 
in the table. On the other hand, U.S. Pat. No. 3,845,970, at column 3, 
discloses a list of solid compositions for generating gas in a shock 
absorption system. Among the components of the various compositions are 
ammonium nitrate and aminoguanidine nitrate. The two materials are not 
disclosed in admixture and, obviously, are not in a eutectic composition. 
Similarly, U.S. Pat. No. 3,954,528 discloses new solid composite gas 
generating compositions. Among the ingredients mentioned are ammonium 
nitrate and triaminoguanidine nitrate. See Examples 2 through 5. However, 
neither the specified components of the aminoguanidine nitrate 
compositions at hand nor any eutectic compositions, are disclosed therein. 
In U.S. Pat. No. 4,111,728, the inventor discloses ammonium nitrate with 
small amounts of guanidine nitrate. See column 2 and the table at columns 
3-4. However, the compositions do not include aminoguanidine nitrate and 
do not characterize any composition as forming a eutectic solution. 
U.S. Pat. No. 5,125,684 also discloses propellant compositions containing 
dry aminoguanidine nitrate and an oxidizer salt containing a nitrate 
anion. However, the disclosure is deficient with respect to the present 
invention since it fails to disclose the specific combination of 
components of this invention and does not mention eutectics. 
Finally, U.S. Pat. No. 5,336,439 concerns salt compositions and 
concentrates used in explosive emulsions. As disclosed at columns 37 and 
38, ammonium nitrate is one of the ingredients for forming the patentee's 
composition, while at column 20, line 51, aminoguanidine is indicated as 
also being an appropriate component. Nevertheless, like the other 
disclosures mentioned, the patent fails to disclose a specific composition 
including the same nitrates as are disclosed herein and clearly does not 
teach a eutectic composition containing said components.

SUMMARY OF THE INVENTION 
The invention herein involves eutectic mixtures of ammonium nitrate (AN), 
guanidine nitrate (GN) and/or aminoguanidine nitrate (AGN), 
triaminoguanidine nitrate (TAGN) or nitroguanidine (NQ) and small amounts 
of polyvinyl alcohol (PVA) and either potassium nitrate (KN) or potassium 
perchlorate (KP) as well as a method of generating an essentially 
particulate-free, non-toxic, odorless and colorless gas for various 
purposes, such as the inflation of an air bag in an automotive vehicle. In 
generating a particulate-free, non toxic, odorless and colorless gas, an 
enclosed chamber having exit ports is provided; a solid eutectic solution 
comprising AN, GN (and/or AGN or TAGN or NQ), KN, (and/or KP), and PVA is 
disposed as a propellant within said chamber; means are then provided for 
igniting said eutectic solution in response to a sudden deceleration being 
detected by a detection device in the chamber, whereby gas is instantly 
generated and conducted through the exit ports of the chamber through a 
diffuser to accomplish a desired function, such as inflating an automotive 
vehicle air bag. 
Eutectic mixture of AN, GN and/or AGN, or TAGN or NQ and minor amounts of 
KN and/or KP, and PVA have been found to eliminate pellet cracking and 
substantially reduce ammonium nitrate phase change due to temperature 
cycling. Although the addition of about 1 to about 2% potassium nitrate to 
an AN/GN eutectic totally eliminates the ammonium nitrate phase change, it 
is not sufficient to prevent cracking of the pressed pellet upon 
temperature cycling. At least about 5% by weight KN, or at least about 9% 
by weight KP, and at least about 3% by weight PVA are required. In 
addition, other chemicals, especially triamino-guanidine nitrate, are used 
in the propellant to aid ignition, give smooth burning, modify burning 
rates and give lower flame temperatures. 
DESCRIPTION OF THE PREFERRED EMBODIMENT 
To achieve the advantages of employing ammonium nitrate, e.g., low cost, 
availability and safety, while avoiding its drawbacks, e.g., cracks and 
voids in the pressed pellet when subjected to temperature cycling, it is 
proposed to mix the ammonium nitrate oxidizer with guanidine nitrate 
and/or aminoguanidine nitrate, triaminoguanidine nitrate or 
nitroguanidine, the potassium nitrate or potassium perchlorate, and the 
polyvinyl alcohol and then form a eutectic solution which avoids some of 
the problems previously encountered and discussed above. The oxidation 
ration (O.sub.R) of the mixture should be slightly less than unity, say 
0.95, where O.sub.R is defined as the ratio of available oxygen in the 
formulation to that required to burn the carbon, hydrogen, and potassium 
to carbon dioxide, water, and potassium oxide respectively. Potassium 
chloride is the product of the composition when potassium perchlorate is 
used in the formulation. The resultant eutectic in the form of a pressed 
pellet results in a propellant grain to produce an essentially 
particulate-free, non-toxic, odorless, and colorless gas for inflating an 
air bag, but without the tendency of the pellet to crack (with the 
eliminated phase change of the AN) due to temperature cycling. 
Conventionally, the propellant is ignited with mixtures of boron and 
potassium nitrate, such as "2C granules", made by Tracor, Inc. (18% boron 
with about 82% KNO.sub.3). 
In addition, it has been discovered that the same eutectic employed to 
generate the gases may also be used as the igniter in the inflator device. 
By so utilizing the same eutectic for igniting the propellant, the 
inventors are able to eradicate particulates that would otherwise be 
present in the exhaust due to the use of "dirty" ignition materials, such 
as boron/potassium nitrate. For use as an igniter, the eutectic is 
provided as a powder, granulate, monolithic composite or any other form 
that may conveniently be disposed in the generator. This use of the noted 
eutectic as an igniter is not limited to its employment in conjunction 
with the same composition as generant, but is effective as a 
general-purpose smokeless igniter. 
The following composition (Comp 93) is well-suited for use as a propellant 
to inflate air bags: 
about 30% by weight guanidine nitrate (GN) 
about 60% by weight ammonium nitrate (AN) 
about 5% by weight potassium nitrate (KN) 
about 5% by weight polyvinyl alcohol (PVA) 
The combination of all four ingredients not only eliminates the phase 
changes of AN as shown in FIG. 1, but also enables pressed pellets of the 
composition to withstand temperature cycling requirements for air bags. 
This latter phenomenon is demonstrated below in the examples. 
The above-described eutectic, when used as the generant in pellet form, as 
well as the igniter in a granular form, enables the use of an inflator 
that delivers hot gas for the purpose of inflating an air bag, the gas 
being non-toxic and essentially particulate-free. This propellant is about 
three times as effective as a comparable azide propellant currently used 
in industry. Because of this, the amount of particulates generated by this 
propellant is only about 2% of that generated in a comparable azide 
propellant. 
By this invention, the propellant or generant, when ignited by the 
initiator causes the non-toxic particulate-free effluent to pressurize the 
cartridge which ruptures the seal and causes the effluent to exhaust 
through the diffuser into an air bag. 
THE DRAWINGS 
To demonstrate the effectiveness of the present propellant system, the 
accompanying drawings are provided wherein: 
FIG. 1 provides a comparison of scanning calorimeter traces of two 
compositions. This has been described above. 
FIG. 2 provides an analysis of the exhaust gas provided by burning about 20 
gm of an aminoguanidine nitrate/ammonium nitrate eutectic propellant. The 
exhaust gas was collected in a 60 liter tank and indicates 1500 ppm of 
carbon dioxide, with a smaller amount of 350 ppm of carbon monoxide. The 
exhaust gas also contains non-toxic amounts of hydrogen cyanide, 
formaldehyde, ammonia and nitrogen oxides. 
FIG. 3 is a drawing of the pyrotechnic generator of the instant invention. 
Since no part of the inflator is reserved for storage capacity, the device 
is smaller than its counterpart hybrid inflator. In this figure, a 
cartridge (21) holds a generant (22), which may be a eutectic solid 
solution of GN/AN with at least 5% by weight KN and at least 3% by weight 
PVA formulated to an oxidizer ratio of about 0.95. At one end of said 
cartridge (21) is an initiator (23) that will combust in response to a 
signal from a sensor (not shown) which generates said signal as a result 
of a change in conditions, e.g., an excessive increase in temperature or a 
sudden deceleration of a vehicle (indicative of a crash), in which the 
inflator is installed. The initiator (23) is kept in place by an initiator 
retainer (24). An C-ring (25) serves as a gasket to render the inflator 
essentially gas tight in the end where the initiator (23) is located. 
The end of the inflator opposite from that containing the initiator (23) 
holds a screen (27) upon which any particulates in the produced gas are 
retained, a spring (29) to maintain dimensional stability of the generant 
bed, and a burst disc (28), which is ruptured when the gas pressure 
exceeds a predetermined value, permitting the gas to escape from the 
cartridge (21) through exit ports (not shown) in cartridge (21) wall near 
the end containing the diffuser (30). To ensure that the expelled gas is 
not released in an unduly strong stream, a diffuser (30) is affixed to the 
discharge end of the inflator. 
THE EXAMPLES 
To illustrate the instant method, the following tests were conducted. In 
these tests, propellant formulations were prepared by dissolving all the 
ingredients in water and mixing down to dryness to form granules suitable 
for pressing. The granules were then compacted into pellets measuring 
about 0.5 inch diameter .times.0.4 inch length. These pellets served as 
test specimens for temperature cycling tests, where they were subjected to 
either 200 cycles in the temperature range 40 to +107.degree. C., or 300 
cycles in the range -30.degree. to +90.degree. C. The cycle time was three 
hours and 20 minute/cycle, consisting of 40 minutes cold, 60 minutes 
warmup, 40 minutes hot, and 60 minutes cooldown. 
Measurements of compressive strength (yield) and pellet diameter were made 
periodically on samples removed from cycling. It was found that many of 
the pellets gained strength during cycling and essentially all suffered 
permanent growth. A growth of greater than about 2% was used to disqualify 
the sample. 
The rule of thumb for stabilizing AN with KN is an AN/KN ratio of about 
85/15, or 5.67. The goal of these experiments was to maximize this ratio 
(i.e., minimize the KN content, which is the source of particulates, as 
K.sub.2 C0.sub.3) 
Amounts up to about 20% wt., based on the propellant of either 
triaminoguanidine nitrate (TAGN) or nitroguanidine (NQ) may be added as 
ballistic modifiers to increase burn rate and lower pressure exponent. 
Test A 
The effect of KN content on the thermal stability of AN/AGN eutectics is 
summarized in FIG. 4. None of these eutectics survived -40/+107.degree. C. 
cycling. All of them survived -30/+90.degree. C. cycling, indicating that 
no more than 3% KN is needed to stabilize these eutectics in this range, 
resulting in an AN/Kn ration of 21.67, and about 2% particulates (as 
K.sub.2 CO.sub.3) in the combustion products. 
When AN is not a part of the mixture, stability in the range 
-40/+107.degree. C. is attained without KN (Comp 103), and the combustion 
products are particulate-free. However, these propellant compositions are 
fuel-rich of the oxidation ratio of 0.95, and are only suitable for hybrid 
inflator systems in which part of the compressed gas is oxygen. 
Test B 
A number of comparisons are available from the data in FIG. 5, where the 
stabilities of selected AN/GN eutectics are summarized. 
First, Comp 111 shows that PVA alone is not enough to stabilize AN. The 
cycling was terminated at 25 cycles because of excessive growth. Later in 
the table, it is shown that KN alone is also not enough. The 50/50 AN/KN 
eutectic disintegrated after only five cycles. 
The effect of KN content is shown in the next four entries (84, 94, 93, and 
92). The changes at 3% KN are excessive, so the lowest acceptable KN 
content in the table is 5%, resulting in about 3.4% particulates in the 
combustion product (as K.sub.2 CO.sub.3). The resulting AN/KN ratio is 20, 
which agrees well with the value of 21.67 in the AN/AGN eutectics, both of 
which are much greater than the 5.67 without either GN, AGN, or PVA in the 
eutectic. 
The effect of PVA content on stability is shown in the next three entries 
(120, 121, and 132). Here the lowest acceptable value is 3%. In Comp 121, 
AN/KN=10.8, but it is possible that a value closer to 20 might be 
acceptable, in which KN.about.3%, resulting in about 2% particulates in 
the combustion products, as K.sub.2 CO.sub.3. The next two entries (125 
and 126) show that the PVA must be dissolved in the eutectic to be 
effective. Adding the PVA to the AN/GN/KN eutectic as a dry powder 
resulted in unacceptable changes during cycling. 
The last three entries (110, 99, and 114) show that KClO.sub.4, is also an 
effective stabilizer, but probably not as good as KN. Between 6 and 9% 
KClO.sub.4 is required. Taking the latter, the resulting AN/KClO.sub.4, 
ratio is 6.1, in approximate (though probably fortuitous) agreement with 
the value of 5.67 for AN/KN alone, (i.e., in the absence of a eutectic). 
The resulting combustion products contain 4.8 particulates (as KCl), 
comparable to the value of 3.4% particulates as K.sub.2 CO.sub.3 reported 
above for eutectics stabilized with KN. 
Similar to that found in Test A, when AN is not a part of the mixture, 
stability in the range -40/+107.degree. C. is attained by the simple 
two-component mixture COMP 87, composed of 65% GN+35% KP. However, 
although this propellant formulation has an oxidation ratio of 0.95 and is 
suitable for use in an all-pyro inflator, it is considerably dirtier than 
COMP 93, with an ash content of 18.8%. 
Only the preferred embodiment of the invention and a few examples of its 
versatility are shown and described in the present disclosure. It is to be 
understood that the invention is capable of use in various other 
combinations and environments and is capable of changes or modifications 
within the scope of the inventive concept as expressed herein. 
Additional objects and advantages of the present invention will become 
readily apparent to those skilled in this art from the description. As 
will be realized, the invention is capable of other and different 
embodiments, and its several details are capable of modifications in 
various obvious respects, all without departing from the invention. 
Accordingly, the drawings and description are to be regarded as 
illustrative in nature, and not as restrictive.