WO2009126182A1

WO2009126182A1 - Monolithic gas generants containing perchlorate-based oxidizers and methods for manufacture thereof

Info

Publication number: WO2009126182A1
Application number: PCT/US2008/079750
Authority: WO
Inventors: Brett Hussey; Gary Lund; Ivan Mendenhall; Roger Bradford
Original assignee: Autoliv Asp, Inc.
Priority date: 2008-04-10
Filing date: 2008-10-13
Publication date: 2009-10-15

Abstract

A gas generant for an inflatable restraint device (for example, an airbag system for a vehicle) is a monolithic compressed solid that has a linear burn rate of greater than or equal to about 1 inch per second at a pressure of about 3,000 pounds per square inch. The gas generant can be in the form of an annular disk having a plurality of apertures. The gas generant has at least one fuel and at least one perchlorate-containing oxidizer and is substantially free of polymeric binder. The monolithic grain gas generant may have a low initial surface area which progressively increases during burning. The gas generant is optionally formed by the methods of the present disclosure that include a spray drying process.

Description

MONOLITHIC GAS GENERANTS CONTAINING

PERCHLORATE-BASED OXIDIZERS AND METHODS FOR MANUFACTURE

THEREOF

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Application No. 61/043,909 filed on April 10, 2008, the entire disclosure of which is incorporated herein by reference in its entirety.

FIELD [0002] The present disclosure relates to inflatable restraint systems and more particularly to pyrotechnic gas generant materials for use in such systems

BACKGROUND

[0003] The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.

[0004] Passive inflatable restraint systems are often used in a variety of applications, such as in motor vehicles. When a vehicle decelerates due to a collision, an inflatable restraint system deploys an airbag cushion to prevent contact between the occupant and the vehicle, thus minimizing occupant injuries. Such devices usually employ an inflator that can include a pyrotechnic gas generant. The gas generants burn very rapidly to generate heated gas that inflates an airbag. The inflatable device generally requires a sustained gas pressure in the airbag (generally over 5-10 pounds per square inch for at least

40 to 150 milliseconds) to restrain the occupant relative to the vehicle. Thus, gas generants provide gases that fulfill these requirements.

[0005] Sometimes, vehicle occupants are not in a predetermined position to receive the benefits of airbag deployment. If the rate of pressure increase in the airbag is too rapid and the amount of pressure generated by the gas generant is excessive, then an out-of-position occupant may not receive the desired benefits of the airbag.

[0006] Improvements in gas generant performance remain desirable. Tailoring the performance of the gas generant in an inflatable device system, such as an airbag, can require a complex design of not only the gas generant, but also hardware systems that control gas flow. It is preferred that gas generants for inflators of inflatable restraint devices rapidly generate gases during combustion at desired pressure levels and rates to achieve superior performance and to improve out-of-position performance. Likewise, gas generant materials are preferably safe for handling, have high gas yields and acceptable flame temperatures, with burning rates appropriate to the generant web thickness, and ideally are of relatively lower expense to manufacture. Gas generants that fulfill these requirements and further minimize the production of byproduct compounds in effluent gases released through the airbag are highly desirable.

SUMMARY

[0007] According to various aspects, the present disclosure provides a gas generant. In certain aspects, a pressed monolithic gas generant for an inflatable restraint device is provided that comprises an annular disk having a plurality of apertures. The plurality of apertures has a ratio of length to diameter of about 3.5 to about 8. An initial surface area of the disk is less than about 13,000 mm². The gas generant has a linear burn rate of greater than or equal to about 1 inch per second (about 25 mm per second) at a pressure of about 3,000 pounds per square inch (about 20.7 MPa). The gas generant comprises a fuel and an oxidizer comprising a perchlorate-containing compound, and is substantially free of polymeric binder.

[0008] In other aspects, the present disclosure provides an inflatable restraint device that comprises an airbag for restraining motion of a vehicle occupant and at least one gas generant for inflating the airbag. The at least one gas generant is in the form of a monolithic annular disk having a plurality of apertures. The gas generant has a linear burn rate of greater than or equal to about 1 inch per second at a pressure of about 3,000 pounds per square inch (about 20.7 MPa) and a gas yield is greater than or equal to about 3 moles/100 g of gas generant. Further, the gas generant comprises a fuel, an oxidizer comprising a perchlorate-containing compound, and is substantially free of polymeric binder. [0009] In yet other aspects, the present disclosure provides a method of making a gas generant for an inflatable restraint device. The method comprises spray drying an aqueous mixture to produce a powder. The aqueous mixture comprises a fuel and an oxidizer comprising a perchlorate-containing compound. The spray dried powder is then pressed to produce a monolithic gas generant grain having an annular disk including a plurality of apertures, where the monolithic gas generant grain has an average linear burn rate of greater than or equal to about 1 inch per second (about 38.1 mm per second) at a pressure of about 3,000 pounds per square inch (about 20.7 MPa) and a gas yield of greater than or equal to about 3 moles/100 g of gas generant.

[0010] Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. DRAWINGS

[0011] The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way.

[0012] Figure 1 is a simplified partial side view of an exemplary passive inflatable airbag device system in a vehicle having an occupant; [0013] Figure 2 is an exemplary partial cross-sectional view of a passenger-side airbag module including an inflator for an inflatable airbag restraint device;

[0014] Figure 3 is an exemplary partial cross-sectional view of a driver- side airbag module including an inflator for an inflatable airbag restraint device; [0015] Figure 4 is an isometric view of a pressed monolith gas generant in accordance with the principles of certain embodiments of the present disclosure;

[0016] Figure 5 is a simplified schematic of an exemplary spray drying process; [0017] Figure 6 illustrates gas generant powder produced by (A) two nozzle spray drying, (B) roll compacting and co-milling, and (C) fountain nozzle spray drying; [0018] Figures 7A and 7B are detailed views (5Ox magnification) of powders formed in accordance with methods (A) and (C) of Figure 5, comparing relative size, appearance, and shape of the respective powders; and

[0019] Figure 8 is a graph of combustion pressure versus time, as well as an inflator tank pressure trace, comparing a pressed monolith gas generant grain according to certain embodiments of the present disclosure with a comparative conventional gas generant.

DETAILED DESCRIPTION

[0020] The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features. The description and any specific examples, while indicating embodiments of the present disclosure, are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. Moreover, recitation of multiple embodiments having stated features is not intended to exclude other embodiments having additional features, or other embodiments incorporating different combinations of the stated features.

[0021] Inflatable restraint devices are used in various types of restraint systems including seatbelt pretensioning systems and airbag module assemblies. These devices and systems may be used in multiple applications in automotive vehicles, such as driver-side, passenger-side, side-impact, curtain, and carpet airbag assemblies. Other types of vehicles including, for example, boats, airplanes, and trains may use inflatable restraints. In addition, other types of safety or protective devices may also employ various forms of inflatable restraints.

[0022] Inflatable restraint devices preferably generate gas in situ from a reaction of a pyrotechnic gas generant contained therein. Inflatable restraint devices typically involve a series of reactions, which facilitate production of gas, to deploy an airbag or actuate a piston. In the case of airbags, upon actuation of the airbag assembly system, the airbag cushion should begin to inflate within a few milliseconds. As discussed herein, exemplary inflatable restraint devices will be referred to as airbag assemblies for purposes of illustration. Figure 1 shows an exemplary driver-side front airbag inflatable restraint device 10. Such driver side, inflatable restraint devices typically comprise an airbag cushion 12 that is stored within a steering column 14 of a vehicle 16. A gas generant contained in an inflator (not shown) in the steering column 14 creates rapidly expanding gas 18 that inflates the airbag 12. The airbag 12 deploys within milliseconds of detection of deceleration of the vehicle 16 and creates a barrier between a vehicle occupant 20 and the vehicle components 22, thus minimizing injuries.

[0023] A typical airbag module 30 includes a passenger compartment inflator assembly 32 and a covered compartment 34 to store an airbag 36. Such devices often use a squib or initiator 40 that is electrically ignited when rapid deceleration and/or collision is sensed. The discharge from the squib 40 usually ignites an initiator or igniter material 42 that burns rapidly and exothermically, in turn igniting a gas generant material 50. The gas generant material 50 burns to produce the majority of gas products that are directed to the airbag 36 to provide inflation.

[0024] Figure 2 shows a simplified exemplary airbag module 30 comprising a passenger compartment inflator assembly 32 and a covered compartment 34 to store an airbag 36. Such devices often use a squib or initiator 40 which is electrically ignited when rapid deceleration and/or collision is sensed. The discharge from the squib 40 usually ignites an igniter material 42 that burns rapidly and exothermically, in turn, igniting a gas generant material 50. The gas generant material 50 burns to produce the majority of gas products that are directed to the airbag 36 to provide inflation. [0025] Gas generants are also known as ignition materials, propellants, gas-generating materials, and pyrotechnic materials. The gas generant 50 may be in the form of a solid grain, a pellet, a tablet, or the like. Impurities and other materials present within the gas generant 50 facilitate the formation of various other compounds during the combustion reaction(s), including additional gases, aerosols, and particulates. Often, a slag or clinker is formed near the gas generant 50 during burning. The slag/clinker often serves to sequester various particulates and other compounds. However, a filter 52 is optionally provided between the gas generant 50 and airbag 36 to remove particulates entrained in the gas and to reduce gas temperature of the gases prior to entering the airbag35. The quality and toxicity of the components of the gas produced by the gas generant 50, also referred to as effluent, are important because occupants of the vehicle are potentially exposed to these compounds. It is desirable to minimize the concentration of potentially harmful compounds in the effluent.

[0026] Figure 3 shows a simplified exemplary driver side airbag module 60 with a covered compartment 62 to store an airbag 64. A squib 66 is centrally disposed within an igniter material 68 that burns rapidly and exothermically, in turn, igniting a gas generant material 70. Filters 72 are provided to reduce particulate in effluent gases entering the airbag 64 as it inflates.

[0027] Various different gas generant compositions (50 or 70) are used in vehicular occupant inflatable restraint systems. As described above, gas generant material selection involves various factors, including meeting current industry performance specifications, guidelines and standards, generating safe gases or effluents, handling safety of the gas generant materials, durational stability of the materials, and cost-effectiveness in manufacture, among other considerations. It is preferred that the gas generant compositions are safe during handling, storage, and disposal, and preferably are azide-free.

[0028] The gas generant typically includes at least one fuel component and at least one oxidizer component, and may include other minor ingredients, that once ignited combust rapidly to form gaseous reaction products {e.g., CO₂, H₂O, and N₂). One or more compounds undergo rapid combustion to form heat and gaseous products; e.g., the gas generant burns to create heated inflation gas for an inflatable restraint device or to actuate a piston. The gas generant may comprise a redox-couple having at least one fuel component. The gas- generating composition optionally includes one or more oxidizing components, which are typically used with under-oxidized fuels, where the oxidizing component reacts with the fuel component in order to generate the gas product. [0029] In accordance with various aspects of the present disclosure, gas generants are provided that have desirable compositions and shapes that result in superior performance characteristics in an inflatable restraint device, while reducing overall cost of gas generant production. In preferred embodiments, the gas generants have a high burn rate (i.e., rate of combustion reaction), a high gas yield (moles/mass of generant), a high achieved mass density, a high theoretical density, a high loading density, are substantially free of binder, and are formed in unique shapes that optimize the ballistic burning profiles of the materials contained therein. Thus, the present disclosure pertains to compositions and methods for making a gas generant, particularly suitable to be used in a monolithic grain shape. In various aspects, a monolithic gas generant composition is formed that comprises at least one fuel and at least one oxidizer. In preferred embodiments, the gas generant composition includes at least one perchlorate-containing oxidizer, which unexpectedly enhances gas generant dynamic performance and effluent behavior, as will be discussed in greater detail below. Further, the gas generant is substantially free of polymeric binder.

[0030] In various aspects, the pressed monolithic gas generant is formed from a gas generant powder formed by a spray drying process. In certain aspects, an aqueous mixture including fuel, at least one oxidizer, including the perchlorate-containing oxidizer, and other optional ingredients is spray dried to produce a gas generant powder. The powder is pressed to produce grains of the gas generant.

[0031] The ballistic properties of a gas generant, such as 50 or 70 shown in Figures 2 and 3, are typically controlled by the gas generant material composition, shape and surface area, as well as the burn rate of the material. Conventional gas generant materials comprise at least one fuel, an oxidizer, and at least one binder. Binders are typically mixed with the various constituents of the gas generant. Binders serve to retain the shape of the gas generant solids, particularly when they are formed via extrusion and/or molding. For example, a dry blended mixture of various gas generant components can be mixed with a liquid binder resin, extruded, and then cured. Alternatively, solid polymeric binder particles can be dissolved in a solvent or heated to the melting point, then mixed with other gas generant components and extruded or molded. However, gas generant solids formed by such methods are only subjected to relatively low to moderate compressive forces due to the nature of the manufacturing methods and relative compressive force applied. Thus, the presence of a binder is required to retain shape and prevent fracture during storage and use.

[0032] Most of the above described formation methods require a binder, such as polymeric binders, including organic film formers, inorganic polymers, thermoplastic and/or thermoset polymers. Examples of common polymeric binders include, but are not limited to: natural gums, cellulosic esters, polyacrylates, polystyrenes, silicones, polyesters, polyethers, polybutadiene, and the like.

[0033] However, the presence of polymeric binders in conventional gas generants poses several potential issues. First, during the combustion of the fuel and oxidizer, the binder is likewise burned and/or volatilized, which potentially generates undesirable byproducts in the gas/effluent. Thus, generation of such byproducts requires appropriate treatment and/or sequestration. Binders are usually carbon-rich and can consume a large portion of the oxidizer during combustion, thus restricting the quantity of other fuel that can be used in formulation. Second, binder resin is a diluent and slows otherwise rapid reaction of the chemical materials in the gas generant. Furthermore, in some circumstances, the burning rate of gas generants having certain binders is so compromised that monolithic shapes have previously not been industrial practicable. Since the presence of the binder slows the rate of reaction of the gas generant, other compounds are often added to compensate for the binder (thereby boosting the reaction rate).

[0034] In certain aspects, the gas generant materials are substantially free of polymeric binder. The term "substantially free" as referred to herein is intended to mean that the compound is absent to the extent that that undesirable and/or detrimental effects are avoided. In the present embodiment, a gas generant that is "substantially free" of binder comprises less than about 5% by weight binder, more preferably less than about 4% by weight, optionally less than about 3% by weight, optionally less than about 2% by weight, optionally less than about 1 % by weight binder, and in certain embodiments comprises 0% by weight of the binder. The gas generant compositions of the present disclosure thus avoid production of undesirable byproduct species potentially generated in effluent by the burning and/or volatilization of the binder resins as the gas generant burns. Further, gas generants that are substantially free of binders have significantly improved burn characteristics (i.e., higher burning rate).

[0035] In various embodiments, the gas generant comprises at least one fuel. The fuel component may be a nitrogen-containing compound and preferably is an azide-free compound. In certain aspects, preferred fuels include tetrazoles and salts thereof (e.g., aminotetrazole, mineral salts of tetrazole), bitetrazoles, 1 ,2,4-triazole-5-one, guanidine nitrate, nitro guanidine, amino guanidine nitrate, and the like. These fuels are combined with one or more oxidizers in order to obtain an acceptable burning rate and production of desirable gaseous species. In certain embodiments, the gas generant comprises at least guanidine nitrate as a fuel.

[0036] In certain embodiments, suitable pyrotechnic materials for the gas generants of the present disclosure comprise non-azide compounds that comprise a substituted basic metal nitrate. The substituted basic metal nitrate can include a reaction product formed by reacting an acidic organic compound with a basic metal nitrate. Examples of suitable acidic organic compounds include, but are not limited to, tetrazoles, imidazoles, imidazolidinone, thazoles, urazole, uracil, barbituric acid, orotic acid, creatinine, uric acid, hydantoin, pyrazoles, derivatives and mixtures thereof. Examples of such acidic organic compounds include 5-amino tetrazole, bitetrazole dihydrate, and nitroimidazole. Generally, suitable basic metal nitrate compounds include basic metal nitrates, basic transition metal nitrate hydroxy double salts, basic transition metal nitrate layered double hydroxides, and mixtures thereof. Suitable examples of basic metal nitrates include, but are not limited to, basic copper nitrate, basic zinc nitrate, basic cobalt nitrate, basic iron nitrate, basic manganese nitrate and mixtures thereof. One particularly preferred gas generant composition includes about 5 to about 60 weight % of guanidine nitrate co-fuel and about 5 to about 95 weight % substituted basic metal nitrate.

[0037] As appreciated by those of skill in the art, such fuel components may be combined with additional components in the gas generant, such as co- fuels or oxidizers. For example, in certain embodiments, a gas generant composition comprises a substituted basic metal nitrate fuel, as described above, and a nitrogen-containing co-fuel or oxidizer, like guanidine nitrate. Suitable examples of gas generant compositions having suitable burn rates, density, and gas yield for inclusion in the gas generants of the present disclosure include those described in U.S. Patent No. 6,958,101 to Mendenhall et al., the disclosure of which is herein incorporated by reference in its entirety. The desirability of use of various co-fuels, such as guanidine nitrate, in the gas generant compositions of the present disclosure is generally based on a combination of factors, such as burn rate, cost, stability (e.g., thermal stability), availability and compatibility (e.g., compatibility with other standard or useful pyrotechnic composition components).

However, any suitable fuels known or to be developed in the art that can provide gas generants having the desired burn rates, gas yields, and density described above are contemplated for use in various embodiments of the present disclosure.

[0038] Other suitable oxidizers for the gas generant composition include, by non-limiting example, alkali (e.g., elements Group 1 of IUPAC Periodic Table, including Li, Na, K, Rb, and/or Cs), alkaline earth (e.g., elements of Group 2 of IUPAC Periodic Table, including Be, Mg, Ca, Sr, and/or Ba), and ammonium nitrates, nitrites, and perchlorates; metal oxides (including Cu, Mo, Fe, Bi, La, and the like); basic metal nitrates (e.g., elements of transition metals of Row 4 of IUPAC Periodic Table, including Mn, Fe, Co, Cu, and/or Zn); transition metal complexes of ammonium nitrate (e.g., elements selected from Groups 3-12 of the IUPAC Periodic Table); and combinations thereof. One or more co-fuel/oxidizers are selected along with the fuel component to form a gas generant that upon combustion achieves an effectively high burn rate and gas yield from the fuel. The gas generant may include combinations of oxidizers, such that the oxidizers may be nominally considered a primary oxidizer, a second oxidizer, and the like. Specific examples of suitable oxidizers include basic metal nitrates such as basic copper nitrate. Basic copper nitrate has a high oxygen-to-metal ratio and good slag forming capabilities upon burn. Ammonium dinitramide is another suitable oxidizing agent. Such oxidizing agents may be respectively present in an amount of less than or equal to about 50% by weight of the gas-generating composition; optionally less than or equal to about 40% by weight; optionally less than or equal to about 30% by weight; optionally less than or equal to about 25% by weight; optionally less than or equal to about 20% by weight; and in certain aspects, less than or equal to about 15% by weight of the gas generant composition.

[0039] In various embodiments of the present disclosure, the gas generant composition comprises an oxidizer comprising a perchlorate-containing compound, in other words a compound including a perchlorate group (CIO₄ ^"). Such perchlorate oxidizer compounds are typically water soluble. By way of non-limiting example, alkali, alkaline earth, and ammonium perchlorates are contemplated for use in gas generant compositions. Particularly suitable perchlorate oxidizers include alkali metal perchlorates and ammonium perchlorates. Thus, suitable perchlorate oxidizer compounds include ammonium perchlorate (NH₄CIO₄), sodium perchlorate (NaCIO₄), potassium perchlorate (KCIO₄), lithium perchlorate (LiCIO₄), magnesium perchlorate (Mg(CIO₄)2), and combinations thereof. In certain aspects, each perchlorate oxidizer is present in the gas generant at less than about 30% by weight. By way of example, a perchlorate containing oxidizer is present in certain embodiments at about 1 % to about 30% by weight; optionally about 2 to about 15% by weight; optionally about 3 to about 10% by weight of the gas generant.

[0040] In certain embodiments, a gas generant comprises at least one fuel component, such as guanidine nitrate, mixed with a combination of oxidizers, including a primary oxidizer of basic copper nitrate and a secondary oxidizer of potassium perchlorate, to form a gas generant.

[0041] Significant improvements in gas generant performance, including higher burn rates are achieved in accordance with the present teachings when perchlorate oxidizers (e.g., potassium perchlorate) are processed via spray drying techniques of the present disclosure. Further, the use of such compositions in monolithic gas generant grains surprisingly results in a combustion pressure which desirably has both significantly reduced maximum pressure (MEOP) relative to pellets and simultaneously substantially increased average operating pressure over the action time of the generant. Increased average operating pressure contributes to reduced trace noxious gas levels during inflator operation, but has been difficult to achieve in actual gas generant products. The ability to use monolithic grain design as a result of the relatively high burning rates obtained with perchlorate-containing oxidizers allows significant increases in average operating pressure to be achieved, without excessive MEOP requirements. Also, the addition of perchlorate-containing oxidizer(s) to the gas generant surprisingly provides much less aggressive early time pressuhzation behavior, while maintaining relatively high average operating pressures, which is advantageous. Thus, the gas generant compositions prepared in accordance with the present teachings provide improved inflator effluent performance, while having desirably reduced cost and weight requirements for the gas generant.

[0042] Other suitable additives include slag forming agents, flow aids, viscosity modifiers, pressing aids, dispersing aids, or phlegmatizing agents that can be included in the gas generant composition. The gas generant compositions optionally include a slag forming agent, such as a refractory compound, e.g., aluminum oxide and/or silicon dioxide. Other suitable viscosity modifying compounds/slag forming agents include cerium oxide, ferric oxide, zinc oxide, titanium oxide, zirconium oxide, bismuth oxide, molybdenum oxide, lanthanum oxide and the like. Generally, such slag forming agents may be included in the gas generant composition in an amount of 0 to about 10 weight %, optionally at about 0.5 to about 5% by weight of the gas generant composition. [0043] Coolants for lowering gas temperature, such as basic copper carbonate or other suitable carbonates, may be added to the gas generant composition at 0 to about 20% by weight. Similarly, press aids for use during compression processing, include lubricants and/or release agents, such as graphite, calcium stearate, magnesium stearate and/or graphitic boron nitride, by way of non-limiting example, can be present in the gas generant at 0 to about 2%. While in certain aspects it is preferred that the gas generant compositions are substantially free of polymeric binders, in certain alternate aspects, the gas generant compositions optionally comprise low levels of certain acceptable binders or excipients to improve crush strength, while not significantly harming effluent and burning characteristics. Such excipients include microcrystalline cellulose, starch, carboxyalkyl cellulose, e.g., carboxymethyl cellulose (CMC), by way of example. When present, such excipients can be included in alternate gas generant compositions at less than 10% by weight, optionally less than about 5% by weight, and optionally less than about 2.8%.

[0044] Additionally, certain ingredients can be added to modify the burn profile of the pyrotechnic fuel material by modifying pressure sensitivity of the burning rate slope. One such example is copper bis-4-nitroimidazole. Agents having such an affect are referred to herein as pressure sensitivity modifying agents and they can be present in the gas generant at 0 to about 10% by weight. Such additives are described in more detail in U.S. Patent Application Serial No. 11/385,376, entitled "Gas Generation with Copper Complexed Imidazole and Derivatives" to Mendenhall et al., the disclosure of which is herein incorporated by reference in its entirety. Other additives known or to be developed in the art for pyrotechnic gas generant compositions are likewise contemplated for use in various embodiments of the present disclosure, so long as they do not unduly detract from the desirable burn profile characteristics of the gas generant compositions.

[0045] In certain aspects, the gas generant may include about 30-70 parts by weight, more preferably 40-50 parts by weight, of at least one fuel (e.g., guanidine nitrate), about 30-60 parts by weight of oxidizers (e.g., basic copper nitrate and potassium perchlorate), and about 0-5 parts by weight of slag forming agents like silica (Siθ2) or equivalents thereof. In certain embodiments, the gas generant may include one or more additional metal oxides such as cupric oxide, molybdenum oxide, iron oxide, bismuth oxide the like in addition to the basic copper nitrate. In addition to potassium perchlorate, or in substitution thereof, co-oxidizers such as ammonium perchlorate, potassium nitrate, strontium nitrite, and sodium nitrate may also be used. Alternate slag promoters that may be used include zinc oxide, aluminum oxide, cerium oxide, and similar compounds. Pressing agents such as calcium or magnesium stearate, graphite, molybdenum disulfide, tungsten disulfide, boron nitride, and mixtures thereof may also be added prior to tableting or pressing.

[0046] The gas-generating composition may be formed from an aqueous dispersion of one or more fuel components that are added to an aqueous vehicle to be substantially dissolved or suspended. The oxidizer component(s) are dispersed and stabilized in the fuel solution either dissolved in the solution or optionally present as a stable dispersion of solid particles. The solution or dispersion may also be in the form of a slurry. The aqueous dispersion or slurry is spray-dried by passing the mixture through a spray nozzle in order to form a stream of droplets. The droplets contact hot air to effectively remove water and any other solvents from the droplets and subsequently produce solid particles of the gas generant composition, as will be described in greater detail below.

[0047] The mixture of components forming the aqueous dispersion may also take the form of a slurry, where the slurry is a flowable or pumpable mixture of fine (relatively small particle size) and substantially insoluble particle solids suspended in a liquid vehicle or carrier. Mixtures of solid materials suspended in a carrier are also contemplated. In some embodiments, the slurry comprises particles having an average maximum particle size of less than about 500 μm, optionally less than or equal to about 200 μm, and in some cases, less than or equal to about 100 μm. Thus, the slurry contains flowable and/or pumpable suspended solids and other materials in a carrier.

[0048] Suitable carriers include aqueous solutions that may be mostly water; however, the carrier may also contain one or more organic solvents or alcohols. In some embodiments, the carrier may include an azeotrope, which refers to a mixture of two or more liquids, such as water and certain alcohols that desirably evaporate in constant stoichiometric proportion at specific temperatures and pressures. The carrier should be selected for compatibility with the fuel and oxidizer components to avoid adverse reactions and further to maximize solubility of the several components forming the slurry. Non-limiting examples of suitable carriers include water, isopropyl alcohol, n-propyl alcohol, and combinations thereof.

[0049] Viscosity of the slurry is such that it can be injected or pumped during the spray drying process. In some embodiments, the viscosity is kept relatively high to minimize water and/or solvent content, for example, so less energy is required for carrier removal during spray drying. However, the viscosity may be lowered to facilitate increased pumping rates for higher pressure spray drying. Such adjustments may be made when selecting and tailoring atomization and the desired spray drying droplet and particle size.

[0050] In some embodiments, the slurry has a water content of greater than or equal to about 15% by weight and may be greater than or equal to about 20%, 30%, or 40% by weight. In some embodiments, the water content of the slurry ranges from about 15% to 85% by weight. As the water content increases, the viscosity of the slurry decreases, thus pumping and handling become easier. In some embodiments, the slurry has a viscosity ranging from about 50,000 to 250,000 centipoise. Such viscosities are believed to be desirable to provide suitable rheological properties that allow the slurry to flow under applied pressure, but also permit the slurry to remain stable.

[0051] In some embodiments, a quantity of silica (Siθ2) is included in the aqueous dispersion, which can act as a slag forming component, but also serves to thicken the dispersion and reduce or prevent migration of solid oxidizer particles in the bulk dispersion and droplets. The silica can also react with the oxidizer during the redox reaction to form a glassy slag that is easily filtered out of the gas produced upon ignition of the gas generant. The silica is preferably in very fine form. In certain embodiments, preferable grades of silica include those having particle sizes of about 7 nm to about 20 nm, although in certain aspects, silica having particles sizes of up to about 50 μm may be employed as well.

[0052] As discussed above, equivalent and equally useful slag and viscosity modifying/promoting agents include aluminum oxide, cerium oxide, ferric oxide, zinc oxide, titanium oxide, zirconium oxide, bismuth oxide, molybdenum oxide, lanthanum oxide and the like. Such redox inert oxides maybe employed individually or as mixtures of two or more individual components. For example, where one oxide has a very fine form (e.g., particle size of less than about 20 nm) useful for improving viscosity of the mixture slurry, another coarser oxide having larger particle sizes may be provided to the mixture to improve slagging properties without interfering with or negatively affecting burning rate. As noted above, in certain embodiments, the aqueous mixture also includes about 0.1 % to about 5.0% of a slag promoting agent, such as silicon dioxide.

[0053] In certain aspects when forming the aqueous dispersion, the composition is mixed with sufficient aqueous solution to dissolve substantially the entire fuel component at the spray temperature; however, in certain aspects, it is desirable to restrict the amount of water to a convenient minimum in order to minimize the amount of water that is to be evaporated in the spray-drying process. For example, the dispersion may have greater than or equal to about 25 weight % to less than or equal to about 50 weight % (on a wet basis, in other words parts water per total of parts water and parts solids x 100). In certain aspects, the water content is greater than or equal to about 30 weight % and less than or equal to about 45 weight %.

[0054] The oxidizer components may be uniformly dispersed in the fuel solution by vigorous agitation to form the dispersion, where the particles of oxidizer are separated to a sufficient degree to form a stable dispersion. In the case of water insoluble oxidizers, the viscosity will reach a minimum upon achieving a fully or near fully dispersed state. A high shear mixer may be used to achieve efficient dispersion of the oxidizer particles. The viscosity of the dispersion should be sufficiently high to prevent any substantial migration (i.e., fall-out or settling) of the solid particles in the mixture.

[0055] The spray drying process is used for forming particles and drying materials. It is suited to continuous production of dry solids in powder, granulate, or agglomerate particle forms using liquid feedstocks of the redox couple components to make the gas generant. Spray drying can be applied to liquid solutions, dispersions, emulsions, slurries, and pumpable suspensions. Variations in spray drying parameters may be used to tailor the dried end- product to precise quality standards and physical characteristics. These standards and characteristics include particle size distribution, residual moisture content, bulk density, and particle morphology.

[0056] Spray drying includes atomization of the aqueous mixture, for example, atomization of the liquid dispersion of redox couple components into a spray of droplets. The droplets are then contacted with hot air in a drying chamber. Evaporation of moisture from the droplets and formation of dry particles proceeds under controlled temperature and airflow conditions. Powder may be continuously discharged from the drying chamber and recovered from the exhaust gases using, for example, a cyclone or a bag filter. The whole process may take no more than a few seconds. In some embodiments, the liquid dispersion or slurry is heated prior to atomization. [0057] A spray dryer apparatus typically includes a feed pump for the liquid dispersion, an atomizer, an air heater, an air disperser, a drying chamber, a system for powder recovery, an exhaust air cleaning system, and a process control system. Equipment, process characteristics, and quality requirements may be adjusted based on individual specifications. Atomization includes forming sprays having a desired droplet size distribution so that resultant powder specifications may be met. Atomizers may employ various approaches to droplet formation and include rotary (wheel) atomizers and various types of spray nozzles. For example, rotary nozzles provide atomization using centrifugal energy, pressure nozzles provide atomization using pressure energy, and two- fluid nozzles provide atomization using kinetic energy.

[0058] Airflow adjustment may be used to control the initial contact between spray droplets and the drying air in order to control evaporation rate and product temperature in the dryer. Co-current airflow moves drying air and droplets/particles through the drying chamber in the same direction. In co- current airflow, product temperature on discharge from the dryer is lower than the exhaust air temperature and the method therefore works well for drying heat sensitive products. Counter-current airflow moves drying air and droplets or particles through the drying chamber in opposite directions and is useful for products that require heat treatment during drying. The temperature of the powder leaving counter-current airflow drying is usually higher than the exhaust air temperature. Mixed flow combines co-current and counter-current airflow so that droplets or particles experience both types of airflow. The mixed flow method is used for heat stable products where coarser powder requirements require the use of nozzle atomizers. Mixed flow methods include spraying upwards into an incoming airflow, or for heat sensitive particles the atomizer sprays downwards toward an integrated fluid bed, and typically the air inlet and outlet are located at the top of the drying chamber.

[0059] The aqueous dispersion of gas generant components may be atomized using a spray nozzle to form droplets of about 40 μm to 200 μm in diameter by forcing the droplets under pressure through a nozzle having one or more orifices of about 0.5 mm to 2.5 mm in diameter. The droplets may be spray-dried by allowing the droplets to fall into or otherwise contact a stream of hot air at a temperature in the range from about 80⁰C to about 250⁰C, preferably about 80°C to about 180⁰C. The outlet and inlet temperatures of the air stream may be different in order to achieve the heat transfer required for drying the droplets. The preceding illustrative air temperature ranges are further indicative of examples of outlet and inlet temperatures, respectively.

[0060] Spray drying a mixture of fuel (for example, guanidine nitrate) and a primary oxidizer (for example, basic copper nitrate), and secondary oxidizer (for example, potassium perchlorate) may be accomplished using various spray drying techniques and equipment. An exemplary simplified spray drying system is shown in Figure 5. A slurry source 252 contains a slurry comprising the individual components of the gas generant, which is fed to a mixing chamber 254. The slurry is forced through one or more atomizing nozzles 256 at high velocity against a counter current stream of heated air. The slurry is thus atomized and the water removed. The heated air is generated by feeding an air source 258 to a heat exchanger 260, which also receives a heat transfer stream 262. The heat transfer stream 262 may pass through one or more heaters 264. The atomization of slurry in the mixing chamber 254 produces a rapidly dried powder that is entrained in an effluent stream 270. The effluent stream 270 can be passed through a collector unit 272, such as a baghouse or electrostatic precipitator, which separates powder/particulates from gas. The powder 274 is recovered from the collector unit 274 and can then be pelletized, compacted, or otherwise fashioned into a shape suitable for use as a gas generant in an inflating device. The exhaust stream 276 from the separator unit 272 can optionally be passed through one or more processes downstream as necessary, such as a scrubber system 280.

[0061] The present methods may employ various spray driers as known in the art. For example, suitable spray drying apparatuses and accessory equipment include those manufactured by Anhydro Inc. (Olympia Fields, IL), BUCHI Corporation (New Castle, DE), Marriott Walker Corporation (Birmingham, Ml), Niro Inc. (Columbia, MD), and Spray Drying Systems, Inc. (Eldersburg, MD). In certain aspects, a suitable spray drying process to form powdered or particulate materials includes those processes described in U.S. Patent 5,756,930 to Chan et al, the relevant portion of which is incorporated herein by reference.

[0062] Particles produced from the spray-dried droplets may comprise aggregates of very fine mixed crystals of the gas generant components, having a primary crystal size of about 0.5 μm to about 5 μm in the thinnest dimension, and preferably about 0.5 μm to about 1 μm. However, water insoluble oxidizer components are preferred as these can be obtained in very small particle sizes and incorporated in the aqueous solution of dissolved fuel component to form a dispersion, thereby reducing the water content required for the aqueous medium. [0063] The dried particles of gas generant may take the form of substantially spherical microporous aggregates of fuel crystals {e.g., guanidine nitrate crystals) having a narrow size distribution within the range required for substantially complete reaction with the oxidizers. For example, the spherical microporous aggregates may be about 20 μm to about 100 μm in diameter, the primary fuel crystals being about 0.5 μm to about 5 μm and generally about 0.5 μm to 1 about μm in the thinnest dimension. Generally, particles of the solid oxidizer(s) are encapsulated by the fuel crystals, where the oxidizer particles serve as crystal growth sites for the fuel component crystals. The spray drying process produces very little ultrafine dust that could be hazardous in subsequent processing operations.

[0064] The dried particles of gas generant may be readily pressed into pellets or grains for use in a gas-generating charge in inflatable restraints; e.g., air-bags. The pressing operation may be facilitated by mixing the spray-dried gas generant particles with a quantity of water or other pressing aid, such as graphite powder, calcium stearate, magnesium stearate and/or graphitic boron nitride, by way of non-limiting example. The water may be provided in the form of a mixture of water and hydrophobic fumed silicon, which may be mixed with the particles using a high shear mixer. The composition may then be pressed into various forms, such as pellets or grains. By way of non-limiting example, suitable gas generant grain densities can be greater than or equal to about 1.8 g/cm³ and less than or equal to about 2.2 g/cm³, however, density varies as a function of materials of composition and press force used during formation. These pellets and granular forms are readily ignited by an igniter, such as an electric squib, or in certain aspects, more efficiently, by an igniferous booster comprising pyrotechnic sheet material. The pyrotechnic sheet material may be formed of an oxidizing film, for example, a film of polytetrafluoroethylene coated with a layer of oxidizable metal, such as magnesium, as described in European Patent Publication No. 0505024 to Graham et al., the relevant portions of which is incorporated by reference.

[0065] In some embodiments, methods of making a gas generant use a processing vessel, such as a mix tank, in order to prepare the gas generant formulation that is subsequently processed by spray drying. For example, the processing vessel may be charged with water, guanidine nitrate, and oxidizers including basic copper nitrate and potassium perchlorate, which are mixed to form an aqueous dispersion. The temperature of the slurry may be equilibrated at about 80⁰C to about 90⁰C for approximately one hour. Additives and components, such as additional fuel components, oxidizer components, slagging aids, and the like may be added to the reaction mixture at this time. The resulting aqueous dispersion is then pumped to the spray drier to form the dry powder or particulate gas generant product. Further processing steps such as blending, pressing, igniter coating, etc. or the like can then be preformed per standard procedures.

[0066] The present spray drying methods produce unexpectedly high burning rates for gas generant compositions containing guanidine nitrate, basic copper nitrate, and about 1 % to about 15% by weight of a co-oxidizer, such as potassium perchlorate. These burn rates are surprising when compared to comparative gas generants formed by using the same components and having substantially the same composition, but prepared using different processes. For example, spray drying of these mixtures may result in compositions exhibiting burning rates at least about 20% greater than a comparative burn rate of a comparative gas generant having substantially the same compositions prepared by a process selected from: mechanically blending followed by roll compacting the individual ingredients, milling, and/or mechanical blending of the potassium perchlorate into a spray dried mixture of basic copper nitrate and guanidine nitrate, which are conventional processes used to form gas generant grains. In certain aspects, gas generant compositions prepared by the present spray drying methods provide the ability to utilize inexpensive ingredients, while exhibiting burn rates comparable to burn rates previously achieved only through incorporation of expensive ingredients such as bitetrazole and aminotetrazole. The present methods and formulations may also include additional additives such as silica or similar inert oxides for promoting slag formation during combustion of the generant.

[0067] Observed increases in linear burning rates of the spray dried compositions are surprising and unexpected in view of other conventional methods of making gas generants. These process-based enhancements can be observed when comparing examples of data obtained from gas generant formulations prepared using three different methods forming two comparative examples and an example (1 ) formed in accordance with the present disclosure. The three methods include: (1 ) Dry blending the components of guanidine nitrate, basic copper nitrate, and potassium perchlorate, then roll-compacting and milling the gas generant product (Comparative Example A).

(2) Spray drying the components of guanidine nitrate and basic copper nitrate, then mechanically blending potassium perchlorate into the mixture to form the gas generant product (Comparative Example B).

(3) Spray drying the components of guanidine nitrate, basic copper nitrate, and potassium perchlorate as a single aqueous mixture to form the gas generant product (Example 1 ). [0068] Results for these three methods are summarized in Tables 1 and 2. As can be seen, example methods (1 ) and (2) listed above gave nearly identical results regardless whether the water soluble fuel and principal oxidizer are spray dried or not. While it might be expected that the spray drying in method (2) increases the linear burning rate as compared to that obtained with the dry blending process of method (1 ), only when the minor oxidizer component of potassium perchlorate is included in the aqueous mixture with the other components and spray dried, as per method (3) of the present teachings, are significant improvements in burning rate achieved. This is further unexpected since potassium perchlorate has only minor solubility in water and the aqueous mixture spray dried in the Example (1 ) formed in accordance with method (3) is also saturated with respect to guanidine nitrate.

a - Base A: Spray dried basic copper nitrate, guanidine nitrate and silica. *^b - basic copper nitrate

*c - guanidine nitrate *^d - potassium perchlorate [0070] Table 2. Performance Comparison of Samples Made Via Different Processes

a - Base B: Spray dried basic copper nitrate, guanidine nitrate and silica. *^b - basic copper nitrate *^c - guanidine nitrate

^*d - potassium perchlorate [0071] With reference to Tables 1 and 2, the present methods may be used to make gas generants having increased burn rates relative to comparative gas generants made by other conventional methods. In certain embodiments, the present methods are used to make grains of gas generant that provide a burning rate at least about 20% greater than a comparative gas generant produced by mechanically blending, roll compacting and milling the same amounts of guanidine nitrate, basic copper nitrate, and secondary oxidizer or a gas generant produced by mechanically blending the same amount of secondary oxidizer into a spray dried mixture of the same amounts of basic copper nitrate and guanidine nitrate.

[0072] As such, the present methods contemplate spray drying of guanidine nitrate, a principal oxidizer (e.g., basic copper nitrate), and a secondary oxidizer (e.g., potassium perchlorate), which results in a gas generant with surprising and unexpected burn rates. Compared with a dry blending method conducted in Comparative Example (A) or a post blending method used to form Comparative Example (B). In accordance with the present teachings, Example (1 ) is prepared by a method of spray drying all three primary gas generant components, which can increase the burn rate by at least about 25% at 3,000 psi (see e.g., Table 2 above for burning rate). These increased burn rates contrast with methods of spray drying the guanidine nitrate and principal oxidizer followed by dry blending of the secondary oxidizer into the spray dried powder, which in certain aspects, does not appear to afford much, if any, advantage over dry blending all components. Therefore, the present methods and compositions demonstrate particular advantages by including the secondary perchlorate- containing oxidizer in the spray drying process.

[0073] Without wishing to be bound by theory, it is believed that including the secondary perchlorate-containing oxidizer during particle formation by spray drying results in particles and/or crystals with structures responsible for the advantageous burn rates. Spray drying may be accomplished, for example, using rotary nozzles, pressure nozzles, and two-fluid nozzles as described herein, and parameters such as pressure, flow rate, and airflow may be optimized to achieve desired particle sizes. Thus, gas generants with improved burn rates may be produced using guanidine nitrate, principal oxidizer, and secondary perchlorate-containing oxidizer by a variety of spray drying techniques.

[0074] In certain aspects, the present methods of making gas generants provide additional unexpected benefits based on the selection of spray drying technique employed. In particular, spray drying methods using a single orifice or fountain nozzle spray head are in certain aspects, particularly advantageous in producing a gas generant product that is easier to handle and further process as compared to powder or particulate formed using other spray drying techniques. For example, in certain aspects, powder produced with a single orifice fountain nozzle has better tableting and pressing characteristics. However, the present teachings also provide advantages in various types of spray drying techniques aside from the single orifice fountain spray drying, including spray drying by using two-fluid nozzles, which are also contemplated. [0075] A single orifice fountain nozzle generally sprays only liquid material. An exemplary two-fluid nozzle spray orifice is described by U.S. Patent 5,756,930 to Chan et al., which can also be employed in accordance with the present teachings to process generant to maximize linear burn rate behavior for compositions so processed. The two-fluid nozzle spray orifice used in Chan et al. combines an air nozzle and a liquid nozzle which are sprayed together. The two-fluid nozzle is, by design, intended to impart very high shear forces to the fluid stream and produces minimal product particle size.

[0076] The product produced by the single orifice fountain nozzle, on the other hand, generally has a substantially larger particle size than that produced from the two-fluid nozzle and is particularly suitable for tableting (i.e., pressing or compacting under pressure) without further processing. In certain aspects, this is advantageous compared to powder produced with the two-fluid nozzle, which generally requires further roll compacting and regrinding after spray drying in order to produce a material which can then be tableted. While either the two-fluid nozzle spray drying and single orifice fountain nozzle are suitable for use in accordance with the present disclosure, in certain aspects, gas generant grains made by pressing material produced with the single orifice fountain nozzle spray dry process are particularly suitable, in that they are generally superior in compaction, density, and homogeneity. Examples of the appearance of these three powders and examples of generant grains produced with the same powders are shown in Figures 6 and 7A-7B. [0077] In certain embodiments, the gas generant produced by spray drying with a single orifice fountain nozzle has a burn rate similar to the gas generant produced by spray drying with a two-fluid nozzle, where each gas generant is produced using the same aqueous mixture of guanidine nitrate, basic copper nitrate, and potassium perchlorate. However, the material produced using the single orifice fountain nozzle results in more rounded particles that are easier to handle and press, as shown by comparative views in Figures 7A and. Figure 7A shows powders formed via spray drying with a two-fluid nozzle and Figure 7B shows powders formed by spray drying with a fountain nozzle, which have a relatively larger particle size and a more rounded shape. Spray dried product particle sizes of about 100 μm to 200 μm may be easier to handle and feed to tablet press, such as those formed in the fountain nozzle spray drying methods.

[0078] In various aspects, the present methods may be used to produce a high burning rate gas generant composition including guanidine nitrate, basic copper nitrate, and from about 1 % to 30% by weight of a secondary perchlorate-containing oxidizer, such as potassium perchlorate. The composition may also include up to about 5% by weight of a slag promoter such as silicon dioxide. The process includes forming an aqueous mixture of the components by first completely dissolving the guanidine nitrate and then adding the basic copper nitrate and potassium perchlorate to the aqueous mixture to produce a slurry. The slurry is spray dried with a single orifice fountain nozzle to produce a freely flowing powder. The resulting powder is pressed into tablets, cylinders, or other geometries to produce grains suitable for use as a gas generant in an inflatable restraint system. [0079] Resulting tablets and pellets produced using material from single orifice fountain nozzle generally have fewer physical defects, such as voids and chips of the gas generant grain or pellet, as compared to tablets and pellets produced using material from two-fluid nozzle.

[0080] In this regard, certain gas generant materials have a compressed monolithic grain shape and further have an actual density that is greater than or equal to about 90% of the maximum theoretical density. According to certain aspects of the present disclosure, the actual density is greater than or equal to about 93%, more preferably greater than about 95% of the maximum theoretical density, and even more preferably greater than about 97% of the maximum theoretical density. In some embodiments, the actual density exceeds about 98% of the maximum theoretical density of the gas generant material. Such high actual mass densities in gas generant materials are obtained in certain methods of forming gas generant grains in accordance with various aspects of the present disclosure, where high compressive force is applied to gas generant raw materials that are substantially free of binder. [0081] In accordance with the present disclosure, the gas generant materials are in a dry powderized and/or pulverized form. The dry powders are compressed with applied forces greater than about 50,000 psi (approximately 350 MPa), preferably greater than about 60,000 psi (approximately 400 MPa), more preferably greater than about 65,000 psi (approximately 450 MPa), and most preferably greater than about 74,000 psi (approximately 500 MPa). The powderized materials can be placed in a die or mold, where the applied force compresses the materials to form a desired grain shape. While not limiting as to any particular theory by which the teachings of the present disclosure operate, it is believed that a high actual density as compared to the theoretical mass density is important because the gas generant grain holds its shape during combustion (rather than fracturing and/or pulverizing), which assists in maintaining the desirable performance characteristics, such as progressive surface area exposure, burn profile, combustion pressure, and the like. These aspects of performance improve both out-of-position occupant performance and eliminate the need for a two-stage driver inflatable restraint device assembly, as will be described in more detail below. [0082] Further, it is preferred that a loading density of the gas generant is relatively high; otherwise a low performance for a given envelope may result. A loading density is an actual volume of generant material divided by the total volume available for the shape. In accordance with various aspects of the present disclosure, it is preferred that a loading density for the gas generant is greater than or equal to about 60%, even more preferably greater than or equal to about 62%. In certain aspects, a gas generant has loading density of about 62 to about 63%.

[0083] Various aspects of the present disclosure provide a gas generant having a monolithic grain shape tailored to create rapid heated gas. The grain shape has a desired surface area and shape to facilitate prolonged reaction and to create preferred gas production profiles at the desired pressures, as will be described in more detail below. The absence of the binder further enables development of desirable burn and pressure profiles. It is the combination of the selected gas generant material composition, initial surface area, shape, and density of the monolithic gas generant grain that maximizes the desired performance results, which is facilitated by the removal of binder that would otherwise impede rapid reaction.

[0084] In accordance with various aspects of the present disclosure, a monolithic gas generant grain is created via certain processing steps to have a specific shape that enables such desirable properties. In certain embodiments, the gas generant is in the form of a single large grain, where a grain shape provides increasing surface area as the grain burns. In certain embodiments, one or more of such monolithic grains can be used in an inflator of an inflatable restraint device. The desired shape of the monolithic grain is linked to ballistic characteristics of the composition. The shape of the monolithic grain augments and controls the burn rate of the gas generant composition. The rate of generation of gas from a gas generant can be expressed by the following equation: m_g = p_gA_byr where "m_g" is a gas generation rate (mass per unit time), "P₉" density of the gas generant, "A_b" = burning area of the surface, "y" is a multiplication factor defined as the generant gas yield in %, and "r" is the mass burning rate, also known as the surface recession rate (length per unit time). The burning rate is an empirically determined function of the gas generant grain composition, and depends upon various factors including initial temperature of the gas generant, combustion pressure, velocity of gaseous combustion products over the surface of the solid, and the gas generant grain shape. A linear burn rate "r_L" for a gas generant material is independent of the surface of the gas generant grain shape and is also expressed in length per time at a given pressure.

[0085] In various embodiments, a desirably high burning rate enables desirable pressure curves for inflation of an airbag. In this regard, an initial surface area of the monolithic grain is relatively low as compared to surface areas of traditional pellets and/or wafers, as will be described in more detail below. However, as the preferred monolithic grain shapes of the present disclosure are burned, more surface area is progressively exposed, thus the amount of the composition combusting (m_g) progressively becomes greater and generates a higher quantity of gas.

[0086] In accordance with various aspects of the present disclosure, the gas generant has a linear burn rate of greater than or equal to about 1.0 inches per second (about 38.1 mm per second) at a pressure of about 3,000 pounds per square inch (about 20,865 kPa). In certain aspects, the gas generant has a linear burn rate of greater than or equal to about 1.1 inches per second (about 28 mm/Sec); optionally greater than or equal to about 1.2 inches per second (about 30.5 mm/Sec); optionally greater than or equal to about 1.3 inches per second (about 33 mm/Sec); optionally greater than or equal to about 1.4 inches per second(about 36 mm/Sec); optionally greater than or equal to about 1.5 inches per second (about 38 mm/Sec); optionally greater than or equal to about 1.6 inches per second (about 41 mm/Sec); optionally greater than or equal to about 1.7 inches per second (about 43 mm/Sec); optionally greater than or equal to about 1.8 inches per second (about 46 mm/Sec); and optionally greater than or equal to about 1.9 inches per second (about 48 mm/Sec); at a pressure of about 3,000 pounds per square inch (psi) (about 20.7 MPa). In certain embodiments, the linear burn rate of the gas generant is greater than or equal to about 2.0 inches per second (about 51 mm/Sec) at a pressure of about 3,000 psi (about 20.7 MPa). In certain embodiments, the burning rate of the gas generant is less than or equal to about 2.1 inches per second (about 53 mm/Sec) at a pressure of 3,000 psi (about 20.7 MPa).

[0087] Additionally, it is preferred that the gas generant has a high mass density in various embodiments. For example, in certain embodiments, the gas generant has a theoretical mass density of greater than about 1.9 g/cm³, preferably greater than about 1.94 g/cm³, and even more preferably greater than or equal to about 2.12 g/cm³.

[0088] Further, in accordance with the present disclosure, the gas yield of the gas generant is relatively high. For example, in certain embodiments, the gas yield is greater than or equal to about 2.4 moles/100 grams of gas generant. In other embodiments, the gas yield is greater than or equal to about 2.5 moles/100 g of gas generant. In certain embodiments, the gas yield is greater than or equal to about 3 moles/100 g of gas generant; optionally greater than or equal to about 3.1 moles/100 g of gas generant; and optionally greater than or equal to about 3.2 moles/100 g of gas generant

[0089] Expressed in another way, the amount of gas produced for a given mass of gas generant present at a specific volume is relatively high. In this regard, the product of gas yield and density is an important parameter for predicting performance of the gas generant. A product of gas yield and density (of the gas generant) is preferably greater than about 5.0 moles/100 cm³, and even more preferably greater than about 5.2 moles/100 cm³, in various embodiments.

[0090] Figure 4 depicts a single monolithic gas generant grain shape 110 according to certain aspects of the present disclosure. The combustion pressure resulting from the burning of a monolithic annular disk grain shape 110 such as that shown in Figure 4 is distinct from that of a conventional pellet (cylindrical shape) or wafer (a toroidal ring shape). The monolithic grain shape 110 shown in Figure 4 is an annular disk. Exemplary dimensions of the grain shape 110 are an inner diameter "a" of about 14 mm, an outer diameter "b" of 41 mm, and a height "c" of about 22 mm. A plurality of apertures 114 extend from a first side 116 of the gas generant grain 110 to a second side 118 of the gas generant grain 110, thus providing open channels through the body 120 of the grain 110 that extend therethrough. As shown, each aperture 114 has a diameter "d" of about 3 mm. The gas generant grain 110 as shown has 30 apertures 114, although different configurations, dimensions, and quantities of the apertures 114 are contemplated. The number, size, and position of the apertures 114 may be varied, as they relate to the desired initial surface area and specific burn rate of the gas generant material. Similarly, the dimensions (a, b, and c) of the disk can also be varied, as appreciated by skilled artisans. For example, where multiple disks are employed as gas generant, the height "c" can be reduced.

[0091] The initial surface area of this grain shape 110 is relatively low, as compared to conventional pellet or wafer shapes; however, the burn rate of the gas generant material is sufficiently high to permit a low initial surface area that burns rapidly to expose additional surface area as the combustion reaction progresses. The initial surface area of the shape of the gas generant grain 110 as shown in Figure 4 is less than 12,000 mm²; specifically it is about 11 ,930 mm². In various embodiments, an initial surface area of the grain shape is less than about 13,000 mm². Traditional grain shapes require a higher initial surface area, for example greater than about 35,000 mm², inter alia, to achieve the necessary burn rate and gas combustion pressure to inflate an airbag cushion appropriately.

[0092] In accordance with the present disclosure, a ratio of the diameter of the each aperture to the length (L/D) is preferably from about 3.5 to about 9. In certain embodiments, the maximum ratio of L/D is 7.5. In the specific example shown in Figure 4, the L/D ratio of each aperture is about 7.3. The ratio of L/D of the plurality of apertures relates to the surface area progression and overall burning behavior of the gas generant. The number of apertures and the ratio of L/D of each aperture relate to the shape or profile of the combustion pressure curve of the gas generant material. [0093] The profile of the combustion pressure curve relates to the improved protection for occupants and it is preferable that the combustion pressure curve is progressive to neutral, in accordance with the principles of the present disclosure. The comparative conventional materials typically have regressive combustion pressure curves. The profile of this pressure curve relates to the amount of surface area of the gas generant which correlates to the mass of generant reacting, hence the mass gas generation rate (m_g) and pressure of gas generated over time. In this regard, a monolithic shape of the gas generant grain 110, similar to that shown in Figure 4, provides a controlled combustion pressure that provides longer, controlled, and sustained combustion pressure at desired levels which is important for improving inflator effluent properties and for occupant safety during deployment of the airbag cushion. [0094] This concept can also be expressed as a "rise rate" which is the rate at which the gas output from an inflator increases pressure (usually measured when the gas output is directed to a closed volume). It is commonly desirable that an inflatable restraint airbag cushion initially inflates in a relatively gradual manner to reduce injury to an occupant (particularly where the occupant is too close to the airbag or "out-of-position") which is then followed by a period where the inflation gas passes into the airbag cushion at a relatively greater or increased pressure rate. A gas generant that creates such inflation is commonly referred to in the art as producing inflation gas in an "S" curve. The gas generants of the present disclosure approach a rise rate having an S curve, which is highly desirable, particularly for out-of-position occupants. These features of the present disclosure will be described in greater detail in the context of Figure 8.

[0095] In accordance with aspects of the various embodiments of the present disclosure, a monolithic grain design provides a lower rise rate, while providing a higher average combustion pressure and superior control over the burning characteristics. Additionally, in preferred embodiments, the absence of polymeric binder in the gas generant as compared to conventional extruded monolithic grains improves burning characteristics.

[0096] As discussed above, the gas generants according to various embodiments of the present disclosure provide improved effluent quality. This may be attributed to several aspects of the present disclosure, including that the gas generant composition is substantially free of polymeric binder and that gas generants including perchlorate containing oxidizers surprisingly reduce maximum operating pressure (MEOP), while increasing average operating pressure that appears to reduce noxious effluent levels during inflator operation. [0097] In accordance with certain embodiments of the present disclosure, the maximum combustion temperature (also expressed as flame temperature) is less than about 2,300 K. In various embodiments, the flame temperature during combustion can range from about 1400 K to about 2300 K. In certain embodiments, the flame temperature is less than about 2,000 K. .

[0098] In certain aspects, the present disclosure provides methods of making a gas generant for an inflatable restraint device that comprises spray drying an aqueous mixture to produce a powder, wherein the aqueous mixture comprises a fuel and an oxidizer comprising a perchlorate-containing compound. The powder is pressed to produce a monolithic gas generant grain having an annular disk including a plurality of apertures. In certain embodiments, the pressed gas generant has an actual density of greater than or equal to about 95% of the maximum theoretical mass density of the gas generant. The pressed monolithic gas generant grain has an average linear burn rate of greater than or equal to about 1 inch per second (about 38.1 mm per second) at a pressure of about 3,000 pounds per square inch (about 20.7 MPa) and a gas yield of greater than or equal to about 3 moles/100 g of gas generant. In certain embodiments, the gas generant has a mass density of greater than or equal to about 1.9 g/cm³ and the plurality of apertures has a ratio of length to diameter of about 3.5 to about 8 and an initial surface area of the disk is less than about 13,000 mm². The gas generant is optionally any of the gas generants described above. In certain aspects, the aqueous mixture optionally comprises water, guanidine nitrate and the oxidizer comprises potassium perchlorate and further comprises a second distinct oxidizer of basic copper nitrate. In certain aspects, the spray drying of the aqueous mixture to produce the powder is performed using a single orifice fountain nozzle. [0099] Table 3 compares effluent generated from a pressed monolithic annular disk shaped gas generant similar to that shown in Figure 4, having about 60% guanidine nitrate, about 26% basic copper nitrate, about 14 % potassium perchlorate oxidizer, and about 0.3% silicon dioxide. The U.S. Council for Automotive Research (USCAR) issues guidelines for maximum recommended levels of effluent constituents in airbag devices. Desirably, the production of these effluents is minimized to at or below these guidelines. The current USCAR guidelines for a driver-side inflatable restraint device are included in Table 3. Also, 1/3 of the USCAR driver-side guidelines for effluent constituents (33% of the recommended levels) are included.

[00100] Table 3 shows averaged effluent analysis during combustion of the gas generant by Fourier Transform Infrared Analysis (FTIR) showing that the nitrogen oxide species, which includes NO, NO2, and NO_x effluent gases are reduced and hence improved. As can be observed, with the exception of carbon monoxide all of the trace gas levels (effluent levels) are well below the USCAR guidelines (and less than 1/3 of the USCAR guidelines), where some fell below detection limits (NO2, NO_x, and hydrogen cyanide). The carbon monoxide levels are within the USCAR guidelines and can easily be adjusted to be within 1/3 USCAR guideline limit by stoichiometric adjustment.

[00101] TABLE 3

[00102] In accordance with various embodiments of the present disclosure, a gas generant is suitable for a single-stage driver inflatable restraint device, as the gas generant has a rapid reaction rate and sufficient sustained combustion pressure to eliminate any need for multiple stages of inflation without endangering occupants in an out-of-position (OOP) condition. Thus, all of the above described hardware and complications can be avoided via the use of the improved gas generants in a single stage according to various embodiments of the present disclosure. Thus, grain shapes similar to those of Figure 4 can be used in a single stage inflator to provide superior OOP performance without the need for second stage operation and associated complex hardware.

[00103] The embodiments of the present disclosure can be further understood by the specific examples contained herein. Specific examples are provided for illustrative purposes of how to make and use the compositions and methods of the present disclosure and, unless explicitly stated otherwise, are not intended to be a representation that given embodiments of this present disclosure have, or have not, been made or tested. Example 2

[00104] In one example, a gas generant having about 60% guanidine nitrate, about 26% basic copper nitrate, about 14 % potassium perchlorate oxidizer, and about 0.3% silicon dioxide of slag forming agent is prepared. 300 Ib of guanidine nitrate is charged to 40 gallons of hot water to form an aqueous solution. 130 Ib of basic copper nitrate is slowly added to the aqueous solution. 15 Ib of silicon dioxide is added to the aqueous solution, which is then mixed for 60 minutes at 200 rpm. The slurried mixture is then spray dried via a single fountain nozzle set-up, as discussed above. The powder particles are collected in a baghouse collector and have an average particle size of 100 μm.

[00105] A release agent (inert carbon, i.e., graphite) is dry blended with the spray dried composition. The blended powder is placed in a pre-formed die having the desired shape, such as the annular disk shape with a plurality of apertures, as shown in Figure 4, for example. The die and powders are placed in a large, high tonnage hydraulic press capable of exerting forces in excess of

50 tons. The raw materials are pressed to form a monolithic gas generant solid.

[00106] Figure 8 is a graph showing combustion pressure versus time for a gas generant monolith formed according to Example 2 compared to a conventional pressed pellet designated Control A. Control A is formed by conventional tablet pressing. The gas generant compositions of both Example 2 and Control A are the same, as described above. [00107] Example 2 and Control A are ignited at the same time (at approximately 1 millisecond). As can be observed from Figure 8, Control A generates an initial combustion pressure of 2,900 psi and peaks within 10 milliseconds. The combustion pressure falls below 2,000 psi for Control A at approximately 15 milliseconds. However, for various airbag applications, it is preferred that the combustion pressure remains over 2,000 psi for at least about 25 to 30 milliseconds, which improves effluent. Thus, if Control A were employed as the gas generant in an inflatable restraint device, it would most likely require a two stage design, with Stage I only being deployed in OOP situations to alleviate OOP forces on the occupant. Deploying only Stage I exposes less initial gas generant surface area, thus proportionately reducing initial mass and inflating flow (the product of moles generated and temperature). Further, the shape of the pressure curve over time for Control A has a large initial slope (a high rise rate, as described above). The shape of the pressure curve is regressive, as the peak pressure is reached within about 5 milliseconds of initial burning. The pressure then regresses (i.e., decreases) for the remainder of burning time (here greater than 75 milliseconds).

[00108] As can be observed, the gas generant of Example 2 creates a combustion pressure that is greater than 2,000 psi for over 35 milliseconds. Further, combustion pressure is lowest at the beginning of operation, which is highly desirable for reducing out-of-position occupant injuries. In various embodiments, it is preferred that the combustion pressure does not exceed about 3,500 psi for considerations related to the structure of the inflator. In various embodiments of the present disclosure, the pressure generated by the gas generant is from about 2,000 psi to about 3,000 psi for at least 30% of the burning period, preferably for at least about 40% of the burning period, preferably for at least about 45-75% of the burning period. These pressure parameters improve effluent quality by reducing undesirable species.

[00109] Later in operation as the gas generant grain burns back, burning surface area increases substantially, allowing the gas production for the inflator to effectively increase. In this regard the initial mass flow and/or inflating flow of the gas generant of Example 2 is much lower with the monolithic grain shape than with pellets traditionally used in pyrotechnic inflators (i.e., in Control A). Thus, the rise rate or slope of pressure increase over time is more gradual than the rise rate of Control A. For example, a slope of the pressure curve is slightly progressive with increasing pressure until the maximum pressure is reached at about 35 milliseconds. After the peak pressure is reached, burning continues and the pressure curve slope decreases. This progressive behavior is highly desirable and is closely related to the surface area, including the number of apertures and their respective L/D ratios, as well as the burn rate, gas yield, and density of the monolith solid gas generant grains of the present disclosure. This combustion pressure profile results in acceptable or improved bag inflation and occupant restraint as shown by linear impactor testing.

[00110] In some collision events, the vehicle occupant may be out-of- position, or in a location not anticipated by the design of the inflatable restraint system. Such occupants may be subject to a higher risk of injury by a deploying airbag because of their improper placement. The higher the initial combustion pressure (or more specifically, the higher the initial inflating flow), the greater the likelihood of injury for an out-of-position occupant. For example, a lower rise rate of the gases produced by the gas generant (a lower slope) relates to improved out-of-position occupant (reduced injury) results. The gas generant monolithic grain of Example 2 has significantly reduced maximum operating pressure (2,600 psi) as compared to a pellet formed of the same composition (2,900 psi), while having significantly increased average operating pressure over the action time of the generant. See for example, the operating pressure of Example 2 is greater than 1 ,500 psi for over 45 milliseconds as compared to less than 20 milliseconds for Control A. lnflator tank traces are also indicated for the monolithic grain of Example 2 and the pellet of Control A, where the initial rate of pressure increase is more gradual for Example 2 from 1 to 50 ms time period as compared to that of the Control A pellet during the same interval, again demonstrating improved dynamic inflator performance. Notably, Example 2 provides a higher maximum tank pressure than Control A, despite having a lower maximum operating pressure of the gas generant and a neutral to progressive combustion profile. The monolithic gas generant grain provides improved dynamic performance behavior and also improves effluent behavior in an inflator device.

[00111] The shape of such pressed monolithic gas generant grains according to various embodiments of the present disclosure not only improves effluents, but it also significantly improves the out-of-position (OOP) characteristics of the module employing an inflator with such a grain.

[00112] High burning rate gas generants (preferably greater than or equal to about 1 in/sec at 3,000 psi) having high mass density (preferably greater than or equal to 1.9 g/cm³) and high gas yield (preferably greater than or equal to about 2.4 moles per 100 grams) are desirable to take full advantage of the shapes providing low initial surface area and progressive surface area during burning. In certain alternate embodiments, similar grain shapes can be extruded to provide similar surface area progressively. In other embodiments, a plurality of monolithic pressed grains can be used as gas generants. Such grains can have reduced dimensions from single monolithic grains.

[00113] Various embodiments of the present disclosure provide a gas generant for use in an inflatable restraint device that provides an initial surface area that is low as compared to conventional wafers and pellets, thus improving OOP behavior. As the grain burns, surface area increases, maintaining or even improving total occupant restraint. The gas generant grain maintains combustion pressure above 2,000 psi for a large duration of burn, while not exceeding 3,500 psi. In other words, in various embodiments, gas generant grains of the present disclosure have considerably higher average combustion pressure than traditional gas generant pellets or wafers. [00114] Further improvement in effluent quality is achieved by other aspects of certain embodiments of the present disclosure, where gas generant grains are formed by pressing the monolithic grain, as opposed to extruding the gas generant formulation. In this regard, the gas generant grain is substantially free of polymeric binder and has robust stability due to formation by application of compressive strength. In certain embodiments, the actual density of the gas generant grain is greater than 95% of the maximum theoretical density. The burn rate, combustion profile, and effluent quality are significantly improved by the absence of binder compositions.

[00115] The present disclosure still further provides pyrotechnic compositions that are economical to manufacture. The present disclosure additionally provides a burn rate enhanced gas generant composition that overcomes one or more of the limitations of conventional gas generants.

Claims

CLAIMS What is claimed is:

1. A pressed monolithic gas generant for an inflatable restraint device comprising an annular disk having a plurality of apertures, wherein the plurality of apertures have a ratio of length to diameter of about 3.5 to about 8, an initial surface area of the disk is less than about 13,000 mm², wherein a linear burn rate of the gas generant is greater than or equal to about 1 inch per second (about 25 mm per second) at a pressure of about 3,000 pounds per square inch (about 20.7 MPa), and wherein the gas generant comprises a fuel and an oxidizer comprising a perchlorate-containing compound, wherein the gas generant is substantially free of polymeric binder.

2. The pressed monolithic gas generant of Claim 1 , wherein said perchlorate-containing compound is selected from the group consisting of ammonium perchlorate (NH₄CIO₄), sodium perchlorate (NaCIO₄), potassium perchlorate (KCIO₄), lithium perchlorate (LiCIO₄), and combinations thereof.

3. The pressed monolithic gas generant of Claim 1 , wherein a gas yield of the gas generant is greater than or equal to about 3 moles/100 g of gas generant.

4. The pressed monolithic gas generant of Claim 1 , wherein said perchlorate-containing compound is a secondary oxidizer and the oxidizer further comprises a distinct primary oxidizer in addition to said perchlorate-containing compound secondary oxidizer.

5. The pressed monolithic gas generant of Claim 4, wherein said gas generant comprises a total amount of a fuel component is about 30 to about 70 parts by weight, a total amount of oxidizer component including said perchlorate- containing compound is about 30 to about 60 parts by weight; and about a total amount of slag forming agent is 0 to about 5 parts by weight.

6. The pressed monolithic gas generant of Claim 4, wherein said fuel component is selected from the ground consisting of guanidine nitrate, aminotetrazole, salts of tetrazole, bitetrazoles, 1 ,2,4-thazole-5-one, nitro guanidine, amino guanidine nitrate, and mixtures thereof; said secondary oxidizer is selected from the group consisting of ammonium perchlorate, sodium perchlorate, potassium perchlorate, lithium perchlorate, and combinations thereof; said primary oxidizer is selected from the group consisting of from alkali, alkaline earth, and ammonium nitrates, and nitrites, metal oxides, basic metal nitrates, transition metal complexes of ammonium nitrate; ammonium dinitramide, and combinations thereof; and said slag forming agent is selected from the group consisting of silicon dioxide (Siθ2), aluminum oxide, cerium oxide, ferric oxide, zinc oxide, titanium oxide, zirconium oxide, bismuth oxide, molybdenum oxide, lanthanum oxide, and combinations thereof.

7. The pressed monolithic gas generant of Claim 4, wherein said fuel component comprises guanidine nitrate, said perchlorate-containing secondary oxidizer comprises potassium perchlorate, said primary oxidizer comprises basic copper nitrate, and said slag forming agent comprises silica (SiO₂).

8. The pressed monolithic gas generant of Claim 1 , wherein the gas generant in a final pressed form has an actual density of greater than or equal to about 95% of the maximum theoretical mass density of the gas generant.

9. The pressed monolithic gas generant of Claim 1 , wherein the gas generant has a flame temperature of less than or equal to about 2300 Kelvin.

10. An inflatable restraint device comprising: an airbag for restraining motion of a vehicle occupant; and at least one gas generant for inflating the airbag in the form of a monolithic annular disk having a plurality of apertures and further having a linear burn rate of greater than or equal to about 1 inch per second at a pressure of about 3,000 pounds per square inch (about 20.7 MPa) and a gas yield greater than or equal to about 3 moles/100 g of gas generant, wherein the gas generant comprises a fuel, an oxidizer comprising a perchlorate-containing compound, and is substantially free of polymeric binder.

11. The device of Claim 10, wherein a combustion pressure in the inflatable device is less than or equal to about 3,000 pounds per square inch (20.7 MPa) and greater than about 2,000 pounds per square inch (13.8 MPa) for at least 25 milliseconds.

12. The device of Claim 10, wherein the gas generant has a maximum expected operating pressure (MEOP) during combustion of less than or equal to about 2,700 psi (about 18.6 MPa).

13. The device of Claim 10, wherein during combustion, the gas generant has a neutral to progressive burn profile from 5 to about 35 milliseconds.

14. The device of Claim 10, wherein said perchlorate-containing compound is selected from the group consisting of ammonium perchlorate (NH₄CIO₄), sodium perchlorate (NaCIO₄), potassium perchlorate (KCIO₄), lithium perchlorate (LiCIO₄), and combinations thereof.

15. A method of making a gas generant for an inflatable restraint device, the method comprising: spray drying an aqueous mixture to produce a powder, wherein said aqueous mixture comprises a fuel and an oxidizer comprising a perchlorate- containing compound; and pressing the powder to produce a monolithic gas generant grain having an annular disk including a plurality of apertures, wherein the monolithic gas generant grain has an average linear burn rate of greater than or equal to about 1 inch per second (about 38.1 mm per second) at a pressure of about 3,000 pounds per square inch (about 20.7 MPa) and a gas yield of greater than or equal to about 3 moles/100 g of gas generant.

16. The method of Claim 15, wherein said aqueous mixture comprises water, said fuel comprises guanidine nitrate and said perchlorate-containing compound is potassium perchlorate and said oxidizer further comprises a second distinct oxidizer comprising basic copper nitrate.

17. The method of Claim 15, wherein said spray drying of the aqueous mixture to produce the powder is performed using a single orifice fountain nozzle.

18. The method of Claim 15, wherein said perchlorate-containing compound is selected from the group consisting of ammonium perchlorate (NH₄CIO₄), sodium perchlorate (NaCIO₄), potassium perchlorate (KCIO₄), lithium perchlorate (LiCIO₄), and combinations thereof.

19. The method of Claim 15, wherein the gas generant has a mass density of greater than or equal to about 1.9 g/cm³ and said plurality of apertures has a ratio of length to diameter of about 3.5 to about 8 and an initial surface area of the disk is less than about 13,000 mm².

20. The method of Claim 15, wherein the pressed gas generant has an actual density of greater than or equal to about 95% of the maximum theoretical mass density of the gas generant.