WO2007038803A2

WO2007038803A2 - Gas generant

Info

Publication number: WO2007038803A2
Application number: PCT/US2006/038955
Authority: WO
Inventors: Sean P. Burns; Jeffrey W. Halpin; Graylon K. Williams; Paresh S. Khandhadia; Deborah L. Hordos; Jason Newell
Original assignee: Automotive Systems Laboratory, Inc.
Priority date: 2005-09-30
Filing date: 2006-10-02
Publication date: 2007-04-05
Also published as: WO2007038803A3; JP2009509908A; US20070084532A1; DE112006002624T5

Abstract

The present invention includes a gas generator 10 that incorporates a gas generatin composition 12 and a scavenging additive 16 in heterogeneous but vapor/gaseous communication with the gas generating composition 12. The scavenging additive retains moisture/contaminants typically evolving over time at relatively higher temperatures. The present invention further includes a gas generating system 180 incorporating the gas generator 10.

Description

GAS GENERANT

CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of U.S. Provisional Application Serial No.

60/722,168 filed on September 30, 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 bis-1(2)H-tetrazol-5-yl)-amine (BTA-1NH3) and derivatives thereof. 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.

Furthermore, some gas generants, such as those formed by the combination of the monoammonium salt of bis-(1(2)H-tetrazol-5-yl)-amine (BTA-1NH3) with ammonium nitrate or phase stabilized ammonium nitrate (PSAN) exhibit many favorable qualities, such as high gas production, and therefore are useful in automotive passenger restraints. BTA-1 NH3 is a high energy, high-nitrogen fuel that exhibits excellent stability and very favorable levels of hygroscopicity and sensitivity. The properties of ammonium nitrate and potassium nitrate when co-precipitated include minimal or no sensitivity when subjected to impact, friction, and electrostatic discharge stimuli. One concern with PSAN-containing propellants as well is that they exhibit significant aggressive behavior with regard to ballistic properties, particularly with respect to USCAR Thermal Shock conditioning when ballistically tested at elevated temperatures (the industry standard is about 85C).

It is also required that airbag inflators be subjected to environmental conditioning, such as high temperature heat aging, thermal aging, thermal cycling, thermal shock, humidity cycling, and so forth. These extreme tests can cause many problems, ranging from failure to inflate the airbag to over-pressurization of the inflator leading to rupture. It is therefore desirable to have a gas generant and inflator system that performs the same regardless of conditioning. The present invention provides a solution to many of these possibilities.

Many gas generants are moisture-sensitive due to the inherent hygroscopicity of many common components including oxidizers such as nitrate salts, naturally occurring hydrates of fuels such as 5-aminotetrazole, bis-(1(2)H-tetrazol-5-yl)-amine, carboxymethyl cellulose and its salts, clays, and others. It is therefore imperative that these types of gas generants are kept dry for repeatable performance.

Moisture or volatile contaminants can be introduced to gas generating systems in many ways.

A few examples include: improperly processed gas generants that contain excess moisture; moisture introduced to the system via humidity during assembly; moisture introduced to the system during environmental conditioning such as high humidity cycling or salt spray; moisture introduced to the system via decomposition of materials within the system such as auto-ignition materials, seals, gaskets, greases, and other gas generator constituents. The use of certain additives, preferably zeolites such as molecular sieves, calcium sulfate (Drierite), or calcium oxide can minimize, and in some cases eliminate any problems that may occur in these situations.

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 cycling requirements as set forth in USCAR standards. SUMMARY OF THE INVENTION

The above-referenced concerns are resolved by gas generators containing gas generating compositions including a fuel such as BTA-1NH3 and an oxidizer such as phase stabilized ammonium nitrate. The gas generator contains a separate additive or contaminant scavenger such as molecular sieves. The separate additive is an adsorbent or absorbent contained within the gas generator to facilitate scavenging of any moisture or volatiles as it evolves or occurs within the propellant bed. It has been found that the addition of adsorbents or absorbents such as molecular sieves has resulted in compositions that are now able to withstand the thermal cycling/thermal shock tests required by USCAR standards.

The use of certain additives, preferably zeolites such as molecular sieves, calcium sulfate (Drierite™), or calcium oxide can minimize, and in some cases eliminate contaminants occurring within the inflator over time. In particular, molecular sieves function to better retain moisture and other contaminants in the presence of relatively high temperatures such as 107 degrees Celsius, thereby ensuring predictable performance of associated inflators. Stated another way, molecular sieves retain moisture/contaminants at elevated temperatures at least as high as 107 degrees Celsius thereby facilitating consistent performance predictability of inflators incorporating one or more types of these additives.

Accordingly, the present invention includes a gas generator containing a gas generating composition and a scavenging additive that is provided within the gas generator in heterogenous relationship with the gas generating composition, and in vapor/gaseous communication therewith.

In further accordance with the present invention, a gas generator and a vehicle occupant protection system incorporating the gas generant composition and scavenging additive 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.

FIG. 3 is a graphical representation of ballistic performance of an inflator not containing an additive such as an adsorbent or absorbent in accordance with the present invention. FIG. 4 is a graphical representation of ballistic performance of an inflator containing an additive such as an adsorbent or absorbent in accordance with the present invention.

DETAILED DESCRIPTION

The present invention includes gas generator or gas generating system that in accordance with the present invention, incorporates an additive separate from and functioning as an adjunct to gas generating compositions containing BTA-1 NH3 or other fuels 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 constituents described below although other constituents known in the art may be employed as well.

As shown in FIG. 1, the gas generant 12 is contained within a gas generant chamber 14 of the inflator 10. The scavenging additive 16 is intermingled or sprinkled in heterogeneous relationship to the gas generant 12. It should be emphasized that the additive 16 need not be mixed directly with the gas generant 12, but is instead added to the bed to act independently of the gas generant 12 thereby scavenging various contaminants and moistures due to decomposition over time for example.

Gas generating compositions of the present invention contain 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 is provided. 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 within the gas generant compositions, and is 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 combined oxidizer component is nevertheless only provided at about 0.1-80 wt% of the gas generant composition.

The gas generant composition contains a first or primary fuel selected from the group containing derivatives of bis-(1(2)H-tetrazol-5-yl)-amine, including its anhydrous acid and its acid monohydrate, mono-ammonium salt of bis-(1(2)H-tetrazol-5-yl)-amine, metal salts of bis- (1(2)H-tetrazol-5-yl)-amine including the potassium, sodium, strontium, copper, boron, zinc salts of BTA-1 NH3, 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-1 H-tetrazoIe; 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 dipotassium 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 is generally W 2

provided at about 0.1-50 wt%, more preferably 0.1-30 wt%.

The gas generant composition may also contain an optional additive selected from the group including silicone compounds; elemental silicon; silicon dioxide; fused silica; silicones such as polydimethylsiloxane; silicates such as potassium silicates; natural minerals such as talc 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 may be included in the gas generant composition and is selected from the group of cellulose derivatives such as cellulose acetate, cellulose acetate butyrate, carboxymethycellulose, salts of carboxymethylcellulose, carboxymethyl cellulose 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 a total gas generant weight.

In accordance with the present invention, the scavenging additive 16 is any additive that will absorb, adsorb, or chemically react with water and/or other volatiles that may be detrimental to stability or performance of gas generants and inflation devices. Typically compounds such as carbon monoxide, nitrogen oxides, ammonia, chlorine, cyanic acid, chlorites, and acids such as HONO, and chloric acid are liberated as the propellant ages. An example of an absorbent includes calcium sulfate as it forms a hydrate. An adsorbent may be selected from molecular sieves, montmorillonite clay, other zeolites, silica gel, and mixtures thereof, as these compounds can adsorb and contain H₂O and NO_x. Molecular sieves are preferred because of their enhanced ability to retain moisture/contaminants at relatively higher temperatures thereby ensuring they do not interfere with the combustion reaction of the gas generant composition. An example of a chemical reactant is calcium oxide as it will react with water to form stable solid products. The scavenging additive 16 is generally provided at about 0.1-10 wt%, and more preferably at about 0.1-5 wt% of the total weight of the gas generant.

Nevertheless, it will be appreciated that gas generators of the present invention preferably contain at least one or more moles of scavenging additive per mole of contaminant that evolves over time. Accordingly, each gas generant composition may be evaluated on an iterative basis as to what amounts of contain inant/volatiles/moisture will evolve or decompose over time, and the amount of scavenging additive may then be determined. In practice, although not required, the scavenging additive should be provided in excess amounts thereby ensuring that each mol of volatile/contaminant/moisture is either adsorbed, absorbed, or reacted with the additive thereby ensuring the integrity of the gas generator performance upon combustion of the gas generant. The weight percent amounts given above are designed with this approach in mind.

An optional binder is selected from the group of cellulose derivatives such as cellulose acetate, cellulose acetate butyrate, carboxymethycellulose, salts of carboxymethylceliulose, carboxymethyl cellulose 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%.

It has been determined that the addition of small amounts of adsorbents or absorbents 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 monoammonium salt of BTA-1NH3, when combined with PSAN, exhibits many favorable qualities for use in automotive passenger restraints and combines to form preferred gas generating compositions. BTA-1 NH3 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.

EXAMPLE 1 : To form comparative compositions, dry mixes of formulations containing the various constituents described above were prepared in a known manner. The dry material (containing less than 0.2% moisture by mass) was then tableted, loaded into inflators, and subjected to USCAR Thermal Shock conditioning (200 Cycles, -40C to 90C). These formulations indicated an increase in aggressive ballistic performance when deployed at 85C. Next, the same process was used to prepare gas generants containing the various constituents described above, and a small amount of additive was added to each inflator. A ratio of 100:1 was used for the mass of gas generant to additive. The additive employed was 13X Type Molecular Sieves because of its inert chemical tendencies and ability to scavenge moisture and other volatiles, and retain these contaminants at relatively higher temperatures such as 107 degrees Celsius. The gas generant contained little moisture (less than 0.2% by mass). After USCAR Thermal Shock conditioning, it was found that the ballistic performance of the inflators was nearly identical to the baseline inflators. The gas generant contained almost exactly the same amount of moisture after conditioning as it did before testing. The increase in ballistic stability due to the addition of molecular sieves is somewhat counterintuitive as is does not appear to be moisture-related phenomena. This unexpected result of the use of additives such as molecular sieves is repeatable, and produces a novel method of stabilizing gas generants that have otherwise been deemed unsuitable for use in the automotive industry.

EXAMPLES 2 and 3:

A composition was made by combining 72.53% PSAN, 27.22% BTA-1NH3, and 0.25% M5 fumed silica and grinding with ceramic media in a Sweco vibratory grinder for 10 minutes. This powder was then pressed into %" diameter by 0.125" thick tablets. Driver side inflators were built with 25.Og of this pressed gas generant into two different configurations: one contained no scavenging additives; the other contained 0.25g of 13X zeolite in the form of 8-12 mesh beads poured directly into the gas generant chamber thereby intermingling the additive in heterogeneous relationship with the gas generating composition. The inflators were sealed and subjected to the USCAR thermal shock specification (200 thermal shock cycles between -40C and +90C). Inflators were pulled out after 50 cycles, 100 cycles, and 200 cycles. All inflators were tested at +85C and compared to the baseline results.

Without the addition of 13X zeolite, and as shown in FIG. 3, the ballistic curves indicate that changes occurred in the gas generant after 50 cycles. After 100 cycles the ballistic performance was very aggressive and did not meet USCAR specification. After 200 cycles the ballistic performance was so aggressive that the inflator ruptured due to extremely high internal pressures.

With the addition of 0.25g of the 13X zeolite, all inflators were in the same range as the baseline curve, or the inflator not subject to thermal shock testing, and therefore easily passed the USCAR specification. The ballistic curves are shown in FIG. 4.

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 or gas generator 10 containing a primary gas generant 12 and a scavenging additive 16 formed as described herein, may be manufactured as known in the art. U.S. Patent 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. It will be appreciated that the additive 16 is in gaseous or vapor communication with the propellant 12 prior to combustion of the gas generant 12. The additive 16 may for example, as shown in FIG. 1 be provided in juxtaposition to the gas generant 12 within the gas generant chamber 14, intermingled or heterogeneously arranged about the gas generant 12.

Referring now to FIG. 2, the exemplary inflator 10 described above may also be incorporated into an 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. The gas generant constituents may be provided by suppliers such as Sigma Aldrich and Fisher. The additives may also be supplied by well known suppliers. For example, the molecular sieves may be supplied by Sigma Aldrich. Furthermore, the molecular sieves may be provided as categorized in 3A, 4A, 5A, and 13X, in bead or powder form, in 4-8 or 8-12 mesh, for example. 13X is particularly preferred.

It should further 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

WHAT IS CLAIMED IS:

1. A gas generator comprising: a gas generating composition comprising a first oxidizer and a first fuel; and a scavenging additive selected from adsorbents, absorbents, chemical reactants, and mixtures thereof, said scavenging additive provided in vapor/gaseous communication with said gas generant composition within said gas generator, in heterogeneous relationship with said gas generant composition.

2. The gas generator of claim 1 wherein said first oxidizer is 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.

3. The gas generator of claim 1 wherein said first fuel is selected from derivatives of bis- (1(2) H-tetrazol-5-yl)-amine including its anhydrous acid, its acid monohydrate, mono- ammonium salt of bis-(1(2) H-tetrazol-5-yl)amine, 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 0.1-50% by weight of the gas generant composition.

4. The gas generator of claim 1 wherein said adsorbents, absorbents, chemical reactants, and mixtures thereof are selected from the group including molecular sieves, zeolites, calcium oxide, and calcium sulfate.

5. The gas generator of claim 1 wherein said gas generating composition further comprises a second additive selected from silicon, silicon dioxide, fused silicon, silicones; silicates; natural minerals including clay, mica, and silica; 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.

6. The gas generator of claim 1 wherein said scavenging additive is provided at about 0.05 to 10% by weight of the gas generant composition.

7. The gas generator of claim 1 wherein said gas generating composition further comprises 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.

8. A gas generating system comprising the gas generator of claim 1.

9. A vehicle occupant protection system comprising the gas generator of claim 1.

10. The gas generator of claim 1 wherein said gas generating composition comprises 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, molecular sieves provided at about 0.1 to 10% by weight of the gas generant composition.

11. The gas generator of claim 1 wherein said first oxidizers are selected from ammonium nitrate, phase stabilized ammonium nitrate, potassium nitrate, strontium nitrate, potassium nitrite, potassium chlorate, potassium perchlorate, ammonium perchloate, iron oxide, copper oxide, basic copper, nitrate, basic iron nitrate and mixture thereof.

12. The gas generator of claim 1 wherein said first 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-1 H-tetrazole and di-ammonium salt of 5,5-bis-1 H-tetrazole; 5-aminotetrazole nitrate; nitraminotetrazole; dipotassium 5-nitraminotetrazole; mono-ammonium nitraminotetrazole and di-ammonium nitraminotetrazole; dicyandiamide; guanidine nitrate; nitroguanidine; azodicarbonamide, azodicarbonamidine di-nitrate; and mixtures thereof.