US20070084531A1

US20070084531A1 - Gas generant

Info

Publication number: US20070084531A1
Application number: US11/540,841
Authority: US
Inventors: Jeffrey Halpin; Sean Burns
Original assignee: Automotive Systems Laboratory Inc
Current assignee: Automotive Systems Laboratory Inc
Priority date: 2005-09-29
Filing date: 2006-09-29
Publication date: 2007-04-19
Also published as: WO2007041384A2; WO2007041384A3; JP2009512613A

Abstract

Gas generant compositions contain the mono-ammonium salt of bis (1(2) H-tetrazol-5-yl)-amine, an oxidizer such as phase stabilized ammonium nitrate, and a first additive selected from fumed oxides such as fumed silica. Gas generators 10 and gas generating systems 200 incorporating the compositions are also contemplated.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser. No. 60/721,873 filed on Sep. 29, 2005.

TECHNICAL FIELD

The present invention relates generally to gas generating systems, and to gas generant compositions employed in gas generator devices for automotive restraint systems, for example.

BACKGROUND OF THE INVENTION

The present invention relates to gas generant compositions that upon combustion produce a relatively small amount of solids and a relatively abundant amount of gas. It is an ongoing challenge to reduce the amount of solids and increase the amount of gas thereby decreasing the filtration requirements for an inflator. As a result, the filter may be either reduced in size or eliminated altogether thereby reducing the weight and/or size of the inflator.
An equally important challenge is to manufacture gas generants that exhibit relatively low sensitivity with regard to impact, friction, or electrostatic discharge stimuli.
Yet another challenge with gas generant compositions that produce relatively small amounts of solids, sometimes known as “smokeless” compositions, is that not all non-metallic constituents contribute to stable ballistic performance when subjected to environmental conditioning. In fact, one fuel that is favored because of its propensity to produce all or mostly gas is the mono-ammonium salt of bis-1(2)H-tetrazol-5-yl)-amine (BTA-1NH3). When combined with other gas generant constituents such as an oxidizer, and formed into a gas generant composition, this fuel contributes to greater amounts of gas upon combustion of the composition. It has nevertheless been discovered that BTA-1NH3 contributes to an unacceptably aggressive ballistic performance as measured after thermal cycling and thermal shock testing defined in SAE International Document SAE/USCAR-24 “USCAR INFLATOR TECHNICAL REQUIREMENTS AND VALIDATION”, herein incorporated by reference.
Accordingly, it would be an improvement in the art to provide compositions that contain BTA-1NH3 that contribute to a “smokeless” gas generant composition, or one that when combusted produces 90% or more of gas as a product, while yet passing all thermal shock requirements as set forth in USCAR standards.

SUMMARY OF THE INVENTION

The above-referenced concerns are resolved by gas generating compositions including BTA-1NH3, an oxidizer such as phase stabilized ammonium nitrate, and a fumed oxide such as fumed silica or fumed alumina. It has been found that the addition of fumed silica or fumed oxides to compositions containing BTA-1NH3 has resulted in compositions that are now able to withstand the thermal cycling/thermal shock tests required by USCAR standards. Other constituents including processing aids such as graphite, may be included in relatively small amounts.
In further accordance with the present invention, a gas generator and a vehicle occupant protection system incorporating the gas generant composition are also included.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional side view showing the general structure of an inflator in accordance with the present invention;
FIG. 2 is a schematic representation of an exemplary vehicle occupant restraint system containing a gas generant composition in accordance with the present invention.
FIGS. 3 and 4 are graphical representations of a composition containing 0.25% by weight fumed silica.
FIGS. 5 and 6 are graphical representations of a composition containing 0.50% by weight fumed silica.
FIGS. 7 and 8 are graphical representations of a composition containing 0.75% by weight fumed silica.

DETAILED DESCRIPTION

The present invention includes gas generant compositions that in accordance with the present invention, incorporate an additive into compositions containing BTA-1NH3 that, when added at relatively low levels, stabilizes the propellant grains when subjected to thermal cycling or thermal shock conditioning, as required for use in the automotive industry. These formulations generally contain the following:
A first oxidizer selected from the group including nonmetal and metal nitrate salts such as ammonium nitrate, phase-stabilized ammonium nitrate, potassium nitrate, strontium nitrate; nitrite salts such as potassium nitrite; chlorate salts such as potassium chlorate; metal and nonmetal perchlorate salts such as potassium or ammonium perchlorate; oxides such as iron oxide and copper oxide; basic nitrate salts such as basic copper nitrate and basic iron nitrate; and mixtures thereof. The first oxidizer is generally provided at about 0.1-80 wt% of the gas generant composition, and more preferably at about 10-70 wt%.
An optional secondary oxidizer may also be provided and selected from the oxidizers described above, and when included is generally provided at about 0.1-50 wt%, and more preferably at about 0.1-30 wt%. The total oxidizer component, that is the combined weight percent of all oxidizers, will nevertheless only range from 0.1 to 80 wt%.
A first or primary fuel consists of mono-ammonium salt of bis-(1(2)H-tetrazol-5-yl)-amine (BTA-1NH3) and is generally provided at about 0.1-50 weight percent or wt%, and more preferably at about 10-30 wt%.
An optional secondary fuel is selected from the group containing derivatives of bis-(1(2)H-tetrazol-5-yl)-amine, including its anhydrous acid and its acid monohydrate, from metal salts thereof including the potassium, sodium, strontium, copper, boron, zinc salts of BTA-1NH3, and complexes thereof; azoles such as 5-aminotetrazole; metal salts of azoles such as potassium 5-aminotetrazole; nonmetal salts of azoles such as mono-or di-ammonium salt of 5, 5′-bis-1H-tetrazole; nitrate salts of azoles such as 5-aminotetrazole nitrate; nitramine derivatives of azoles such as 5-nitraminotetrazole; metal salts of nitramine derivatives of azoles such as di-potassium 5-nitraminotetrazole; nonmetal salts of nitramine derivatives of azoles such as mono- or di-ammonium 5-nitraminotetrazole and; guanidines such as dicyandiamide; salts of guanidines such as guanidine nitrate; nitro derivatives guanidines such as nitroguanidine; azoamides such as azodicarbonamide; nitrate salts of azoamides such as azodicarbonamidine dinitrate; and mixtures thereof, and when included is generally provided at about 0.1-49.9 wt%, more preferably 0.1-30 wt%. The total fuel component, that is the combined amount of all of the fuels of the composition, will nevertheless only range from 0.1-50 wt%, and more preferably about 0.1-30 wt%.
In accordance with the present invention, a first or primary additive is selected from the group of fumed metal oxides including fumed silica and fumed alumina, and mixtures thereof, and is generally provided at about 0.05-10 wt%, and more preferably at about 0.05-5 wt%.
All percentages for the constituents described herein are presented as weight percents of a total gas generant weight.
An optional second additive is selected from the group including silicon compounds including elemental silicon, silicon dioxide, and fused silica; silicones such as polydimethylsiloxane; silicates such as potassium silicates; natural minerals such as talc, mica, and clay; lubricants such as graphite powder or fibers, magnesium stearate, boron nitride, molybdenum sulfide; and mixtures thereof, and when included is generally provided at about 0.1-10%, and more preferably at about 0.1-5%.
An optional binder is selected from the group of cellulose derivatives such as cellulose acetate, cellulose acetate butyrate, carboxymethycellulose, salts of carboxymethylcellulose, carboxymethylcellulose acetate butyrate; silicone; polyalkene carbonates such as polypropylene carbonate and polyethylene carbonate; and mixtures thereof, and when included is generally provided at about 0.1-10%, and more preferably at about 0.1-5%.
All percentages for the constituents described herein are presented as weight percents of the total gas generant weight.
It has been determined that the addition of small amounts of fumed metal oxides, such as fumed silica (M-5 Grade provided by the Cabot Corporation), to these formulations provides a gas generant which exhibits all of the favorable properties listed above, and, more importantly, exhibits stable ballistic performance when subjected to thermal cycling or thermal shock conditioning.
The mono-ammonium salt of BTA-1NH3, when combined with PSAN, exhibits many favorable qualities for use in automotive passenger restraints, and therefore forms preferred gas generating compositions. BTA-1NH3 is a high energy, high-nitrogen fuel which exhibits excellent stability and very favorable levels of hygroscopicity and sensitivity. The properties of ammonium nitrate and potassium nitrate, for example, are well known throughout the propellant industry. PSAN, more specifically, exhibits no sensitivity when subjected to impact, friction, or electrostatic discharge stimuli.
Dry mixes of formulations containing these materials were made. The raw materials were ground together for 15 minutes in a Sweco vibratory mill. The dry material was then tableted, loaded into inflators, and subjected to USCAR Thermal Shock conditioning (200 Cycles, −40 C to 90 C). These formulations showed an increase in ballistic performance when deployed at +85 C.
Next, the same process was used to make gas generants containing the above listed materials with 0.25%, 0.5%, and 0.75% M-5 Silica by weight. After USCAR Thermal Shock conditioning, it was found that the ballistic stability of the gas generant increased with the amount of fumed silica, thereby providing the stability required for use in the automotive industry. Iterative analysis of various amounts of fumed silica in various compositions determined the amount of fumed silica or fumed oxide employed to assure acceptable ballistic performance. Accordingly, gas generant compositions made in accordance with the present invention exhibit many favorable characteristics for use in the auto industry, while avoiding many of the drawbacks of gas generants listed in the prior art.

EXAMPLE 1

Silica-free Mixtures

A smokeless gas generant was produced by mixing Phase-Stabilized Ammonium Nitrate (PSAN) containing 10% by weight Potassium Nitrate, with bis-(1(2)H-tetrazol-5-yl)-amine, mono-ammonium salt (BTA-1NH3). The mixture was substantially in stoichiometric balance. The components were ground dry for about 15 minutes within a Sweco Vibratory Jar Mill.
The resultant mixture was pressed into tablets, as is standard in the industry. The tablets were then loaded into a single-stage driver-side inflator for ballistic evaluation. Several inflators were deployed to provide baseline data while other inflators were subjected to thermal shock conditioning per SAE International Document SAE/USCAR-24 “USCAR INFLATOR TECHNICAL REQUIREMENTS AND VALIDATION”. Upon completion of 200 thermal shock cycles, several inflators were deployed to compare with baseline ballistics. As required by USCAR specifications, inflators were deployed at temperatures of −40 C, +23 C, and +85 C. The −40 C and +23 C deployments match the baseline data fairly well, while the +85 C ballistic data showed a significant increase in performance. The increase was large enough to cause the test series to fail to meet the USCAR specification and therefore makes the inflators unsuitable for use in the automotive industry.

Example 2

High Silica Levels

To alleviate this problem, varying amounts of fumed silica were added to the mixture. The fumed silica was commercially available as M-5 Grade provided by Cabot Corporation. Initially, the fumed silica was added at levels between about 3-6% by mass. The stoichiometric balance of the fuel and oxidizer and processing were kept the same. The resultant gas generants were then ballistically evaluated via the same method described above. After thermal shock conditioning, no change had occurred in the ballistic performance. However, the addition of such a large amount of “inert” material detracted from or inhibited the energy of the system and therefore made the formulations not as desirable.

Example 3

Low (Preferred) Silica Levels

Next, the amount of fumed silica was reduced to test what level was required to pass thermal shock conditioning. Three new mixtures were made via the same processing using 0.25%, 0.5%, and 0.75% fumed silica by mass. The new mixtures were tableted and ballistically evaluated via the same method as described above.
0.25% Silica (Mixture 1)
The ballistic data for the mixture containing 0.25% silica is illustrated in FIGS. 3 and 4. The pressure was measured inside the inflator and inside a 60-L tank during deployment prior to thermal shock tests and is represented in FIG. 3. FIG. 4 illustrates the results of combusting this mixture after thermal shock testing where a spike in the pressure indicates excessively aggressive ballistic performance. One of the thermal shock inflator pressures actually increased high enough to fracture the inflator body. These results are unfavorable for use in the automotive industry. A few tablets were weighed and measured to determine density both before and after thermal shock conditioning. The crush strength was also measured for comparison. This data is shown in Table 1.
0.5% Silica (Mixture 2)
The ballistic data for the mixture containing 0.5% silica can be seen in FIGS. 5 and 6. The pressure was measured inside the inflator and inside a 60-L tank during deployment prior to thermal shock tests and is represented in FIG. 5. FIG. 6 illustrates the results of combusting this mixture after thermal shock testing wherein the pressure curves indicate excessively aggressive ballistic performance. The pressure was measured inside the inflator and inside a 60-L tank during deployment. The data from the thermal shock inflators showed an improvement over the mixture containing 0.25% silica. This improvement, however, was not sufficient to make the inflators viable for use according to the USCAR specification. A few tablets were weighed and measured to determine density both before and after thermal shock conditioning. The crush strength was also measured for comparison. This data is shown in Table 1.
0.75% Silica (Mixture 3)
The ballistic data for the mixture containing 0.75% silica can be seen in FIGS. 7 and 8. The pressure was measured inside the inflator and inside a 60-L tank during deployment prior to thermal shock tests and is represented in FIG. 7. FIG. 8 illustrates the results of combusting this mixture after thermal shock testing wherein the pressure indicates consistent ballistic performance before and after thermal shock. The pressure was measured inside the inflator and inside the 60-L tank during deployment. The data from the thermal shock inflators again showed an improvement over the mixture containing 0.5% silica. Stated another way, the ballistic performance after thermal shock indicates a minimal change. Analysis of this data determined these inflators acceptable according to the USCAR specification. A few tablets were weighed and measured to determine density both before and after thermal shock conditioning. The crush strength was also measured for comparison. This data can be seen in Table 1.

TABLE 1

Physical Properties of 0.25″ OD × 0.125″ Tablets

Propellant Density (g/cm{circumflex over ( )}3) Crush Strength (kp)

Mixture 1

0.25% Baseline 1.65 21.0

0.25% Thermal Shock 1.60 18.3

Mixture 2

0.5% Baseline 1.65 23.2

0.5% Thermal Shock 1.60 21.0

Mixture 3

0.75% Baseline 1.66 24.6

0.75% Thermal Shock 1.61 22.9

The results of this experiment are counterintuitive according to the density and crush strength of the individual tablets. All three mixtures appear to be nearly identical in density and crush strength, both before and after thermal shock. Accordingly, it is not apparent that the use of fumed silica in the varying amounts would improve the ballistic properties as described. Mixture 3, however, performs significantly better than Mixture 2, which performs significantly better than Mixture 1.
As shown in FIG. 1, an exemplary inflator incorporates a dual chamber design to tailor the force of deployment an associated airbag. In general, an inflator containing a primary gas generant 12 formed as described herein, may be manufactured as known in the art. U.S. Pat. Nos. 6,422,601, 6,805,377, 6,659,500, 6,749,219, and 6,752,421 exemplify typical airbag inflator designs and are each incorporated herein by reference in their entirety.
Referring now to FIG. 2, the exemplary inflator 10 described above may also be incorporated into a gas generating system or airbag system 200. Airbag system 200 includes at least one airbag 202 and an inflator 10 containing a gas generant composition 12 in accordance with the present invention, coupled to airbag 202 so as to enable fluid communication with an interior of the airbag. Airbag system 200 may also include (or be in communication with) a crash event sensor 210. Crash event sensor 210 includes a known crash sensor algorithm that signals actuation of airbag system 200 via, for example, activation of airbag inflator 10 in the event of a collision.
Referring again to FIG. 2, airbag system 200 may also be incorporated into a broader, more comprehensive vehicle occupant restraint system 180 including additional elements such as a safety belt assembly 150. FIG. 2 shows a schematic diagram of one exemplary embodiment of such a restraint system. Safety belt assembly 150 includes a safety belt housing 152 and a safety belt 100 extending from housing 152. A safety belt retractor mechanism 154 (for example, a spring-loaded mechanism) may be coupled to an end portion of the belt. In addition, a safety belt pretensioner 156 containing propellant 12 and autoignition 14 may be coupled to belt retractor mechanism 154 to actuate the retractor mechanism in the event of a collision. Typical seat belt retractor mechanisms which may be used in conjunction with the safety belt embodiments of the present invention are described in U.S. Pat. Nos. 5,743,480, 5,553,803, 5,667,161, 5,451,008, 4,558,832 and 4,597,546, incorporated herein by reference. Illustrative examples of typical pretensioners with which the safety belt embodiments of the present invention may be combined are described in U.S. Pat. Nos. 6,505,790 and 6,419,177, incorporated herein by reference.
Safety belt assembly 150 may also include (or be in communication with) a crash event sensor 158 (for example, an inertia sensor or an accelerometer) including a known crash sensor algorithm that signals actuation of belt pretensioner 156 via, for example, activation of a pyrotechnic igniter (not shown) incorporated into the pretensioner. U.S. Pat. Nos. 6,505,790 and 6,419,177, previously incorporated herein by reference, provide illustrative examples of pretensioners actuated in such a manner.
It should be appreciated that safety belt assembly 150, airbag system 200, and more broadly, vehicle occupant protection system 180 exemplify but do not limit gas generating systems contemplated in accordance with the present invention. Further, the compositions described above do not limit the present invention.
It should be understood that the preceding is merely a detailed description of various embodiments of this invention and that numerous changes to the disclosed embodiments can be made in accordance with the disclosure herein without departing from the scope of the invention. The preceding description, therefore, is not meant to limit the scope of the invention. Rather, the scope of the invention is to be determined only by the appended claims and their equivalents.

Claims

1. A gas generant composition comprising:

a first oxidizer selected from metal and nonmetal nitrates; metal nitrites, metal and nonmetal perchlorates, metal oxides, basic metal nitrates, said first oxidizer provided at about 0.1 to 80% by weight of the gas generant composition;

a first fuel consisting of mono-ammonium salt of bis-(1(2) H-tetrazol-5-yl)amine, mono-ammonium salt, said first fuel provided at about 0.1 to 50% by weight of the gas generant composition; and

a first additive selected from fumed metal oxides, said first additive provided in amounts no greater than 10% by weight of the gas generant composition.

2. The gas generant composition of claim 1 further comprising:

a second fuel selected from derivatives of bis-(1(2) H-tetrazol-5-yl)-amine including its anhydrous acid, its acid monohydrate, metal salts, and complexes thereof; azoles, metal salts of azoles, nonmetal and metal salts of nitramine derivatives of azoles; guanidines; salts of guanidines; nitro derivatives of guanidines; azoamides; nitrate salts of azoamides, and mixtures thereof; said second fuel provided at about 0.1-49.9% by weight of the gas generant composition.

3. The gas generant composition of claim 1 further comprising:

a second additive selected from silicon, silicon dioxide, fused silicon, silicones; silicates; natural minerals including clay, mica, and talc; lubricants including graphite, magnesium stearate, boron nitride, molybdenum sulfide; and mixtures thereof, said second additive provided at about 0.1 to 10% by weight of the gas generant composition.

4. The gas generant composition of claim 1 further comprising a secondary oxidizer.

5. The gas generant composition of claim 1 further comprising a binder selected from cellulose derivatives, polyalkenes carbonates, and mixtures thereof, said binder provides at about 0.1 to 10% by weight of the gas generant composition.

6. A gas generator containing the composition of claim 1.

7. A vehicle occupant protection system containing the composition of claim 1.

8. The gas generant composition of claim 1 comprising the mono-ammonium salt of bis (1(2) H-tetrazol-5-yl)-amine at about 0.1 to 50% by weight of the gas generant composition, phase stabilized ammonium nitrate at about 0.1 to 80% by weight of the gas generant composition, and fumed silica at about 0.1 to 10% by weight of the gas generant composition.

9. The gas generant composition of claim 1 wherein said first oxidizer is selected from ammonium nitrate, phase stabilized ammonium nitrate, potassium nitrate, strontium nitrate, potassium nitrite, potassium chlorate, potassium perchlorate, ammonium perchlorate, iron oxide, copper oxide, basic copper nitrate, basic iron nitrate, and mixtures thereof.

10. The gas generant composition of claim 2 wherein said second fuel is selected from a potassium, sodium, strontium, copper, boron and zinc salt of bis-(1(2)H-tetrazol-5-yl)-amine; 5-aminotetrazole; potassium 5-aminotetrazole; mono-ammonium salt of 5,5-bis-1H-tetrazole and di-ammonium salt of 5,5-bis-1 H-tetrazole; 5-aminotetrazole nitrate; nitraminotetrazole; di-potassium 5-nitram inotetrazole; mono-ammonium nitram inotetrazole and d i-am monium nitram inotetrazole; dicyandiamide; guanidine nitrate; nitroguanidine; azodicarbonamide, azodicarbonamidine di-nitrate; and mixtures thereof.

11. The gas generant composition of claim 1 wherein said fumed metal oxide is selected from fumed silica and fumed alumina.

12. The gas generant composition of claim 1 wherein said fumed metal oxide is fumed silica provided at about 0.75% by weight of the gas generant composition.