Patent Application
20090008003
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.
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.
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-1NH3 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 is that they exhibit less-than desirable behavior with regard to ballistic properties, particularly with respect to thermal shock conditioning when ballistically tested at elevated temperatures.
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. 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 thermal cycling standards.
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 typical thermal cycling/thermal shock tests.
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.
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-1NH3 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
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-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 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 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. 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 NOx. 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 contaminant/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 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.
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-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.
To form comparative compositions, dry mixes of formulations containing the various constituents described above were prepared in a known manner. The dry material was then tableted, loaded into inflators, and subjected to Thermal Shock conditioning (100 Cycles, −40 C to 90 C). These formulations indicated an increase in aggressive ballistic performance when deployed at 85 C.
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, if any, moisture. After 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.
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 ¼″ diameter by 0.125″ thick tablets. Driver side inflators were built with 25.0 g of this pressed gas generant into two different configurations: one contained no scavenging additives; the other contained 0.25 g 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. As used throughout this specification and in the claims, the term “heterogeneous” is meant to indicate that the additive or 13X zeolite is added to a bed of pre-mixed substantially homogeneous gas generant composition made as known in the art. The gas generant is preferably but not necessarily pressed into tablets for example, with the 13X zeolite “heterogeneously” distributed about the tablets, in separate and proximate orientation to the gas generant. As such, the term “heterogeneous” is meant to convey a separate but proximate orientation between the gas generating composition and the 13X zeolite, when viewed from a macroscopic perspective. Stated yet another way, the term “heterogeneous” is meant to convey that the 13X zeolite is not a part of the gas generant composition, but is instead separately added to the bed of homogeneous gas generant composition already in the inflator. The term “heterogeneous” is not meant to convey separate and proximate orientation between the gas generant composition and the 13X zeolite when viewed from a microscopic perspective, or when viewed as the solid particulate separation between an oxidizer and a fuel, for example, that one would see when viewing solid particulates that make up a homogeneous gas generant composition. As such, the term “heterogeneous” would not describe or apply to a desiccant or desiccating constituent contained within a gas generant composition. The inflators were sealed and subjected to the thermal shock specification (100 thermal shock cycles between −40 C and +90 C). Inflators were pulled out after 50 cycles and 100 cycles. All inflators were tested at +85 C and compared to the baseline results.
Without the addition of 13X zeolite, and as shown in
With the addition of 0.25 g 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 desired thermal shock specifications. The ballistic curves are shown in
As shown in
Referring now to
Referring again to
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.
This application is a continuation-in-part of co-pending U.S. application Ser. No. 11/542,302 and claims the benefit thereof.
| Number | Date | Country | |
|---|---|---|---|
| 60722168 | Sep 2005 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11542302 | Oct 2006 | US |
| Child | 12217327 | US |
