Patent Grant
6887326
The present invention relates to nontoxic gas generating compositions which upon combustion, rapidly generate gases that are useful for inflating occupant safety restraints in motor vehicles and specifically, the invention relates to nonazide gas generants that produce combustion products having not only acceptable toxicity levels, but that also exhibit a relatively high gas volume to solid particulate ratio at acceptable flame temperatures, and, operate at relatively lower vessel pressures.
The present invention relates to nontoxic gas generating compositions which upon combustion, rapidly generate gases that are useful for inflating occupant safety restraints in motor vehicles and specifically, the invention relates to nonazide gas generants that produce combustion products having not only acceptable toxicity levels, but that also exhibit a relatively high gas volume to solid particulate ratio at acceptable flame temperatures. Additionally, the compositions of the present invention readily ignite and sustain combustion at burn rates heretofore thought to be too low for automotive airbag applications.
The evolution from azide-based gas generants to nonazide gas generants is well documented in the prior art. The advantages of nonazide gas generant compositions in comparison with azide gas generants have been extensively described in the patent literature, for example, U.S. Pat. Nos. 4,370,181; 4,909,549; 4,948,439; 5,084,118; 5,139,588 and 5,035,757, the discussions of which are hereby incorporated by reference.
In addition to a fuel constituent, pyrotechnic nonazide gas generants contain ingredients such as oxidizers to provide the required oxygen for rapid combustion and reduce the quantity of toxic gases generated, a catalyst to promote the conversion of toxic oxides of carbon and nitrogen to innocuous gases, and a slag forming constituent to cause the solid and liquid products formed during and immediately after combustion to agglomerate into filterable clinker-like particulates. Other optional additives, such as burning rate enhancers or ballistic modifiers and ignition aids, are used to control the ignitability and combustion properties of the gas generant.
One of the disadvantages of known nonazide gas generant compositions is the amount and physical nature of the solid residues formed during combustion. The solids produced as a result of combustion must be filtered and otherwise kept away from contact with the occupants of the vehicle. It is therefore highly desirable to develop compositions that produce a minimum of solid particulates while still providing adequate quantities of a nontoxic gas to inflate the safety device at a high rate.
It is known that the use of ammonium nitrate as an oxidizer contributes to the gas production with a minimum of solids. To be useful, however, gas generants for automotive applications must be thermally stable when aged for 400 hours or more at 107° C. The compositions must also retain structural integrity when cycled between −40° C. and 107° C.
Generally, gas generant compositions using ammonium nitrate are thermally unstable propellants that produce unacceptably high levels of toxic gases, CO and NOx for example, depending on the composition of the associated additives such as plasticizers and binders. Known ammonium nitrate compositions are also hampered by poor ignitability, delayed burn rates, and significant performance variability. Several prior art compositions incorporating ammonium nitrate utilize well known ignition aids such as BKNO3 to solve this problem. However, the addition of an ignition aid such as BKNO3 is undesirable because it is a highly sensitive and energetic compound.
Yet another concern is the pressure requirements for complete combustion of various nonazide compositions containing phase stabilized ammonium nitrate. For certain compositions containing phase stabilized ammonium nitrate, the inflator must be manufactured with a more robust design, such as heavier and thicker walls, to accommodate the relatively greater pressure needed to sustain combustion and minimize the potential for performance variation. This adds to the raw material requirements and to the manufacturing complexity. A reduction in the pressure requirements would therefore constitute a substantial improvement in the art.
The gas generant compositions described in Poole et al, U.S. Pat. Nos. 4,909,549 and 4,948,439, use tetrazole or triazole compounds in combination with metal oxides and oxidizer compounds (alkali metal, alkaline earth metal, and pure ammonium nitrates or perchlorates) resulting in a relatively unstable generant that decomposes at low temperatures. Significant toxic emissions and particulate are formed upon combustion. Both patents teach the use of BKNO3 as an ignition aid.
The gas generant compositions described in Poole, U.S. Pat. No. 5,035,757, result in more easily filterable solid products but the gas yield is unsatisfactory.
Chang et al, U.S. Pat. No. 3,954,528, describes the use of triaminoguanidine nitrate (“TAGN”) and a synthetic polymeric binder in combination with an oxidizing material. The oxidizing materials include ammonium nitrate (“AN”) although the use of phase stabilized ammonium nitrate (“PSAN”) is not suggested. The patent teaches the preparation of propellants for use in guns or other devices where large amounts of carbon monoxide and hydrogen are acceptable and desirable.
Grubaugh, U.S. Pat. No. 3,044,123, describes a method of preparing solid propellant pellets containing AN as the major component. The method requires use of an oxidizable organic binder (such as cellulose acetate, PVC, PVA, acrylonitrile and styrene-acrylonitrile), followed by compression molding the mixture to produce pellets and by heat treating the pellets. These pellets would certainly be damaged by temperature cycling because commercial AN is used and the composition claimed would produce large amounts of carbon monoxide.
Becuwe, U.S. Pat. No. 5,034,072, is based on the use of 5-oxo-3-nitro-1,2,4-triazole as a replacement for other explosive materials (HMX, RDX, TATB, etc.) in propellants and gun powders. This compound is also called 3-nitro-1,2,4-triazole-5-one (“NTO”). The claims appear to cover a gun powder composition which includes NTO, AN and an inert binder, where the composition is less hygroscopic than a propellant containing ammonium nitrate. Although called inert, the binder would enter into the combustion reaction and produce carbon monoxide making it unsuitable for air bag inflation.
Lund et al, U.S. Pat. No. 5,197,758, describes gas generating compositions comprising a nonazide fuel which is a transition metal complex of an aminoarazole, and in particular are copper and zinc complexes of 5-aminotetrazole and 3-amino-1,2,4-triazole which are useful for inflating air bags in automotive restraint systems, but generate excess solids.
Wardle et al, U.S. Pat. No. 4,931,112, describes an automotive air bag gas generant formulation consisting essentially of NTO (5-nitro-1,2,4-triazole-3-one) and an oxidizer wherein said formulation is anhydrous.
Ramnarace, U.S. Pat. No. 4,111,728, describes gas generators for inflating life rafts and similar devices or that are useful as rocket propellants comprising ammonium nitrate, a polyester type binder and a fuel selected from oxamide and guanidine nitrate.
Boyars, U.S. Pat. No. 4,124,368, describes a method for preventing detonation of ammonium nitrate by using potassium nitrate.
Mishra, U.S. Pat. No. 4,552,736, and Mehrotra et al, U.S. Pat. No. 5,098,683, describe the use of potassium fluoride to eliminate expansion and contraction of ammonium nitrate in transition phase.
Chi, U.S. Pat. No. 5,074,938, describes the use of phase stabilized ammonium nitrate as an oxidizer in propellants containing boron and useful in rocket motors.
Canterberry et al, U.S. Pat. No. 4,925,503, describes an explosive composition comprising a high energy material, e.g., ammonium nitrate and a polyurethane polyacetal elastomer binder, the latter component being the focus of the invention.
Hass, U.S. Pat. No. 3,071,617, describes long known considerations as to oxygen balance and exhaust gases.
Stinecipher et al, U.S. Pat. No. 4,300,962, describes explosives comprising ammonium nitrate and an ammonium salt of a nitroazole.
Prior, U.S. Pat. No. 3,719,604, describes gas generating compositions comprising aminoguanidine salts of azotetrazole or of ditetrazole.
Poole, U.S. Pat. No. 5,139,588, describes nonazide gas generants useful in automotive restraint devices comprising a fuel, an oxidizer and additives.
Chang et al, U.S. Pat. No. 3,909,322, teaches the use of nitroaminotetrazole salts with pure ammonium nitrate as gun propellants and gas generants for use in gas pressure actuated mechanical devices such as engines, electric generators, motors, turbines, pneumatic tools, and rockets.
Bucerius et al, U.S. Pat. No. 5,198,046, teaches the use of diguanidinium-5,5′-azotetrazolate with KNO3 as an oxidizer, for use in generating environmentally friendly, non-toxic gases, and providing excellent thermal stability.
Onishi et al, U.S. Pat. No. 5,439,251, teaches the use of a tetrazole amine salt as an air bag gas generating agent comprising a cationic amine and an anionic tetrazolyl group having either an alkyl with carbon number 1-3, chlorine, hydroxyl, carboxyl, methoxy, aceto, nitro, or another tetrazolyl group substituted via diazo or triazo groups at the 5-position of the tetrazole ring. The focus of the invention is on improving the physical properties of tetrazoles with regard to impact and friction sensitivity, and does not teach the combination of a tetrazole amine salt with any other chemical.
Lund et al, U.S. Pat. No. 5,501,823, teaches the use of nonazide anhydrous tetrazoles, derivatives, salts, complexes, and mixtures thereof, for use in air bag inflators.
Highsmith et al, U.S. Pat. No. 5,516,377, teaches the use of a salt of 5-nitraminotetrazole, a conventional ignition aid such as BKNO3, and pure ammonium nitrate as an oxidizer, but does not teach the use of phase stabilized ammonium nitrate.
Therefore, the objects of the invention include providing high yield (gas/mass>90%) gas generating compositions that produce large volumes of non-toxic gases with minimal solid particulates, that are thermally and volumetrically stable from −40° C. through 110° C., that contain no explosive components, and that ignite without delay and sustain combustion in a repeatable manner.
The aforementioned concerns are solved by providing a nonazide gas generant for a vehicle passenger restraint system employing bis-(1(2)H-tetrazol-5-yl)-amine (BTA) at about 10-50% by weight of the composition, and phase stabilized ammonium nitrate (PSAN) as an oxidizer at about 30-90 weight percent of the composition. Preferred stabilizing agents for the PSAN include potassium nitrate and potassium perchlorate, at 10-15% by weight of the PSAN, but may include other known stabilizing agents in amounts sufficient to stabilize the ammonium nitrate.
An optional and preferred secondary fuel is selected from the group consisting of amine salts of tetrazoles and triazoles having a cationic amine component and an anionic component. The anionic component comprises a tetrazole or triazole ring, and an R group substituted on the 5-position of the tetrazole ring, or two R groups substituted on the 3- and 5-positions of the triazole ring. The R group(s) is selected from hydrogen and any nitrogen-containing compounds such as amino, nitro, nitramino, tetrazolyl and triazolyl groups. The cationic amine component is selected from an amine group including ammonia, hydrazine, guanidine compounds such as guanidine, aminoguanidine, diaminoguanidine, triaminoguanidine, dicyandiamide, nitroguanidine, nitrogen subsituted carbonyl compounds such as urea, carbohydrazide, oxamide, oxamic hydrazide, bis-(carbonamide) amine, azodicarbonamide, and hydrazodicarbonamide, and amino azoles such as 3-amino-1,2,4-triazole, 3-amino-5-nitro-1,2,4-triazole, 5-aminotetrazole and 5-nitraminotetrazole. The secondary fuel when present ranges from about 0.1-30% by weight of the gas generating composition
The gas generants may yet further contain a secondary metallic oxidizer selected from alkali metal and alkaline earth metal nitrates and perchlorates. One of ordinary skill will readily appreciate that other oxidizers such as metallic oxides, nitrites, chlorates, peroxides, and hydroxides may also be used. The metallic oxidizer when present ranges from about 0.1-20% by weight of the gas generating composition.
The gas generants may yet further contain an inert component such as an inert mineral selected from the group containing silicates, silicon, diatomaceous earth, and oxides such as silica, alumina, and titania. The silicates include but are not limited to silicates having layered structures such as talc and the aluminum silicates of clay and mica; aluminosilicates; borosilicates; and, other silicates such as sodium silicate and potassium silicate. The inert component when present ranges from about 0.1-10% by weight of the gas generating composition.
In accordance with the present invention, a gas generant contains the following constituents given in weight percents of the total composition. A primary fuel is selected from a substituted tetrazole or substituted bitetrazole occurring as a natural hydrate, such as bis-(1(2)H-tetrazol-5-yl)-amine or 5-aminotetrazole at 10-50%, and more preferably at 25-32%.
When employed, preferred high nitrogen nonazide secondary include, in particular, amine salts of tetrazole and triazole selected from the group including monoguanidinium salt of 5,5′-Bis-1H-tetrazole (BHT·1GAD), diguanidinium salt of 5,5′-Bis-1H-tetrazole (BHT·2GAD), monoaminoguanidinium salt of 5,5′-Bis-1H-tetrazole (BHT·1AGAD), diaminoguanidinium salt of 5,5′-Bis-1H-tetrazole (BHT·2AGAD), monohydrazinium salt of 5,5′-Bis-1H-tetrazole (BHT·1HH), dihydrazinium salt of 5,5′-Bis-1H-tetrazole (BHT·2HH), monoammonium salt of bis-(1(2)H-tetrazol-5-yl)-amine (BTA·1NH3), monoammonium salt of 5,5′-bis-1H-tetrazole (BHT·1NH3), diammonium salt of 5,5′-bis-1H-tetrazole (BHT·2NH3), mono-3-amino-1,2,4-triazolium salt of 5,5′-bis-1H-tetrazole (BHT·1ATAZ), di-3-amino-1,2,4-triazolium salt of 5,5′-bis-1H-tetrazole (BHT·2ATAZ), diguanidinium salt of 5,5′-Azobis-1H-tetrazole (ABHT·2GAD), and monoammonium salt of 5-Nitramino-1H-tetrazole (NAT·1NH3). The secondary nonazide fuel generally comprises 10-65%, and preferably comprises 20-55%, by weight of the total gas generant composition.
A generic nonmetal salt of tetrazole as shown in Formula I includes a cationic component, Z, and an anionic component comprising a tetrazole ring and an R group substituted on the 5-position of the tetrazole ring. A generic nonmetal salt of triazole as shown in Formula II includes a cationic component, Z, and an anionic component comprising a triazole ring and two R groups substituted on the 3- and 5-positions of the triazole ring, wherein R1 may or may not be structurally synonymous with R2. An R component is selected from a group including hydrogen or any nitrogen-containing compound such as an amino, nitro, nitramino, or a tetrazolyl or triazolyl group from Formula I or II, respectively, substituted directly or via amine, diazo, or triazo groups. The compound Z forms a cation by displacing a hydrogen atom at the 1-position of either formula, and is selected from an amine group including ammonia, hydrazine; guanidine compounds such as guanidine, aminoguanidine, diaminoguanidine, triaminoguanidine, and nitroguanidine; amides including dicyandiamide, urea, carbohydrazide, oxamide, oxamic hydrazide, Bi-(carbonamide)amine, azodicarbonamide, and hydrazodicarbonamide; and substituted azoles including 3-amino-1,2,4-triazole, 3-amino-5-nitro-1,2,4-triazole, 5-aminotetrazole, 3-nitramino-1,2,4-triazole, and 5-nitraminotetrazole; and azines such as melamine.
The foregoing primary and optional secondary fuels may initially be dry-mixed with phase stabilized ammonium nitrate (PSAN). PSAN is generally employed in a concentration of about 30-90%, and more preferably 60-75%, by weight of the total gas generant composition. The ammonium nitrate is preferably stabilized with potassium nitrate, as described in Example 16, and as taught in co-owned U.S. Pat. No. 5,531,941, entitled, “Process For Preparing Azide-Free Gas Generant Composition”, and granted on Jul. 2, 1996, incorporated herein by reference. The PSAN comprises 85-90% AN and 10-15% KN and is formed by any suitable means such as co-crystallization of AN and KN, so that the solid-solid phase changes occurring in pure ammonium nitrate (AN) between −40° C. and 107° C. are prevented. Although KN is preferably used to stabilize pure AN, one skilled in the art will readily appreciate that other stabilizing agents may be used in conjunction with AN.
The gas generants, if desired, further contain a metallic oxidizer selected from alkali metal and alkaline earth metal nitrates and perchlorates at about 0-20%, and more preferably at about 0-10% by weight of the gas generant composition. One of ordinary skill will readily appreciate that other oxidizers such as metallic oxides, nitrites, chlorates, peroxides, and hydroxides may also be used. The metallic oxidizer when present constitutes about 0.1-20%, and more preferably 0.1-10%, by weight of the gas generating composition.
The gas generants, if desired, yet further contain an inert component selected from the group containing silicates, silicon, diatomaceous earth, and oxides such as silica, alumina, and titania. The silicates include but are not limited to silicates having layered structures such as talc and the aluminum silicates of clay and mica; aluminosilicate; borosilicates; and other silicates such as sodium silicate and potassium silicate. The inert component is present at about 0.1-10%, and more preferably at about 0.1-2%, by weight of the gas generating composition.
A preferred embodiment contains 65.3% of ammonium nitrate and 7.26% of potassium nitrate coprecipitated as PSAN, 13.72% of monoammonium salt of bis-(1(2)H-tetrazol-5-yl)-amine (BTA-1NH3), and 13.72% of BTA.
When utilized, the combination of the metallic oxidizer and the inert component results in the formation of a mineral containing the metal from the metallic oxidizer. For example, the combination of clay, which is primarily aluminum silicate (Al2Si4O10) and quartz (SiO2) with strontium nitrate (Sr(NO3)2) results in a combustion product consisting primarily of strontium silicates (SrSiO4 and Sr3SiO5). It is believed that this process aids in sustaining the gas generant combustion at all pressures and thus prevents inflator “no-fires”.
Burn rates of gas generants as described above may be lower than the industry standard of 0.40 ips at 1000 psi. Nevertheless, these compositions quite unexpectedly ignite and sustain combustion much more readily than other gas generants having burn rates below 0.40 ips at 1000 psi, and in some cases, perform better than gas generants having burn rates greater than 0.40 ips.
Optional ignition aids, used in conjunction with the present invention, are selected from nonazide fuels including triazoles, triazolone, aminotetrazoles, tetrazoles, or bitetrazoles, or others as described in U.S. Pat. No. 5,139,588 to Poole, the teachings of which are herein incorporated by reference. Conventional ignition aids such as BKNO3 are no longer required because a gas generant containing a tetrazole or triazole based fuel, phase stabilized ammonium nitrate, a metallic oxidizer, and an inert component exhibits improved ignitability of the propellant and also provides a sustained burn rate with repeatable combustible performance.
The manner and order in which the components of the gas generating composition of the present invention are combined and compounded is not critical so long as a uniform mixture is obtained and the compounding is carried out under conditions which do not cause decomposition of the components employed. For example, the materials may be wet blended, or dry blended and attrited in a ball mill or Red Devil type paint shaker and then pelletized by compression molding. When dry blended, high energy fuels such as BTA are added as a hydrate to minimize sensitivity. The materials may also be ground separately or together in a fluid energy mill, sweco vibroenergy mill or bantam micropulverizer and then blended or further blended in a v-blender prior to compaction.
The gas generant constituents from the present invention may be manufactured by known methods or supplied by known suppliers. For example, but not by way of limitation, Toyo Kasie Kogyo Co. of Takasago-city, Japan may provide the fuels, hydrated and nonhydrated, and other constituents of the present invention.
The present invention is illustrated by the following examples, wherein the components are quantified in weight percent of the total composition unless otherwise stated. Values for examples 1-3 and 16-20 were obtained experimentally. Examples 18-20 provide equivalent chemical percentages as found in Examples 1-3 and are included for comparative purposes and to elaborate on the laboratory findings. Values for examples 4-15 are obtained based on the indicated compositions. The primary gaseous products are N2, H2O, and CO2, and, the elements which form solids are generally present in their most common oxidation state. The oxygen balance is the weight percent of O2 in the composition which is needed or liberated to form the stoichiometrically balanced products. Therefore, a negative oxygen balance represents an oxygen deficient composition whereas a positive oxygen balance represents an oxygen rich composition.
When formulating a composition, the ratio of PSAN to fuel is adjusted such that the oxygen balance is between −4.0% and +1.0% O2 by weight of composition as described above. More preferably, the ratio of PSAN to fuel is adjusted such that the composition oxygen balance is between −2.0% and 0.0% O2 by weight of composition. It can be appreciated that the relative amount of PSAN and fuel will depend both on the additive used to form PSAN as well as the nature of the selected fuel.
In Tables 1 and 2 below, PSAN is phase-stabilized with 15% KN of the total oxidizer component in all cases except those marked by an asterisk. In that case, PSAN is phase-stabilized with 10% KN of the total oxidizer component.
In accordance with the present invention, these formulations will be both thermally and volumetrically stable over a temperature range of −40° C. to 110° C.; produce large volumes of non-toxic gases; produce minimal solid particulates; ignite readily and burn in a repeatable manner; contain no toxic, sensitive, or explosive starting materials; and, be non-toxic, insensitive, and non-explosive in final form.
Phase-stabilized ammonium nitrate (PSAN) consisting of 85 wt % ammonium nitrate (AN) and 15 wt % potassium nitrate (KN) was prepared as follows. 2125 g of dried AN and 375 g of dried KN were added to a heated jacket double planetary mixer. Distilled water was added while mixing until all of the AN and KN had dissolved and the solution temperature was 66-70° C. Mixing was continued at atmospheric pressure until a dry, white powder formed. The product was PSAN. The PSAN was removed from the mixer, spread into a thin layer, and dried at 80° C. to remove any residual moisture.
The PSAN prepared in example 16 was tested as compared to pure AN to determine if undesirable phase changes normally occurring in pure AN had been eliminated. Both were tested in a DSC from 0° C. to 200° C. Pure AN showed endotherms at about 57° C. and about 133° C., corresponding to solid-solid phase changes as well as a melting point endotherm at about 170° C. PSAN showed an endotherm at about 118° C. corresponding to a solid-solid phase transition and an endotherm at about 160° C. corresponding to the melting of PSAN.
Pure AN and the PSAN prepared in example 16 were compacted into 12 mm diameter by 12 mm thick slugs and measured for volume expansion by dilatometry over the temperature range −40° C. to 140° C. When heating from −40° C. to 140° C. the pure AN experienced a volume contraction beginning at about −34° C., a volume expansion beginning at about 44° C., and a volume: contraction beginning at about 90° C. and a volume expansion beginning at about 130° C. The PSAN did not experience any volume change when heated from −40° C. to 107° C. It did experience a volume expansion beginning at about 118° C.
Pure AN and the PSAN prepared in example 16 were compacted into 32 mm diameter by 10 mm thick slugs, placed in a moisture-sealed bag with desiccant, and temperature cycled between −40° C. and 107° C. 1 cycle consisted of holding the sample at 107° C. for 1 hour, transitioning from 107° C. to −40° C. at a constant rate in about 2 hours, holding at −40° C. for 1 hour, and transitioning from −40° C. to 107° C. at a constant rate in about 1 hour. After 62 complete cycles, the samples were removed and observed. The pure AN slug had essentially crumbled to powder while the PSAN slug remained completely intact with no cracking or imperfections.
The above example demonstrates that the addition of KN up to and including 15 wt % of the co-precipitated mixtures of AN and KN effectively removes the solid-solid phase transitions present in AN over the automotive application range of −40° C. to 107° C.
A mixture of PSAN and BHT·2NH3 was prepared having the following composition in percent by weight: 76.43% PSAN and 23.57% BHT·2NH3. The weighed and dried components were blended and ground to a fine powder by tumbling with ceramic cylinders in a ball mill jar. The powder was separated from the grinding cylinders and granulated to improve the flow characteristics of the material. The granules were compression molded into pellets on a high speed rotary press. Pellets formed by this method were of exceptional quality and strength.
The burn rate of the composition was 0.48 inches per second at 1000 psi. The burn rate was determined by measuring the time required to burn a cylindrical pellet of known length at a constant pressure. The pellets were compression molded in a ½″ diameter die under a 10 ton load, and then coated on the sides with an epoxy/titanium dioxide inhibitor which prevented burning along the sides.
The pellets formed on the rotary press were loaded into a gas generator assembly and found to ignite readily and inflate an airbag satisfactorily, with minimal solids, airborne particulates, and toxic gases produced. Approximately 95% by weight of the gas generant was converted to gas. The ignition aid used contained no booster such as BKNO3, but only high gas yield nonazide pellets such as those described in U.S. Pat. No. 5,139,588.
As tested with a standard Bureau of Mines Impact Apparatus, the impact sensitivity of this mixture was greater than 300 kp·cm. As tested according to U.S. D.O.T. procedures pellets of diameter 0.184″ and thickness of 0.080″ did not deflagrate or detonate when initiated with a No. 8 blasting cap.
A mixture of PSAN and BHT·2NH3 was prepared having the following composition in percent by weight: 75.40% PSAN and 24.60% BHT·2NH3. The composition was prepared as in Example 18, and again formed pellets of exceptional quality and strength. The burn rate of the composition was 0.47 inches per second at 1000 psi.
The pellets formed on the rotary press were loaded into a gas generator assembly. The pellets were found to ignite readily and inflate an airbag satisfactorily, with minimal solids, airborne particulates, and toxic gases produced. Approximately 95% by weight of the gas generant was converted to gas.
As tested with a standard Bureau of Mines Impact Apparatus, the impact sensitivity of this mixture was greater than 300 kp·cm. As tested according to U.S. Department of Transportation procedures, pellets of diameter 0.250″ and thickness of 0.125″ did not deflagrate or detonate when initiated with a No. 8 blasting cap.
A mixture of PSAN and BHT·2NH3 was prepared having the following composition in percent by weight: 72.32% PSAN and 27.68% BHT·2NH3. The composition was prepared as in example 18, except that the weight ratio of grinding media to powder was tripled. The burn rate of this composition was found to be 0.54 inches per second at 1000 psi. As tested with a standard Bureau of Mines Impact Apparatus, the impact sensitivity of this mixture was greater than 300 kp·cm. This example demonstrates that the burn rate of the compositions of the present invention can be increased with more aggressive grinding. As tested according to U.S.D.O.T. regulations, pellets having a diameter of 0.184″ and thickness of 0.090″ did not deflagrate or detonate when initiated with a No. 8 blasting cap.
In accordance with the present invention, the ammonium nitrate-based propellants are phase stabilized, sustain combustion at pressures above ambient, and provide abundant nontoxic gases while minimizing particulate formation. Because the nonmetal salts of tetrazole and triazole, in combination with PSAN, are easily ignitable, conventional ignition aids such as BKNO3 are not required to initiate combustion.
Furthermore, due to reduced sensitivity and in accordance with U.S.D.O.T. regulations, the compositions readily pass the cap test at propellant tablet sizes optimally designed for use within the air bag inflator. As such, a significant advantage of the present invention is that it contains nonhazardous and nonexplosive starting materials, all of which can be shipped with minimal restrictions.
Comparative data of the prior art and that of the present invention are shown in Table 3 to illustrate the gas generating benefit of utilizing the tetrazole and triazole amine salts in conjunction with PSAN.
As shown in Table 3, and in accordance with the present invention, PSAN and amine salts of tetrazole or triazole produce a significantly greater amount of gas per cubic centimeter of gas generant volume as compared to prior art compositions. This enables the use of a smaller inflator due to a smaller volume of gas generant required. Due to greater gas production, formation of solids are minimized thereby allowing for smaller and simpler filtration means which also contributes to the use of a smaller inflator.
In yet another aspect of the invention, it has also been discovered that certain gas generating compositions containing PSAN and a nonmetal salt of tetrazole or a nonmetal salt of triazole may exhibit poor ignitability and incomplete combustion thereby resulting in an inadequate rate of gas production and/or in “no-fires”. As shown in Examples 21-27 in. Table 4, by adding a metallic oxidizer and an inert component in the percentages given above, silicates are formed thereby improving ignitability and sustaining combustion at all pressures.
In Examples 21-27, the phase stabilized ammonium nitrate (PSAN) contained 10% KN by weight and was prepared by cocrystallization from a saturated water solution at about 80° C. The diammonium salt of 5,5′-Bi-1H-tetrazole (BHT·2NH3), strontium nitrate, clay, and nitroguanidine (NQ) were purchased from an outside supplier.
Each material was dried separately at 105° C. The dried materials were then mixed together and tumbled with alumina cylinders in a large ball mill jar. After separating the alumina cylinders, the final product was collected: 1500 g of homogeneous, pulverized powder. The powder was formed into granules to improve the flow properties, and then compression molded into pellets (0.184″ diameter, 0.090″ thick) on a high speed tablet press. The tablets were loaded into inflators and fired inside a 60L tank and a 100 ft3 tank. The 60L tank was used to determine the pressure over time and to measure the amount of solids that were expelled from the inflator during deployment. The 100 ft3 tank was used to determine the levels of certain gases as well as the amount of airborne particulates produced by the inflator. Table 1 summarizes the results for each of the compositions.
Examples 21-24 are shown for comparative purposes. Example 21 contains PSAN and BHT-2NH3. Example 22 contains PSAN, BHT-2NH3, and NQ. Example 23 contains PSAN, BHT-2NH3, and strontium nitrate (a metallic oxidizer). Example 24 contains PSAN, BHT-2NH3, and clay (an inert component). In accordance with the present invention, Examples 25 and 26 contain PSAN, BHT-2NH3, strontium nitrate as a metallic oxidizer, and clay as an inert component. Finally, Example 27 contains PSAN, BHT-2NH3, strontium nitrate as a metallic oxidizer, and clay as an inert component, but in amounts other than as described above. Applicants have discovered that adding the metallic oxidizer and an inert component to the compositions of Examples 21 and 22 (and similar compositions as taught hereinabove), results in sustained combustion and optimum ignitability. Nevertheless, one of ordinary skill in the art will readily appreciate that redesigning the inflator to operate at a higher combustion pressure, for example, would still make the compositions of Examples 21 and 22 useful in an automotive airbag application.
As shown in Table 4, Examples 21-27 are typical high yield gas generants that produce large volumes of gases with minimal solid particulates. The gas conversion is the percent by weight of solid gas generant that is converted to gas after combustion. Although the gas conversion of Examples 25 and 26 is slightly lower than in Examples 21-24 and 27, there are no significant differences in the amount of solids produced by an inflator in a 60L tank. This demonstrates that the compositions of Examples 25 and 26 are essentially high yield gas generants despite a slight decrease in the gas conversion as compared to Examples 21-24 and 27. All of the Examples presented in Table 4 are thermally and volumetrically stable from −40° C. to 110° C., and contain no explosive components.
It has been discovered that in certain inflator designs, the compositions of Examples 21-23 (and similar compositions as described above) can sometimes experience a “no-fire” situation whereby only a portion of the gas generant is combusted. This is unacceptable for airbag operations demanding a specific rate of gas production, and therefore requires more complicated inflators operable at higher pressures. On the other hand, the compositions of Examples 25-27 when fired consistently result in complete combustion without delay.
Burn rate data is presented to further describe the advantages of combining PSAN, a nonmetal salt of tetrazole or a nonmetal salt of triazole, a metallic oxidizer, and an inert component. The burn rate model Rb=aPn was assumed to apply, where Rb=burn rate, a=a constant, P=pressure, and n=the pressure exponent. Note that the relationship between the burn rate and pressure, and hence a and n, can change as a function of pressure. When this occurs, there is a “break” in the burn rate vs. pressure curve, indicating a transition to a different combustion mechanism. Ideally, a gas generant composition should have a single burning mechanism over the entire inflator operating pressure. In addition, the gas generant should ignite easily and sustain combustion over these pressures. FIG. 1 illustrates the “break” in the pressure exponent of a gas generant. In FIG. 1, the burn rate vs. pressure curves for Examples 21-23 and 26 are presented. Note that the composition of Example 26 when combusted shows no “breaks” thereby indicating a single mechanism of combustion, maintained and occurring in all of the inflator operating pressures.
At pressures above about 3000 psi, all of the compositions ignite easily and sustain combustion. As the pressure decreases below 2000-3000 psi, Examples 21-23 experience a significant increase in the pressure exponent. This indicates a transition to a combustion mechanism that is much more dependent on pressure. At this point, a small decrease in pressure can dramatically reduce the burning rate of the gas generant and eventually cause it to extinguish. In fact, it has been found that certain inflators containing compositions 21-23 sometimes do not function properly because only a small portion of the gas generant has been consumed. This phenomena was also observed at very low pressures. When ignited at atmospheric with a propane torch, compositions 21-23 began to burn, but always extinguished. Furthermore, these compositions did not ignite and burn to completion at 100 psi when tested in a burn rate apparatus.
In contrast, as shown in FIG. 1 (note the absence of a “break” in the curve of composition 26), composition 26 ignites and burns easily and has the same pressure exponent from 0-4500 psi. When ignited with a propane torch at atmospheric pressure, composition 26 ignited easily and burned slowly to completion. At 100 psi in a burn rate apparatus, composition 26 ignited and burned completely. Inflators containing composition 26 functioned properly on all occasions with easy ignitability, and complete and steady consumption of the gas generant. Inflator operating characteristics were relatively equivalent when composition 25 was used. Note that despite low levels of a metallic oxidizer and an inert component, and burn rate properties similar to compositions 21-23, composition 27 functions at the inflator level with complete consumption of the gas generant.
Composition 24 contains PSAN, the primary fuel (BHT-2NH3), and an inert component. “No-fires” or combustion delays were not a problem at the inflator level. However, this formulation produces high levels of undesirable gases. Compared to Examples 21-23, and 25-27, composition 24 has a similar CO level, but much higher levels of ammonia, NO, and NO2, making the composition unsuitable for automotive applications. This indicates the importance of the metallic oxidizer in preventing the production of toxic gases.
X-ray diffraction (XRD) was completed on the solid residue from compositions 23-26. The major phases are presented in Table 4. The use of Sr(NO3)2 alone in composition 23 results in the formation of mainly SrCO3 with problems of inflator “no-fires”. The use of clay alone in composition 24 results in the formation of mainly K2CO3 with problems of high levels of toxic effluents at the inflator level. The use of both Sr(NO3)2 and clay in compositions 25 and 26 results in the formation of mainly strontium silicate, Sr2SiO4, without occurrence of “no-fires” or highly toxic effluent levels.
In sum, Examples 21-27 demonstrate that the addition of both the metallic oxidizer and inert component to PSAN and the primary fuel is necessary to form a metallic silicate product during the combustion process. The result is a high-gas yield generant that is readily ignitable and burns to completion at all operating pressures, and yet produces minimal solid particulates and minimal toxic gases.
Examples 28-32 illustrate how the required combustion operating pressure within a 60L tank is reduced as the composition changes in accordance with the present invention. In particular, as the amount of the high energy fuel, BTA, is increased the pressure requirements are reduced. Accordingly, compositions containing BTA or a similar substituted tetrazole or substituted bitetrazole naturally forming a hydrate tend to reduce the operating pressure requirements needed for sustained and complete combustion. Furthermore, compositions containing a high energy fuel such as BTA are processed by conventional methods, able to be dehydrated by conventional methods without compromising homogeneity or tablet structure, and are safe to process at temperatures required for dehydration (as necessary).
In yet another aspect of the invention, a preferred method of forming a composition containing BTA, a secondary fuel, and PSAN includes the following steps:
Processing compositions containing the primary high energy fuel in this manner facilitates less restrictive transportation requirements, particularly if the hydrate is shipped to the inflator manufacturing site and then combined as detailed in the six steps given above.
It was found that dehydration before pressing of formulations including PSAN and hydrated high energy fuels reduces drying temperatures and times and is necessary for producing a tablet propellant which passes current automobile air bag test specifications. The wet mix process produces granular product, which, in formulations including hydrated fuels was too high in moisture content for the desired applications. Several attempts were made to dry the material further in an oven (4-24 hours at temperatures ranging from 85-125 degrees Celsius). The granular material produced in wet mix operations was analyzed by Karl Fischer (KF) methods and found to contain as much as 1%, by mass, of moisture. This material was then oven dried for 24 hours at 105c and found to have a moisture content of >0.5% by KF method. A second 24-hour drying at 105c was run and material showed no moisture loss. This material was then dried for 18 hours at 125c and found to have a final moisture content of 0.4%, by KF method. This procedure shows that compacted propellant (either granules or tablets) does not allow for sufficient dehydration of the hydrated fuels by conventional methods at safe drying temperatures. It was found that, in these formulations, ARC self-heating began at temperatures around 160c. As a rule of thumb, these formulations should be processed with a 50-degree safety factor, limiting the maximum drying to 110c. To avoid the concern described above, the wet material must be first ground to a powder. Then it can be easily dried in an oven at a reasonable and safe temperature (12 hours at 105c). This is the preferred procedure if the wet mix process is used.
The powder produced in dry mixing was dehydrated to less than 0.2% (12 hours at 105c was sufficient), by mass, of moisture and the material was pressed in both powder form, and after slugging and granulation. Both pressings produced tablets suitable for air bag testing.
It was also shown that pressing propellants including a hydrated fuel and PSAN before dehydration causes several problems. After 12 hours of drying at 105c the tablets had grown large crystalline structures on their surfaces. It is believed that the water of hydration dissolves the ammonium nitrate as it escapes from the tablet and deposits the AN as crystals on the surface. These crystals were analyzed by DSC and found to be AN. This produces tablets of propellant which are not homogeneous throughout and also caused the tablets to expand, and lose density and crush strength. These physical changes resulted to make the propellant unsafe to test in automotive air bag inflators.
While the foregoing examples illustrate the use of preferred fuels and oxidizers it is to be understood that the practice of the present invention is not limited to the particular fuels and oxidizers illustrated and similarly does not exclude the inclusion of other additives as described above and as defined by the following claims.
This application claims the benefit of provisional application 60/369,775 filed on Apr. 4, 2002.
| Number | Name | Date | Kind |
|---|---|---|---|
| 5197758 | Lund et al. | Mar 1993 | A |
| 5500059 | Lund et al. | Mar 1996 | A |
| 5501823 | Lund et al. | Mar 1996 | A |
| Number | Date | Country | |
|---|---|---|---|
| 20040016480 A1 | Jan 2004 | US |
| Number | Date | Country | |
|---|---|---|---|
| 60369775 | Apr 2002 | US |
