Gas generating compositions

A low-solids gas generating composition, comprising a mixture of a fuel selected for the group consisting of guanidine nitrate, nitroguanidine, cellulose, cellulose acetate, hexamine, and mixtures thereof, and an oxidizer selected from the group consisting of ceric ammonium nitrate, lithium nitrate, lithium perchlorate, sodium perchlorate, phase stabilized ammonium nitrate, a combination of ammonium nitrate with potassium nitrate, potassium perchlorate, or mixtures thereof, such that the combination is a solid solution, a mixture of ammonium perchlorate and at least one alkali metal salt, and mixtures thereof, where the fuel is not nitroguanidine when the oxidizer includes ammonium nitrate. The combination of ammonium nitrate with other salts in solid solution is intended to phase stabilize the ammonium nitrate. The oxidizer-fuel mixture is within about 4 percent of stoichiometric balance. Useful alkali metal salts include lithium carbonate, lithium nitrate, sodium nitrate, potassium nitrate, and mixtures thereof. The preferred oxidizers for the gas generating composition of the invention are ceric ammonium nitrate, lithium nitrate, lithium perchlorate, sodium perchlorate, a mixture of ammonium perchlorate and at least one alkali metal salt, and mixtures thereof. The preferred fuels are guanidine nitrate, nitroguanidine, and mixtures thereof. In addition, the gas generating composition may include an energizing agent, such as RDX or HMX. The gas generating composition of the invention may further comprise sub-micron fumed silica to reduce moisture contamination and serve as a processing and powder flow aid and/or a binder, and may be in the form of pressed pellets, grains, or granules.

FIELD OF THE INVENTION 
The invention generally relates to gas generating compositions or gas 
generants, such as those used in "air bag" passive restraint systems. In 
particular, the invention relates to non-azide gas generants having a low 
solids output on combustion. 
BACKGROUND OF THE INVENTION 
The inflator in a vehicle air bag passive restraint system provides the gas 
required to deploy and fill the air bag in a matter of milliseconds when 
an actuation signal is received by the system. The air bag inflator must 
perform properly during an accident at any point in the useful life of the 
vehicle. The fact that an inflator may be required to rapidly fill an air 
bag after 10 or more years of storage places a number of constraints on 
inflator design, which are dictated by the required performance of the 
restraint system, i.e., the time required for the full deployment of the 
air bag, reliability (including environmental exposure and storage life), 
the safety and health of vehicle occupants, air bag volume, and the 
interface between the restraint system and the vehicle. The inflator 
specification that results from these constraints defines the form, fit, 
and function criteria for the inflator. 
The restraint system performance is dictated, in part, by the need to fill 
and deploy the air bag in a matter of milliseconds. Under representative 
conditions, only about 60 milliseconds elapse between the primary impact 
of a vehicle in an accident and the secondary impact of the driver or 
passenger (herein after "an occupant") with a portion of the vehicle 
interior. Therefore, a very rapid generation or release of gas is required 
to fill the bag, and prevent the secondary impact. The amount and rate of 
gas generation or release is determined by the volume of the air bag 
required for the vehicle and the time between primary and secondary 
impacts. 
In addition, to meet environmental and occupant safety and health 
requirements, the inflation gas produced by the inflator should be 
non-toxic and non-noxious when the inflator is functioned in an air bag 
module in a typical vehicle. The gas generated or released must also have 
a temperature that is sufficiently low to avoid burning the occupant and 
the air bag, and it must be chemically inert, so that the mechanical 
strength and integrity of the bag are not degraded by the gas. 
The stability and reliability of an inflator gas generant over the life of 
the vehicle are extremely important. The gas generant must be stable over 
a wide range of temperature and humidity conditions, and should be 
resistant to shock, so that the propellant pellets, grains, granules, etc. 
maintain mechanical strength and integrity during the life of the vehicle. 
Vehicle manufacturers have developed a number of quantitative tests to 
determine whether an air bag restraint system will operate reliably when 
needed during any part of a vehicle's useful life. Although these tests 
and the performance requirements that an inflator should meet in these 
tests vary somewhat from manufacturer to manufacturer, the design criteria 
of all the vehicle manufacturers are essentially the same. 
In a typical prior art passive restraint system the inflation gas is 
nitrogen, which is produced by the decomposition reaction of a gas 
generant containing a metal azide, typically sodium azide (NaN.sub.3). The 
metal azide is the fuel and the principal gas generating compound in the 
gas generant used in the inflator. A typical metal azide gas generant is 
disclosed in U.S. Reissue Pat. No. Re. 32,584. 
The gas produced in sodium azide based inflators is all nitrogen. Because 
there is no carbon in the fuel, oxides of nitrogen, NO.sub.x, can be 
controlled easily by running the propellant under slightly fuel rich 
conditions. In contrast, the combustion of gas generants containing 
carbon, nitrogen, and oxygen, when formulated to be fuel rich, results in 
the production of carbon monoxide (CO), a toxic gas. If excess oxygen is 
present in such a composition to assure the complete oxidation of CO to 
carbon dioxide, the excess oxygen will react with nitrogen at the 
propellant combustion temperature to form oxides of nitrogen, which can 
also be toxic. Therefore, the mixture of oxidizer and fuel must approach a 
stoichiometric balance in gas generants of this type to avoid the 
production of toxic gases. 
Inflator designs based on sodium azide have been shown to meet the 
requirements of vehicle manufacturers, and are used today in most passive 
restraint systems. However, there are disadvantages to this technology, 
including the production of large quantities of hot, solid particulates 
during combustion, which results in added complexity and cost in the 
inflator design. The relatively high toxicity of the raw sodium azide 
(oral rat LD.sub.50 of about 45 mg/kg), which must be handled during the 
inflator manufacturing process, can also create a disposal problem at the 
end of the useful life of the vehicle. Because typical gas generants used 
in inflators produce solid particulates, filters must be incorporated into 
the inflator to separate the hot particulates from the gas prior to 
exhausting the gas from the inflator into the air bag. Filters are 
required in virtually all driver and passenger side air bag inflators that 
incorporate purely pyrotechnic gas generants, including sodium azide based 
air bag inflators. The solids produced during the combustion of the gas 
generant are separated from the gas stream to prevent exposure of vehicle 
occupants to excessive or toxic levels of airborne particulates during and 
after air bag deployment. The need for filters, as well as the toxicity of 
the sodium azide, adds to the cost of producing a typical prior art 
inflator. 
Pyrotechnic compositions typically comprise a fuel and an oxidizer or, in 
the case of monopropellants, such as nitrocellulose, a fuel having an 
integral oxidizer. Most pyrotechnic oxidizers produce significant amounts 
of solids in the process of decomposing to provide an oxidizing agent, 
such as oxygen.. As it is important to provide an inflator gas having a 
temperature sufficiently low to avoid burning the air bag or the vehicle 
occupants, gas generants that burn faster and better at lower 
temperatures, but tend to produce significant quantities of particulates, 
are often utilized in air bag inflators. 
Oxidizers that produce low amounts of particulates during the combustion of 
a gas generant are available, but often produce toxic byproducts. For 
example, gas generants that use ammonium perchlorate as the sole oxidizer 
typically burn rapidly without producing large amounts of solid 
particulates. However, ammonium perchlorate produces large quantities of 
hydrogen chloride (HCl) during combustion, and exceeds the toxicity limits 
placed on an inflator gas for a vehicle air bag. Thus, gas generants 
containing ammonium perchlorate cannot be used in vehicle passive 
restraint systems without some means of trapping or neutralizing the HCl 
produced during combustion. 
"Hybrid" inflators that use stored pressurized gas for part of the inflator 
gas supply are another means used to control solid particulate production, 
since smaller amounts of solid particulate producing gas generant can be 
used to obtain the same inflator gas output. In addition, the stored 
pressurized gas, which is typically an inert gas mixed with oxygen to 
supplement combustion and decrease the level of toxics, cools the gas that 
flows from the inflator, and results in a greater degree of condensation 
and solidification within the inflator. Thus, the amount of particulates 
introduced into the air bag and the vehicle interior is reduced. 
The combination of greater condensation of solids within the inflator and 
the reduction in the total amount of solids produced eliminates the need 
for filters in hybrid inflators. However, hybrid inflators have two main 
disadvantages: 1. They are typically larger and heavier, and 2. They have 
decreased reliability resulting from storing a pressurized gas over the 
lifetime of the vehicle. 
U.S. Pat. No. 5,538,567 discloses a gas generating propellant, which 
produces nitrogen, carbon dioxide, and steam on combustion, consisting 
essentially of guanidine nitrate, a flow enhancer, such as carbon black, a 
binder, such as calcium resinate, and an oxidizer selected from the group 
consisting of potassium perchlorate and ammonium perchlorate. The 
production of only nitrogen, carbon dioxide, steam, and minor amounts of 
hydrogen and carbon monoxide is disclosed. However, only a single 
composition comprising potassium perchlorate is exemplified. There is no 
example of compositions incorporating ammonium perchlorate, which produces 
significant quantities of hydrogen chloride (HCl) during combustion. 
U.S. Pat. No. 5,545,272 discloses a gas generating composition consisting 
essentially of about 35 to 55 percent by weight nitroguanidine and about 
45 to 65 percent by weight phase stabilized ammonium nitrate, and may 
include a flow enhancer or a molding facilitator. The phase stabilizer is 
typically a potassium salt. Although ammonium nitrate produces clean 
non-toxic gases, and is free of solids upon combustion, ammonium nitrate 
has a crystal transition or phase stability problem, resulting from the 
four phase transitions ammonium nitrate crystals undergo over the 
temperature range typically experienced in storage. Each of these 
transitions results in a change of crystal volume, which may cause a slow 
breakup of propellant grains during thermal cycling from high to low 
temperature. However, ammonium nitrate crystals can be "phase stabilized" 
using additives, such as potassium perchlorate and potassium nitrate. The 
effectiveness of these additives varies depending upon the particular 
additive used. However, most of the known additives useful as phase 
stabilizers produce solids upon combustion, and, thus, increase the 
production of solids by the propellant. 
The present invention is directed to low solids producing gas generants 
that minimize or eliminate the need for inflator filters or other means 
for separating solids from the gases produced. 
SUMMARY OF THE INVENTION 
The present invention relates to a low-solids gas generating composition, 
comprising a mixture of a fuel selected from the group consisting of 
guanidine nitrate, nitroguanidine, cellulose, cellulose acetate, hexamine, 
and mixtures thereof, and an oxidizer selected from the group consisting 
of ceric ammonium nitrate, lithium nitrate, lithium perchlorate, sodium 
perchlorate, phased stabilized ammonium nitrate, a combination of ammonium 
nitrate with potassium nitrate, potassium perchlorate, or mixtures 
thereof, such that the combination is a solid solution, a mixture of 
ammonium perchlorate and at least one alkali metal salt, and mixtures 
thereof, where the fuel is not nitroguanidine when the oxidizer comprises 
ammonium nitrate. The oxidizer-fuel mixture is within about 4 percent of 
stoichiometric balance, and produces low solids on combustion. Useful 
alkali metal salts include lithium carbonate, lithium nitrate, sodium 
nitrate, potassium nitrate, and mixtures thereof. The combination of 
ammonium nitrate with other salts in solid solution is intended to phase 
stabilize the ammonium nitrate. 
The preferred oxidizers for the gas generating composition of the invention 
are ceric ammonium nitrate, lithium nitrate, lithium perchlorate, sodium 
perchlorate, a combination of ammonium nitrate with potassium nitrate, 
potassium perchlorate, or mixtures thereof, such that the combination is a 
solid solution, a mixture of ammonium perchlorate and at least one alkali 
metal salt, and mixtures thereof. The most preferred fuels are guanidine 
nitrate, nitroguanidine, and mixtures thereof. However, other preferred 
compositions use cellulose, cellulose acetate, hexamine, and mixtures 
thereof as fuels. 
In addition, the gas generating composition may include an energizing 
agent, such as hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) or 
octahydro-1,3,5,7-tetranitro-1,3,5,7-tetraazocine (HMX). The gas 
generating composition of the invention may further comprise a sub-micron 
(i.e., having an average particle size of less than about 1 .mu.m) fumed 
silica, such as Cabosil.RTM., to reduce moisture contamination and serve 
as a processing and powder flow aid. The compositions of the invention may 
be in the form of pressed pellets, grains, granules, or powder, and may 
also include a binder. 
The preferred gas generating compositions of the invention include the 
following mixtures: 
Guanidine nitrate and an oxidizer comprising a combination of ammonium 
nitrate and potassium perchlorate, and sub-micron fumed silica; more 
preferably, from about 45 to about 54 percent guanidine nitrate, from 
about 26 to about 52 percent ammonium nitrate, and from about 3 to about 
20 percent potassium perchlorate; most preferably, about 50 percent 
guanidine nitrate, about 39 percent ammonium nitrate, and about 11 percent 
potassium perchlorate. 
Guanidine nitrate and a combination of ammonium nitrate and potassium 
nitrate; more preferably from about 37 to about 46 percent guanidine 
nitrate, from about 34 to about 60 percent ammonium nitrate, and from 
about 3 to about 20 percent potassium nitrate; most preferably about 46 
percent guanidine nitrate, about 49 percent ammonium nitrate, and about 6 
percent potassium nitrate. 
Guanidine nitrate and an oxidizer comprising a mixture of ammonium 
perchlorate and sodium nitrate; more preferably from about 54 to about 67 
percent guanidine nitrate and from about 33 to about 46 percent oxidizer, 
wherein the oxidizer comprises ammonium perchlorate and sodium nitrate in 
a mole ratio of about 1 mole of ammonium perchlorate to about 1 to about 4 
moles of sodium nitrate; most preferably, about 59 percent guanidine 
nitrate, about 23 percent ammonium perchlorate, and about 18 percent 
sodium nitrate. 
Guanidine nitrate and an oxidizer comprising a mixture of ammonium 
perchlorate and lithium carbonate; more preferably, from about 41 to about 
57 percent guanidine nitrate and from about 43 to about 59 percent 
oxidizer, wherein the oxidizer comprises ammonium perchlorate and lithium 
carbonate in a mole ratio of about 1 mole of ammonium perchlorate to about 
1 to about 1.4 moles of lithium carbonate; most preferably; about 47 
percent guanidine nitrate, about 40 percent ammonium perchlorate, and 
about 13 percent lithium carbonate. 
Guanidine nitrate and an oxidizer comprising a mixture of ammonium 
perchlorate and lithium nitrate; more preferably, from about 56 to about 
68 percent guanidine nitrate and from about 32 to about 44 percent 
oxidizer, wherein the oxidizer comprises ammonium perchlorate and lithium 
nitrate in a mole ratio of about 1 mole of ammonium perchlorate to about 1 
to about 8 moles of lithium nitrate; most preferably, about 61 percent 
guanidine nitrate, about 24 percent ammonium perchlorate, and about 15 
percent lithium nitrate. 
Guanidine nitrate and an oxidizer comprising a mixture of ammonium 
perchlorate and potassium nitrate; more preferably, from about 50 to about 
64 percent guanidine nitrate and from about 36 to about 50 percent 
oxidizer, wherein the oxidizer comprises ammonium perchlorate and 
potassium nitrate in a mole ratio of about 1 mole of ammonium perchlorate 
to about 1 to about 2 moles of potassium nitrate; most preferably, about 
57 percent guanidine nitrate, about 23 percent ammonium perchlorate, and 
about 20 percent potassium nitrate. 
Guanidine nitrate and ceric ammonium nitrate; more preferably, from about 
51 to about 65 percent guanidine nitrate and from about 35 to about 49 
percent ceric ammonium nitrate; most preferably, about 56 percent 
guanidine nitrate and about 44 percent ceric ammonium nitrate. 
Guanidine nitrate and lithium nitrate; more preferably, from about 61 to 
about 70 percent guanidine nitrate and from about 30 to about 39 percent 
lithium nitrate; most preferably, about 68 percent guanidine nitrate and 
about 32 percent lithium nitrate. 
Guanidine nitrate and lithium perchlorate; more preferably, from about 65 
to about 75 percent guanidine nitrate and from about 25 to about 35 
percent lithium perchlorate; most preferably, about 68 percent guanidine 
nitrate and about 32 percent lithium perchlorate. 
Nitroguanidine and an oxidizer comprising a mixture of ammonium perchlorate 
and lithium carbonate; more preferably, from about 38 to about 53 percent 
nitroguanidine and from about 47 to about 62 percent oxidizer, wherein the 
oxidizer comprises ammonium perchlorate and lithium carbonate in a mole 
ratio of about 1 mole of ammonium perchlorate to about 1 to about 1.3 
moles of lithium carbonate; most preferably, about 44 percent 
nitroguanidine, about 42 percent ammonium perchlorate, and about 14 
percent lithium carbonate. 
Nitroguanidine and an oxidizer comprising a mixture of ammonium perchlorate 
and sodium nitrate; more preferably, from about 50 to about 62 percent 
nitroguanidine, and from about 38 to about 50 percent oxidizer, wherein 
the oxidizer comprises ammonium perchlorate and sodium nitrate in a mole 
ratio of about 1 mole of ammonium perchlorate to about 1 to about 3 moles 
of sodium nitrate; most preferably, about 55 percent nitroguanidine, about 
26 percent ammonium perchlorate, and about 19 percent sodium nitrate. 
Nitroguanidine and lithium perchlorate; more preferably, from about 62 to 
about 71 percent nitroguanidine, and from about 29 to about 38 percent 
lithium perchlorate; most preferably, about 65 percent nitroguanidine and 
about 35 percent lithium perchlorate. 
Nitroguanidine and ceric ammonium nitrate; more preferably, from about 48 
to about 61 percent nitroguanidine and from about 39 to about 52 percent 
ceric ammonium nitrate; most preferably, about 53 percent nitroguanidine 
and about 47 percent ceric ammonium nitrate. 
Cellulose and an oxidizer comprising a mixture of ammonium perchlorate and 
sodium nitrate; more preferably, from about 22 to about 28 percent 
cellulose and from about 72 to about 78 percent oxidizer, wherein the 
oxidizer comprises ammonium perchlorate and sodium nitrate in a mole ratio 
of about 1 mole of ammonium perchlorate to about 1 to about 4 moles of 
sodium nitrate; most preferably, about 24 percent cellulose, about 43 
percent ammonium perchlorate, and about 33 percent sodium nitrate. 
Hexamine and an oxidizer comprising a mixture of ammonium perchlorate and 
sodium nitrate; more preferably, from about 14 to about 18 percent 
hexamine and from about 82 to about 86 percent oxidizer, wherein the 
oxidizer comprises ammonium perchlorate and sodium nitrate in a mole ratio 
of about 1 mole of ammonium perchlorate to about 1 to about 4 moles of 
sodium nitrate; most preferably, about 16 percent hexamine, about 48 
percent ammonium perchlorate, and about 36 percent sodium nitrate. 
Cellulose acetate and an oxidizer comprising a mixture of ammonium 
perchlorate and sodium nitrate; more preferably, from about 20 to about 25 
percent cellulose acetate and from about 75 to about 80 percent oxidizer, 
wherein the oxidizer comprises ammonium perchlorate and sodium nitrate in 
a mole ratio of about 1 mole of ammonium perchlorate to about 1 to about 4 
moles of sodium nitrate; most preferably, about 22 percent cellulose 
acetate, about 44 percent ammonium perchlorate, and about 34 percent 
sodium nitrate. 
Guanidine nitrate, RDX and/or HMX, and an oxidizer comprising a mixture of 
ammonium perchlorate and sodium nitrate; from about 12 to about 52 percent 
guanidine nitrate, from about 15 to about 45 percent RDX and/or HMX, and 
from about 31 to about 45 percent oxidizer, wherein the oxidizer comprises 
ammonium perchlorate and sodium nitrate in a mole ratio of about 1 mole of 
ammonium perchlorate to about 1 to about 4 moles of sodium nitrate; most 
preferably, about 30 percent guanidine nitrate, about 32 percent RDX 
and/or HMX, about 21 percent ammonium perchlorate, and about 17 percent 
sodium nitrate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Unless otherwise stated, all references to "percent" or "%" mean percent by 
weight based on the total weight of the composition. 
As used herein, the term "stoichiometric balance" means that the ratio of 
oxidizer to fuel is such that upon combustion of the composition all of 
the fuel is fully oxidized, and no excess of oxygen is produced. A "near 
stoichiometric balance" is one in which the ratio of oxygen mass surplus 
or deficit to total mixture mass is within about 4 percent of a 
stoichiometric balance. 
As used herein, the terms "low solids" and "low levels of solids" mean 
that, upon combustion, the gas generant produces substantially lower 
solids than gas generants used in prior art pyrotechnic inflators, such as 
sodium azide based inflators, which produce about 60 percent solids on 
combustion. The gas generants of the invention typically produce less than 
about 30 percent solids. This is advantageous in that it minimizes or 
eliminates the need for a filter in the inflator, thus, simplifying 
inflator design. 
The compositions of the invention are low solids producing gas generants. 
Preferred embodiments of the invention are well suited as non-azide gas 
generants for use in filterless vehicle air bag inflators, that is, gas 
generants that do not require a metal azide as a necessary component. The 
compositions disclosed herein produce low levels of solids during 
combustion, and minimize or eliminate the need for filters or hybrid 
operation. 
An example of a filterless inflator is provided in parent U.S. application 
Ser. No. 08/402,103, which is incorporated herein by reference. The 
inflator described in the above identified application comprises a 
contained volume, a source of gas for producing an inflation gas, an 
initiating system for initiating the conversion of the source of gas to 
the inflation gas, and an exhaust orifice that provides an exhaust path 
and controls the flow of the inflation gas. The source of gas is typically 
a mixture of a fuel and oxidizer that is stable, and will not ignite until 
the initiating system ignites the mixture to produce the inflation gas. 
A typical inflator functions by converting an electrical or mechanical 
initiating signal into the generation of a precisely controlled quantity 
of gas at precisely controlled rates. Generally, this is accomplished by 
an inflator pyrotechnic train, which comprises an `initiation` device 
called an initiator, an enhancer charge, and a main gas generant charge, 
all of which are contained in the body of the inflator. In response to the 
initiating signal, the initiator ignites and produces a hot gas, 
particulates, and/or flame. The flame output of the initiator is typically 
small, and often requires enhancement to ignite the main gas generant 
charge. The initiator flame ignites the enhancer charge, which is a hot 
burning propellant, and augments the initiator output sufficiently to 
ignite the main gas generant charge. Once ignited, the gas generant burns 
to produce the hot gas required at a rate sufficient to fill the air bag 
module in the required time. 
Propellant compositions according to the invention are useful as both 
enhancers and gas generants. The claimed compositions provide a relatively 
clean gas that meets the requirements of the automotive air bag market, 
and produce entrained solids in a quantity that is sufficiently low so as 
to not require the use of filters or supplemental stored gas. Although it 
is desirable in many applications for both the gas generant and the 
enhancer to have a low solids output during combustion, it is particularly 
important for the gas generant, which is the principal source of gas for 
the inflator output. 
The fuels of the invention, guanidine nitrate, CH.sub.6 N.sub.4 O.sub.3, 
nitroguanidine, CH.sub.4 N.sub.4 O.sub.2, hexamethylene tetramine 
(hexamine), cellulose, and cellulose acetate, are hydrocarbons, containing 
only carbon, hydrogen, oxygen, and nitrogen. These fuels provide clean 
combustion products when properly mixed with an appropriate oxidizer. Most 
oxidizers used in the air bag industry produce significant quantities of 
solids. Therefore, the amount of solids produced by the combustion of the 
generant compositions of the invention is determined by the amount of 
oxidizer in the propellant. Guanidine nitrate and nitroguanidine require a 
minimum quantity of oxidizer, and thus, produce low solids on combustion. 
Additionally, hexamethylene tetramine (hexamine), cellulose, and cellulose 
acetate, are energetic fuels, which, when used in enhancer mixtures, 
produce sufficiently low quantities of solids to allow their use in 
filterless inflators. 
Oxidizers useful in the invention that produce low solids are ceric 
ammonium nitrate, lithium nitrate, lithium perchlorate, sodium 
perchlorate, phase stabilized ammonium nitrate, a combination of ammonium 
nitrate with potassium nitrate, potassium perchlorate, or mixtures 
thereof, such that the combination is a solid solution, a mixture of 
ammonium perchlorate and at least one alkali metal salt, and mixtures 
thereof, where the fuel is not nitroguanidine when the oxidizer comprises 
ammonium nitrate. The combination of ammonium nitrate with other salts in 
solid solution is intended to phase stabilize the ammonium nitrate. The 
preferred oxidizers are ceric ammonium nitrate, lithium nitrate, lithium 
perchlorate, sodium perchlorate, a mixture of ammonium perchlorate and at 
least one alkali metal salt, and mixtures thereof. 
With an ammonium perchlorate oxidizer, a highly alkaline material must be 
produced during combustion of the gas generant to neutralize or scavenge 
HCl produced during combustion. The alkali metal salts of the invention, 
Li.sub.2 CO.sub.3, LiNO.sub.3, NaNO.sub.3, and KNO.sub.3, burn to form the 
corresponding alkali metal oxides (i.e.: Li.sub.2 O, Na.sub.2 O, and 
K.sub.2 O), which, in turn, being extremely alkaline, react with the HCl 
to form the alkali metal chloride and water. The metal oxides produced by 
the combustion of salts of metals other than the group IA alkali metals 
are typically not basic enough to effectively scavenge HCl. Alkali metal 
salts are used with an ammonium perchlorate oxidizer in the compositions 
of the invention to meet representative gas toxicity requirements. As one 
of ordinary skill in the art will recognize, an ammonium perchlorate based 
oxidizer system can use a single alkali metal salt or multiple alkali 
metal salts mixed in any proportion, as long as the total amount of alkali 
metal oxide produced during combustion is at least sufficient to scavenge 
all of the HCl produced. One of ordinary skill in the art will also 
recognize that an excess amount of salt can be utilized, as long as the 
resulting composition is low solids producing. 
The preferred guanidine nitrate and nitroguanidine gas generant fuels and 
the enhancer fuels of nitroguanidine, hexamethylene tetramine (hexamine), 
cellulose, cellulose acetate, or guanidine nitrate, mixed with at least 
one of RDX and/or HMX, require a minimum amount of oxidizer, thus reducing 
solids production. With the appropriate choice of an oxidizer, the 
preferred fuels provide gas generants and enhancers that produce low 
solids during combustion. 
For a particular fuel, a broad range of operating temperatures can be 
obtained by varying the oxidizer used, while maintaining a near 
stoichiometric balance between the fuel and oxidizer. The sodium and 
lithium perchlorate oxidizers provide gas generants with the highest flame 
temperatures, while the combination of ammonium perchlorate and lithium 
carbonate gives the lowest flame temperature with a particular fuel. The 
ammonium perchlorate/sodium nitrate system provides a flame temperature 
somewhere between these other systems, and, of the low hygroscopicity 
oxidizer systems, also provides gas generants with the lowest solids. This 
system provides a combination of low solids, moderate energy, and 
controllable hygroscopicity. 
Although the gas generant compositions of the invention can function as 
either the main gas generant or the enhancer charge in a pyrotechnic 
inflator, the functions of these charges differ, which dictates 
differences in the formulation of the composition used for each charge. 
To meet representative toxicity requirements, the sum of the charges in a 
vehicle air bag inflator must approach a near stoichiometric balance of 
oxidizer and fuel. For practical systems, the oxygen balance of the system 
must be within about 4 percent of the theoretical stoichiometric balance, 
or the gases produced will contain too much CO or NO.sub.x, depending on 
whether excess fuel or excess oxidizer is present. However, as long as the 
entire system is close to a stoichiometric balance, and any divergence in 
the main charge is compensated for by an opposite divergence in the 
enhancer, the individual charges need not be in stoichiometric balance. 
For example, the main gas generant charge can be fuel rich if the enhancer 
charge is oxidizer rich, and the entire system is within about 4 percent 
of a stoichiometric balance. In general, however, having all charges in 
stoichiometric balance provides a lower level of toxic compounds in the 
inflator effluent gases. In light of these requirements for temperature, 
stoichiometry, and solids production, guanidine nitrate and nitroguanidine 
are preferred fuels. These fuels produce less than about 18 percent solids 
with the oxidizers listed above, and can produce significantly less than 
18 percent solids when used with certain oxidizer combinations discussed 
above, such as ammonium perchlorate/sodium nitrate. Preferred, 
non-limiting guanidine nitrate and nitroguanidine based main charge gas 
generants that meet the requirements discussed above are listed below. 
Guanidine Nitrate/Ammonium Nitrate (phase stabilized) 
49.4% Guanidine Nitrate 
38.8% Ammonium Nitrate 
11.2% Potassium Perchlorate (Phase stabilizer for AN) 
0.6% Sub-micron Fumed Silica 
Guanidine Nitrate/Ammonium Nitrate (phase stabilized) 
45.5% Guanidine Nitrate 
49.0% Ammonium Nitrate 
5.5% Potassium Nitrate (Phase stabilizer for AN) 
Guanidine Nitrate/Ammonium Perchlorate/Sodium Nitrate 
58.5% Guanidine Nitrate 
23.5% Ammonium Perchlorate 
17.8% Sodium Nitrate 
0.2% Sub-micron Fumed Silica 
Guanidine Nitrate/Ammonium Perchlorate/Lithium Carbonate 
47.4% Guanidine Nitrate 
39.5% Ammonium Perchlorate 
13.1% Lithium Carbonate 
Guanidine Nitrate/Ammonium Perchlorate/Lithium Nitrate 
60.6% Guanidine Nitrate 
24.3% Ammonium Perchlorate 
14.9% Lithium Nitrate 
0.2% Sub-micron Fumed Silica 
Guanidine Nitrate/Ammonium Perchlorate/Potassium Nitrate 
56.7% Guanidine Nitrate 
22.8% Ammonium Perchlorate 
20.5% Potassium Nitrate 
Guanidine Nitrate/Ceric Ammonium Nitrate 
56.5% Guanidine Nitrate 
43.5% Ceric Ammonium Nitrate 
Guanidine Nitrate/Lithium Nitrate 
67.4% Guanidine Nitrate 
32.3% Lithium Nitrate 
0.3% Sub-micron Fumed Silica 
Guanidine Nitrate/Lithium Perchlorate 
68.2% Guanidine Nitrate 
31.5% Lithium Perchlorate 
0.3% Sub-micron Fumed Silica 
Nitroguanidine/Ammonium Perchlorate/Lithium Carbonate 
43.6% Nitroguanidine 
42.4% Ammonium Perchlorate 
14.0% Lithium Carbonate 
Nitroguanidine/Ceric Ammonium Nitrate 
52.8% Nitroguanidine 
47.2% Ceric Ammonium Nitrate 
Sub-micron fumed silica, such as Cabosil.RTM., a product of Cabot 
Corporation of Tuscola, Ill., is typically added to compositions 
containing a hygroscopic ingredient. Cabosil.RTM. and similar very fine, 
sub-micron particle size, high surface area fumed silicas, minimize 
contamination by moisture, and act as a flow aid when the compositions are 
in a powdered form prior to pressing into grains or pellets. 
Some of the fuels of the invention do not possess very good binding 
characteristics, and, thus, may require a binder for the formation of 
pellets, grains, or granules. 
With the exception of the guanidine nitrate/ammonium nitrate/potassium 
perchlorate composition, which is stoichiometrically balanced, the 
preferred compositions, listed above, all contain sufficient oxidizer to 
produce a 1 percent by mass excess of oxygen. However, as noted above, a 
greater variation from stoichiometric balance is acceptable, as long as 
the oxygen balance is within about 4 percent of the theoretical 
stoichiometric balance. 
When critical factors, such as hygroscopicity, flame temperature, 
mechanical stability of propellant grains, and minimum solids production, 
are considered, the most preferred main gas generant propellants is the 
guanidine nitrate/ammonium perchlorate/sodium nitrate ("GN/AP/SN") 
composition set forth above. This composition provides good ballistic 
performance when pressed into aspirin sized tablets, and, if properly 
implemented in a vehicle air bag inflator, so that significant 
condensation of solids occurs in the inflator, does not require an 
inflator filter. Tablets comprising the GN/AP/SN composition have good 
mechanical strength and stability following thermal cycling. The GN/AP/SN 
propellant is relatively non-hygroscopic, and is readily produced under 
reasonable temperature and humidity conditions. 
In a typical inflator, the main gas generant charge is ignited by the 
combustion of the enhancer charge, and both charges produce the hot gas 
necessary to pressurize the inflator and fill the air bag. The enhancer 
charge should be readily ignited by a standard initiator, even at low 
ambient temperatures, and should burn hot. The flame temperature should be 
at least as hot as those produced by the main gas generant charge, and 
preferably hotter. In a typical inflator, the mass of the enhancer charge 
is much less than that of the main gas generant charge. Accordingly, the 
percentage of solids produced by combustion of the enhancer charge can be 
higher than that of the main gas generant charge. In practice, the solids 
production of an enhancer charge should be less than about 50 percent, but 
is preferably less than about 20 percent. 
As with the main propellant charge, the enhancer propellant should be close 
to a stoichiometric balance for oxidizer and fuel to meet the gas toxicity 
requirements for vehicle air bag inflators. However, a stoichiometric 
balance is not as critical for the enhancer propellant compared to the 
main gas generant because the output of the enhancer is small in 
comparison to the total inflator output. 
The preferred enhancer charge fuels are guanidine nitrate with RDX or HMX 
as an energizing agent, nitroguanidine, cellulose, cellulose acetate, and 
hexamine. An energizing agent, as used herein, refers to fuels which can 
be added to the compositions of the invention to increase flame 
temperature, and, potentially, increase burn rate and improve 
igniteability. Preferred, non-limiting enhancer charge compositions are 
listed below. 
Guanidine Nitrate/RDX and/or HMX/Ammonium Perchlorate/Sodium Nitrate 
30.0% Guanidine Nitrate 
32.3% RDX and/or HMX 
21.3% Ammonium Perchlorate 
16.2% Sodium Nitrate 
0.2% Sub-micron fumed silica. 
Nitroguanidine/Ammonium Perchlorate/Sodium Nitrate 
54.8% Nitroguanidine 
25.6% Ammonium Perchlorate 
19.4% Sodium Nitrate 
0.2% Sub-micron fumed silica. 
Nitroguanidine/Lithium Perchlorate 
64.8% Nitroguanidine 
34.8% Lithium Perchlorate 
0.4% Sub-micron fumed silica. 
Cellulose/Ammonium Perchlorate/Sodium Nitrate 
24.4% Cellulose 
42.8% Ammonium Perchlorate 
32.5% Sodium Nitrate 
0.3% Sub-micron fumed silica. 
Cellulose Acetate/Ammonium Perchlorate/Sodium Nitrate 
22.4% Cellulose Acetate 
43.9% Ammonium Perchlorate 
33.4% Sodium Nitrate 
0.3% Sub-micron fumed silica. 
Hexamine/Ammonium Perchlorate/Sodium Nitrate 
15.7% Hexamine 
47.7% Ammonium Perchlorate 
36.2% Sodium Nitrate 
0.4% Sub-micron fumed silica. 
Preferably, the same components are used in the enhancer and main gas 
generant charges. For guanidine nitrate, RDX or HMX can be added to 
enhance the burn rate and combustion temperature of the propellant. The 
most preferred enhancer charges comprise the most preferred main gas 
generant charge compositions described above with a portion of the fuel 
replaced with either RDX or HMX. The preferred amount of RDX or HMX is 
about 15 to about 45 percent by weight, most preferably about 32 percent. 
To maintain a propellant that is in a near stoichiometric balance, the 
percentage of oxidizer must be adjusted to compensate for the change in 
fuel composition. The most preferred enhancer charge composition is the 
guanidine nitrate composition listed above. 
For nitroguanidine based gas generants, NH.sub.4 ClO.sub.4 /NaNO.sub.3, 
NH.sub.4 ClO.sub.4 /LiNO.sub.3, and NH.sub.4 ClO.sub.4 /KNO.sub.3 or 
LiClO.sub.4 oxidizers typically do not require an energizing agent, such 
as RDX or HMX, because these compositions burn at a sufficiently high 
temperature. 
The preferred enhancer compositions have a near stoichiometrically balanced 
oxidizer/fuel ratio. When used in a filterless driver side vehicle 
inflator, the preferred enhancer compositions are used in the form of 
granules. 
The following non-limiting example is merely illustrative of the preferred 
embodiments of the present invention, and is not to be construed as 
limiting the invention, the scope of which is defined by the appended 
claims. 
EXAMPLE 
A filterless, driver side air bag inflator was assembled using the enhancer 
and gas generant charges according to the invention. A main gas generant 
charge was formed by mixing 58.5 percent guanidine nitrate, 23.5 percent 
ammonium perchlorate, and 17.8 percent sodium nitrate, with 0.2 percent 
sub-micron fumed silica, and pressing the mixture into pellets having a 
density of about 1.6 to 1.65 g/cc. Twenty one grams of these pellets were 
then loaded into the main charge cup of the inflator. 
The enhancer charge was prepared by pressing 32.3 percent RDX, 30.0 percent 
guanidine nitrate, 21.3 percent ammonium perchlorate, 16.2 percent sodium 
nitrate, and 0.2 percent sub-micron fumed silica into 1.3 to 2.5 cm 
diameter pellets or slugs having a thickness of about 0.5 cm and a density 
of about 1.6 to 1.7 g/cc. The slugs were then granulated and sieved to 
produce granules of the enhancer. The required quantity of the granules 
were loaded into the enhancer cup assembly of the inflator. 
The inflator was equipped with a standard automotive air bag initiator, 
containing a zirconium/potassium perchlorate charge. The inflator was also 
equipped with an autoignition material, having an autoignition temperature 
of about 150.degree..+-.5.degree. C. 
When initiated by the air bag initiator, the enhancer and main gas generant 
charges rapidly generate substantially pure, non-toxic gases without any 
harmful side products at a temperature that is not harmful to vehicle 
occupants. A 60 liter closed tank performance test was performed with the 
inflator described above. In this test, the gas generated on initiation 
was exhausted into a closed 60 liter tank. At temperatures ranging from 
-30.degree. C. to 80.degree. C., the inflator produced pressures in the 
tank in excess of 200 kPa in less than 30 ms. A graph of these results is 
presented in FIG. 1. 
Generally, any size vehicle air bag can be inflated in the requisite time 
by employing sufficient amounts of enhancer and main generant charges, 
where the ratio of the volume of the air bag and the required amount of 
gas generant is approximately constant. 
While it is apparent that the invention disclosed herein is well calculated 
to fulfill the objects stated above, it will be appreciated that numerous 
modifications and embodiments may be devised by those skilled in the art. 
Therefore, it is intended that the appended claims cover all such 
modifications and embodiments as falling within the true spirit and scope 
of the present invention.