Gas generating composition for automobile airbags

BACKGROUND OF THE INVENTION
1. Field of Invention
The invention relates to gas generating compositions delivering a non-toxic gas, such as nitrogen, for filling automobile restraint airbags. More particularly, the invention relates to a composition of an alkali metal azide in combination with a heavy metal sulfide and initiating oxidizers to fill the airbag with nitrogen gas.
2. Brief Description of the Related Art
The development of automobile air bags to restrain occupants upon impact in a collision is a landmark in the field of automobile occupant safety. The devices are designed to deploy when vehicles travelling at 12 mph or greater experience sudden impact. The airbag is inflated and provides a soft barrier between the occupant and the interior of the vehicle, thereby averting serious or fatal injuries to an occupant.
Typically, the airbag system fitted in an automobile consists of a sensor, which picks up the crash pulse and with the aid of a booster composition sets off a gas generating composition housed in a module. The released gas fills up a fabric bag forming a barrier between the occupant and the interior of the vehicle. The sensors used operate either on mechanical or electro-mechanical principles. In a mechanical sensor a primer is set off, whereas in an electromechanical sensor an electro-explosive device (i.e., a squibb) is set off. In turn, the squibb sets off a booster composition (Boron-KN0.sub.3) which activates the gas generating composition. The earliest gas generating compositions generated carbon-dioxide, but the state of the art is to generate nitrogen as the preferred airbag filling gas. Representative of the early nitrogen gas generating compositions for automobile airbags are those described in the U.S. Pat. No. 3,741,585 to Hendrickson et al. The state of the art gas generating compositions at the present time comprise an alkali metal azide, an oxidizer, and other additives. The gas generating compositions in use ordinarily use sodium azide as the preferred fuel. A variety of oxidizers have also been used.
Ideally, a gas generating composition for use in airbags should be a solid material, easily formed into pellets. Further, it should be non-hygroscopic and comprised of constituents which are obtainable in a relatively high degree of purity. The gas generating reaction should be easily controllable and generate the gas at the required rates and pressures. Also, the gas should produce a minimal amount of toxic gas residuals like carbon monoxide and oxides of nitrogen. The solids or slag residues formed during the reaction should be minimal and substantially retained in the combustion zone. Particles of the solid residues should be capable of being arrested in the filter system of the device. Most importantly, the slag residues should be non-toxic and generated in minimal amounts for ultimate disposal.
The gas-generating reaction should further be capable of being modified for different particular applications by either change of the physical parameters of the constituents or by use of suitable additives.
SUMMARY OF THE INVENTION
The invention comprises a solid composition which, upon ignition, decomposes into nitrogen gas and non-toxic solid particulates, and which comprises;
a metal azide;
an equivalent weight of a heavy metal sulfide; and
an oxidizing agent selected from the group consisting of a metal oxide, an alkaline metal nitrate, or an alkaline metal perchlorate.
The composition is a low explosive, useful as a nitrogen gas-generating means to inflate airbag components in automobile driver/passenger restraint systems.
The term "low explosive" as used herein means a composition which undergoes autocombustion at rates which are low as compared with the rates of detonation of high explosives.
The use of the compositions of the invention permits modification, control, and activation of the gas-generating reaction. The solid residue particulates carried in the gas stream are within acceptable limits.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION
The metal azides which may be employed in preparing the compositions of the invention are well known as are methods of their preparation. Representative of the metal azides are the alkaline metal azides such as lithium azide, sodium azide, potassium azide; and the alkaline earth metal azides such as calcium azide, barium azide, magnesium azide and the like. The metal azide functions as a fuel, which upon ignition releases nitrogen gas.
The preferred metal azide used as fuel is sodium azide, which has 63% non-toxic nitrogen by weight. Sodium azide is a solid which can be ground into advantageous particle sizes with commercially available comminuting machines. Advantageously, the metal azide has particle sizes within the range of from 5 to 100 microns, preferably 10 to 25 microns.
Though a number of heavy metal sulfides can be used, the preferred heavy metal sulfides are iron sulfides such as ferrous sulfide, iron disulfide and the like. Preferred is ferrous sulfide. To obtain the most advantageous compositions of the invention the iron sulfide should have particle sizes within the range of from about 1 to about 50 microns, preferably 1 to 20 microns.
The control of the particle size of the constituent ingredients used in the compositions of the invention impact upon overall performance, of the gas generating composition particularly in relation to rate of combustion and the time-pressure profile of gas-release. Smaller grain sizes have increased surface area and burn more rapidly. The surface area and density of the compositions may be controlled to meet diverse end uses which should have minimal solids residues.
When initiated to autocombustion, the two ingredients described above react to release nitrogen gas and a residue of non-toxic, finely divided particulate matter which is readily excluded from the nitrogen gas stream.
The reaction may be initiated by the energy provided by a suitable booster material such as Boron-KN0.sub.3. Since the reaction is exothermic, it is self-sustaining. With sodium azide as a representative azide, the reaction can be shown schematically by the equation
2NaN.sub.3 .fwdarw.2Na+3N.sub.2 .uparw.+10.2 kcals (I)
The sodium metal is scavenged in a second step by the heavy metal sulfide, for example ferrous sulfide.
In the second step, the sulfide of iron reacts with the sodium metal to form non-toxic sodium sulphide and iron metal according to the schematic formula:
2Na+FeS.fwdarw.Na2S+Fe (II)
In the case of iron disulfide, the reaction takes place according to the following equation:
FeS.sub.2 +4Na.fwdarw.2Na.sub.2 S+Fe. (III)
By employing the azide and the sulfide reactants in stoichiometric proportions, i.e.; equal equivalent weights, the end products of reaction (II) form a high density solids mixture of non-toxic, finely divided particles that are readily retained in the combustor zone. Only a minute quantity of this solid residue is liable to escape in the high velocity nitrogen gas stream, and even in this instance the escaping solids may be retained within the combustor zone by a series of filters conventionally employed in surrounding the combustor zone. This results in very low levels of slag particulates entering the airbag and is one of the advantages of present invention. Conversely, in most of the gas generating compositions used prior hereto in airbags, sodium metal is converted into sodium oxide, which combines with additives to form a large quantity of slag. It is difficult to make this reaction occur with high efficiency while arresting the large residue of metal oxide particulates in the filter system.
It will be appreciated that the reaction (II) between sodium and ferrous sulfide by itself is generally slow and would not usually be appropriate for an airbag inflating composition. However, we have found that the reaction II can be initiated and accelerated in the presence of a small proportion of an oxidizer such as a metal oxide, an alkaline metal nitrate, an alkaline metal perchlorate and the like. As an oxidizer, potassium perchlorate and ammonium perchlorate are preferred. In the case of ammonium perchlorate, the products are all gaseous and hence do not contribute to particulate residues. Advantageously, the particle sizes of the oxides are within the range of 2 to 30 microns.
Representative of advantageous alkaline metal perchlorates are potassium perchlorate, sodium perchlorate, ammonium perchlorate and the like.
Representative of alkaline metal nitrates are potassium nitrate, sodium nitrate and the like.
The preferred oxidizer is potassium nitrate.
Similarly, high explosive compounds can be used to activate the reaction. High temperature stable, high explosives like nitroguanidine, cyclonite (RDX) and cyclotetramethylenetetranitramine (HMX) can be used in (small percentages) to initiate the reaction between the sodium and the iron sulfide.
Other additives which can be added to the compositions of the invention with advantage are minor proportions of processing aids that would enhance flow and pelletizing such as magnesium silicate and aluminum oxide. Lubricants are conventionally added. Examples of solid lubricants are molybdenum disulfide. As a lubricant, molybdenum disulfide is preferred, since it reacts with the sodium from step (I) in the reaction described above, to produce molybdenum metal and sodium sulfide products. These products in small quantity are not objectionable residues. Other useful additives include ground sulphur or atomized metal powders like aluminum to increase the heat of reaction and ignition capability. These additives are used in conventional proportions, generally not more than about 1-5% by weight of the total composition.
The ingredients of the compositions of the invention may be mixed in available commercial mixers with explosion-proof fittings. The compositions may be pelletized in multi-station rotary pellet presses to the desired weight, thickness and density.
The following examples and preparations describe the manner and process of making and using the invention and set forth the best mode contemplated by the inventor of carrying out the invention but are not to be construed as limiting the invention. Where reported, the following tests were carried out:
A method of assessing the gas generating compositions for diverse end uses is to load them in inflator housings that form a part of an airbag module. Testing is carried out in a static pressure tank of known volume by igniting the composition as used in the airbag system. The pressure-time (P-T) profile, as well as measurement of the toxic residuals in the gas and the particulates, are obtained by washing the tank, filtering, and weighing. Various manufacturers have used different volumes of the static tank and correlated the results to real-time conditions. In the experiments carried out on the gas generating compositions of the invention, a one cubic foot tank was used. To better represent real-time situations, 100 cubic foot is regarded within the industry as representing the interior volume of an automobile. Therefore, the result using the one cubic foot tank is reduced by a factor of 0.01 to approximate a 100 cubic foot volume.
All proportions are reported as percentage by weight.
PROCEDURE
Sodium azide and ferrous sulfide were ground to a selected particle size and mixed together in predetermined proportions with molybdenum disulfide as a lubricant. Magnesium silicate and aluminum oxide were added as flow assisting agents to obtain a homogeneous mix. The mixture was pelletized in a multi-station rotary pelleting press and pelletized to a desired weight, dimension and density.
EXAMPLES 1-5
These examples illustrate the effect of different additives on the functioning characteristics of the composition of the invention. The additives are identified in Table I, below.
TABLE I______________________________________Composition 1 2 3 4 5______________________________________Sodium Azide 58.5 58.0 58.5 58.5 58.0Ferrous Sulfide 38.7 38.0 38.5 38.5 38.0Potassium Nitrate 2.0 3.0Molybdenum Disulfide 1.0 1.0 1.0 1.0 1.0Aluminum 1.0Iron Oxide (Fe.sub.2 O.sub.3) 2.0DNPT* 2.0Sulfur 1.8Load in gms 63 63 63 63 63Pellet Weight in mgms 160 130 130 130 130P-max in test tank (Kpa) 240, 235, 249, 241, 247, 242 243 247 245 242Time for P-max (in 45.7, 43.1, 37.1, 44.9, 38.5,milisecs.) 48.9 68.9 34.5 39.3 32.7Particulate in m. gms 224, 208 141, 72, 65, 199 99 64 47______________________________________ *DNPT = Dinitroso Pentamethylene Tetramine.
EXAMPLES 6-8
The functioning characteristics of the compositions of the invention can be modified by the addition of a high explosive base charge for detonation. The effect of the use of a typical high explosive, like nitroguanidine is illustrated in Table II below and would typify the effect of other high explosives like cyclotrimethylenetrinitramine or cyclonite (RDX) and cyclotetramethylenetetranitramine (HMX). The high explosives, when added, are added in proportions of from about 0.1 to 2 percent by weight.
TABLE II______________________________________Composition 6 7 8______________________________________Sodium Azide 58.0 58.0 58.0Ferrous Sulfide 38.0 38.0 38.0Molybdenum 1.0 1.0 1.0DisulfidePotassium Nitrate 3.0 2.5 2.0Nitroguanidine 0.5 1.0Load in gms. 78 78 78Pellet Weight in 160 160 160m. gms.P-max in K. Pas. 362, 354 367, 383, 364 371, 367, 369dp/dt 16.1 17.3, 20.10, 18.1 14.4, 14.9, 17.0Time for P-max in 49.0, 45.8 41.6, 45.6, 48.6 46.6, 48.8, 53.0m. secsParticulates in 49, 154 287, 228, 309 210, 237, 276m. gms______________________________________
EXAMPLES 9-12
Particle size control aids in providing consistent, repeatable and desired functioning characteristics. The effect of variation of particle size of the main constituents, namely sodium azide and ferrous sulfide is illustrated in Table III, below.
TABLE III______________________________________Composition 9 10 11 12______________________________________Sodium Azide 58 58 58 58 (10-21.mu.) (13-21.mu.) (60.mu.) (60.mu.)Ferrous Sulfide 38 38 38 38 (2-5.mu.) (13-15.mu.) (2-8.mu.) (13-15.mu.)Molybdenum 1.0 1.0 1.0 1.0DisulfidePotassium Nitrate 3.0 3.0 3.0 3.0Load in gms. 78 78 78 78Pellet Wt. in 160 160 160 160M. gms.P-max in K-Pas 362, 354 331, 343, 352, 358, 327, 297, 338 369 310dp/dt 16.1, 16.0 9.8, 11.1, 7.3, 7.3, 6.1, 5.0, 9.6 8.0 5.1Time for P-max in 49.0, 48.8 80.8, 73.2 98.6, 96.8, 98.7, 98.4m. secs. 96.6Particulate in 149, 154 113, 109, 538, 318, 625, 286,m. gms. 102 296 110______________________________________
Particle size of the azide component is smaller in Example 9 relative to the other examples. Example 9 also exhibits a faster pressure/time response relative to the other examples. Smaller particle size effects response time in a favorable manner.
EXAMPLES 13-14
The functioning characteristics of the compositions of the invention can be effected by altering the surface area of the propellant available for burning. The effect of this parameter on the functioning characteristics of the composition of the invention is given in Table IV, below.
TABLE IV______________________________________Composition 13 14______________________________________Sodium Azide 58.0 58.0Ferrous Sulfide 38.0 38.0Molybdenum Disulfide 1.0 1.0Potassium Nitrate 3.0 3.0Load in gms 78 78Pellet Weight in mgms 160 160Surface Area Available in SQ 436 623mms/gm PropellantP-max in Kpas 334, 338 362, 354dp/dt 10.7, 11.0 16.1, 16.0Time for Pmax in miliseconds 64.6, 67.2 49.0, 48.8Total Particulates in mili-grams 177, 135 149, 154______________________________________
Increased surface area results in a faster pressure time response, and therefore influences response time in a favorable manner. Advantageously, the surface area available is within the range of from about 200 to 1000 mms/gms, preferably 400 to 800.
EXAMPLES 15-16
The density of the pellets has considerable effect on the functioning characteristics of the composition. This example illustrates the effect of this parameter on the composition of the invention and detailed in Table V, below.
TABLE V______________________________________Composition 15 16______________________________________Sodium Azide 58.0 58.0Ferrous Sulfide 38.0 38.0Molybdenum Disulfide 1.0 1.0Potassium Nitrate 3.0 3.0Load in gms 78 78Pellet Weight in miligms 160 160Density of Pellets in gms/cc 2.0 2.25Pmax in Kpas 403, 397, 399 362.3, 354.4dp/dt 22.1, 24.4, 24.8 16.1, 16.0Time for Pmax in miliseconds 39.0, 38.0, 41.0 49.0, 48.8Total Particulates in miligms 204, 268 149, 154______________________________________
A density range of from about 1.5 to 2.75 gms/cc is advantageous, preferably 2.0 to 2.15.
EXAMPLES 17-19
By varying the load of the propellant used, the functioning characteristics can be altered. The effect of varying the load of the propellant is set forth in Table VI.
TABLE VI______________________________________Composition 17 18 19______________________________________Sodium Azide 58.0 58.0 58.0Ferrous Sulfide 38.0 38.0 38.0Molybdenum Disulfide 1.0 1.0 1.0Potassium Nitrate 3.0 3.0 3.0Load in gms 63 78 86Pellet Weight in milligms 160 160 160P-max in K. Pas. 267, 262 362, 354 402, 403dp/dt 10.1, 9.2 16.1, 16.0 21.6, 19.6Time for P-max in millisecs 58.6, 57.4 49.0, 48.8 41.2, 43.2Total Particulates in 144, 90 149, 154 267, 319milligms______________________________________
While pressure-time response is somewhat slower for heavier loads, higher maximum pressures are realized in relatively shorter periods of time.
EXAMPLE 20
Sodium azide and ferrous sulfide can be mixed together in equal equivalent weight proportions after comminuting them to desired particle sizes, along with molybdenum disulfide as a lubricant. A gas generating composition of this kind has the following functioning characteristics.
TABLE VII______________________________________Sodium Azide 59.5Ferrous Sulfide 39.5Molybdenum Disulfide 1.0Load in gms 78Weight of Pellet in milligms 160P-max in K-Pas 345, 346, 350dp/dt 14.8, 13.8, 13.2Time of Pmax in milliseconds 50.8, 51.0, 56.4Particulates in milligms 292.3, 350.7, 345.0______________________________________
EXAMPLES 21-24
Ferrous sulfide may be replaced with iron disulfide. The reaction takes place in a manner as indicated earlier with the formation of an innocuous solid as slag containing iron and sodium sulfide. A typical composition made in this manner and tested under different loads and conditions, has results as indicated in Table VIII, below.
TABLE VIII______________________________________Composition 21 22 23 24*______________________________________Sodium Azide 67.0 67.0 67.0 67.0Iron Disulfide 31.0 31.0 31.0 31.0Magnesium Silicate** 1.0 1.0 1.0 1.0Aluminum Oxide 1.0 1.0 1.0 1.0Load in gms. 65 69.5 68.0 69.5Pellet Wt. in M. gms. 160 160 160 160P-max in K-Pas 331, 333, 373, 319, 350, 333, 336 367 322 354dp/dt 13.5, 12.5, 15.4, 16.6, 12.7, 12.9, 13.2 15.0 16.9 14.1Time for P-max in m. secs. 57.8, 59.2, 55.2, 46.2, 59.6, 62.2, 59.4 54.0 46.0 53.0Total Particulate in 204.9, 412, 95.7, 136.7,m. gms. 235.0, 303 115.2 167.7 185.0, 242______________________________________ *Utilized a modified filter system, different from Examples 21 and 22. Whereas Examples 21 and 22 were conducted with a 25.mu. screen as the final particulate control filter, Examples 23 and 24 were conducted with an additional 40.mu. screen in front of the 25.mu. screen. **MAGNESOL .RTM., Reagent Chemical and Research Inc., 124 River Road, Middlesex, New Jersey, Technical Brochure Rev. 1, July 1986.
EXAMPLES 25-26
The potassium nitrate oxidizer used to activate the composition can be replaced by potassium perchlorate after grinding it to a desired size. A typical composition made using potassium perchlorate and its effect on the functioning characteristics at various loads are illustrated in Table IX, below.
TABLE IX______________________________________Composition 25 26______________________________________Sodium Azide 59.0 59.0Ferrous Sulfide 39.0 39.0Molybdenum Disulfide 1.0 1.0Potassium Perchlorate 1.0 1.0Pellet wt. in mgs 160 160Load in gms 78 gm 92 gmDensity of Pellet in gms/cc 2.25 2.25Pmax in K-Pas 354, 347, 346 401, 413, 418dp/dt 14.8, 13.5, 13.1 14.5, 14.4, 16.6Time for Pmax in m. secs 45.4, 45.4, 47.0 52.4, 54.4, 47.0Total Particulate in m. gms 209, 208, 216 305, 336, 332Test condition Ambient Ambient______________________________________
EXAMPLES 27-28
The potassium nitrate oxidizer used to activate the composition can be replaced by ammonium perchlorate after grinding it to a desired size. A typical composition made using ammonium perchlorate and its effect on the functioning characteristics at various loads is illustrated in Table X, below.
TABLE X______________________________________Composition 27 28______________________________________Sodium Azide 59.0 59.0Ferrous Sulfide 39.5 39.5Ammonium Perchlorate 1.5 1.5Pellet wt. in mgs 220.0 220.0Load in gms 76 gm 86 gmDensity of Pellet in gms/cc 2.25 2.25Pmax in K-Pas 334, 325, 330 416, 413, 416dp/dt 16.1, 17.1, 14.1 23.1, 21.8, 20.2Time for Pmax in m. secs 54.6, 48.2, 54.4 47.4, 50.8, 47.2Total Particulate in m. gms 563, 467, 658 766, 712, 741Test Condition Ambient Ambient______________________________________

Number	Name	Date
3741585	Hendrickson et al.	Jun 1973
3947300	Passauer et al.	Mar 1976
4062708	Goetz	Dec 1977
4370181	Lundstrom et al.	Jan 1983
4533416	Poole	Aug 1985
4834818	Kazumi et al.	May 1989
4931111	Poole et al.	Jun 1990
5089069	Ramaswamy et al.	Feb 1992

Gas generating composition for automobile airbags

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