Nitroguanidine based gas generant containing mica
Gas generants in an inflator have nitroguanidine as a fuel and mica as a slagging agent. A gas generant with nitroguanidine has many desirable properties such as little hygroscopicity, a high conversion efficiency, and a suitable burn rate. Mica is a beneficial ingredient to the gas generant because it reduces the amount of undesirable gases as well as reduces the amount of solid combustion particles from escaping the inflator
 The present invention generally relates to gas generating compositions utilized for inflation of an occupant safety restraints in motor vehicles. In particular, the present invention relates to gas generating compositions containing the fuel, nitroguanidine and the particulate reducer, mica.BACKGROUND OF THE INVENTION
 Vehicle airbags have been developed to afford protection to occupants involved in a car crash. During a crash, an airbag is filled with inflation gas, and the inflated airbag protects an occupant by acting as a cushion. A common method of providing inflation gas to a vehicle airbag is by utilizing a pyrotechnic inflator. Simply, a pyrotechnic inflator operates by the rapid burning of a gas generant to produce inflation gas.
 Previously, sodium azide was commonly utilized as a gas generant for inflators because the burning of sodium azide produces gas rich in nitrogen gas. However, gas generants containing sodium azide have a number of drawbacks such as dangerous decompositional explosion, formation of explosive compounds by the reaction with heavy metals, and environmental pollution caused by mass disposal. For all of these reasons, the person skilled in the art has sought to replace (avoid selecting) sodium azide as the fuel for the gas generant.
 Non azide based gas generants have been developed to overcome the drawbacks associated with azide based gas generants. Co-owned U.S. Pat. No. 6,071,364 to Canterberry et al., which is incorporated by reference in its entirety herein, teaches a non-azide formulation with mica. The addition of mica in the range of 5-25% by weight to a non azide based gas generant reduces the amount of solid particles exiting the inflator and also reduces the amount of production of undesired toxic gases such as nitrogen oxides (NOx) and carbon monoxide (CO). The preferred gas generant in U.S. Pat. No. 6,071,364 is 5-amino tetrazole for the fuel, potassium nitrate and strontium nitrate for the oxidizer, and mica. This gas generant has a good burn rate velocity and ballistic properties, however this formulation also has some drawbacks, namely its hygroscopicity.
 The present invention relates to an improved gas generant formulation containing mica that is less hygroscopic.SUMMARY OF THE INVENTION
 The gas generant composition of the present invention comprises between about 15 and 70 wt % nitroguanidine, between about 20 and 80 wt. % oxidizer, and between about 5 and about 25 wt. % of mica. The preferred composition comprises about 50 wt. % of nitroguanidine.DETAILED DESCRIPTION OF THE INVENTION
 The present invention is an improvement of the gas generant composition taught in U.S. Pat. No. 6,071,364. The preferred composition in U.S. Pat. No. 6,071,364 includes 5-amino tetrazole, potassium nitrate, strontium nitrate, and mica. 5-amino tetrazole is hygroscopic and thus the gas generant containing 5-amino tetrazole absorbs moisture. One molecule of 5-amino tetrazole crystallizes with one molecule of water to from a monohydrate structure. The problem associated with the hydration of 5-amino tetrazole is that it has a crystal structure change associated with the addition of water. The anhydrous 5-amino tetrazole has one crystal habit while the monohydrate has another. This problem is compounded by the fact that water is easily removed form the hydrated 5-amino tetrazole at relatively low temperatures. The movement of water into and out of the crystal and the associated habit change results in a disintegration of the propellant tablet's integrity. A loss of tablet integrity may result in loss of density and crumbling, which can cause a loss of ballistic control.
 The gas generants of the present invention contain nitroguanidine, an oxidizer, mica, a processing aid, and a burn rate catalyst. Nitroguanidine (CH4N4O2) is a highly energetic fuel rich in nitrogen; nitroguanidine has a low negative oxygen balance (−30.7%). The gas generant composition in accordance with the present invention comprises between about 15 wt. % and 70 wt. % nitroguanidine with the preferred composition containing about 50 wt. %.
 Nitroguanidine is less hygroscopic than 5-amino-tetrazole, and thus the gas generant according to the present invention absorbs less moisture. Another advantage of the formulation in the present invention is the production of less noxious gases than the gas generant with 5-amino tetrazole. A possible explanation for the difference is the presence of two oxygen atoms on each nitroguanidine molecule allowing a higher collision frequency probability between the oxygen (serving as the oxidizer) and the carbon during the combustion process. This is made possible by the fact that the oxygen of the nitroguanidine is attached to a nitrogen two atoms away from the carbon. On the other hand, 5-amino tetrazole does not contain any oxygen atoms. Another benefit of the present invention is the improved gas conversion efficiency. Similarly, this property can be explained by the presence of the oxygen atoms on the nitroguanidine molecule. Since a portion of the fuel serves as an oxidizer, less potassium nitrate/strontium nitrate mixture needs to be added to the fuel. The metal ions from the oxidizers are responsible for lowering the gas conversion rate because they form solid oxide particles, and thus the formulation in the present invention, which has less oxidizer than a 5 amino-tetrazole based gas generant will produce a higher percentage of combustion gas.
 Unprocessed nitroguanidine has at least two distinct native crystal arrangements: alpha and beta. In the alpha arrangement, the crystals have a long white lustrous needle appearance and are tough. While the beta arrangement has crystals in the shape of thin elongated plates. The alpha arrangement is the desired crystal arrangement for applications in the propellant and explosive industries.
 When nitroguanidine (alpha arrangement) is pressed into a pellet or tablet, its needles bend or become distorted. During the standard test of thermal cycling, the energy supplied to the gas generant causes the nitroguanidine needles to revert back to their original geometry or native conformation. This results in the pellets growing in size. One solution to the foregoing problem is to add a binder to the gas generant. The binder prevents the gas generant pellet from growing during thermal cycling by securing the nitroguanidine needles to their reduced geometry. Another means of stabilizing the size or density of the gas generant containing nitroguanidine is by grinding the nitroguanidine to amorphous crumbs. A suitable grinding machine for this operation is the Palla mill or the vibrating ball mill. The process of grinding nitroguanidine is discussed in co-owned published patent application 2002 0096236 A1, which is incorporated herein in its entirety by reference.
 Oxidizers useful in the composition of the present invention include the alkaline earth metal nitrates, alkaline metal nitrates, chlorates, perchlorates, and oxides. The preferred oxidizer system in the present invention is a mixture of potassium nitrate and strontium nitrate.
 Another component of the composition in the present invention is a processing aid. Those skilled in the art understand that depending on the particular oxidizers and fuels utilized, certain processing aids may be helpful in removing the gas generants from the pellet punch during pelletizing of the gas generant. Examples of processing aids are silica and boron nitride, but other processing aids may be employed.
 Mica is another component added to the gas generant. Mica is discussed in a co-owned patent, U.S. Pat. No. 6,071,364, which is incorporated herein in its entirety. Mica is a group of minerals in the phyllosilicate subclass, and since micas are true phyllosilicates, they are composed of sheets of silicate tetrahedrons. The minerals in the mica group are characterized are constructed of extremely thin cleavage flakes and characterized by near perfect basal cleavage, and a high degree of flexibility, elasticity, and toughness. The various micas, although structurally similar, vary in chemical composition. The properties of mica derive from the periodicity of weak chemical bonding alternating with strong bonding. Representative of the minerals of the mica group are muscovite, phlogopite, biotite, lepidolite, and other such as fluorophlogopite. In general, the silicon to aluminum ratio is about 3:1. Any naturally occurring mica is useful in the gas generant composition of the present invention. However, those micas containing halogen atoms such as lepidolite and fluorophlogopite are not preferred. The presence of halogen atoms in certain of the mica group minerals may result in the production of combustion gases containing undesirable halogen ions. The mica useful in the present invention is ground having a particle size ranging from 2 to 100 microns. This ground mica is also referred to as flake mica. In the present invention muscovite mica with a particle size in the range of 2-25 microns is preferred.
 Mica is employed in the present invention to reduce the amount of solid particles exiting the airbag inflator. The preferred oxidizer system in the present invention comprises a mixture of potassium nitrate and strontium nitrate. During the combustion of the gas generant, the metal ions from the oxidizer system forms various metal oxidizes. Mica is employed to reduce the amount of metal oxides from exiting the inflator. It is theorized that mica reacts with the metal oxides and metal ions to yield a product that condenses on the metal filter. Thus, the metal filter for an inflator acts as a heat sink to limit the amount of solid particles that exit the inflator.
 A burn rate catalyst or enhancer is optionally added to the composition of the present invention to increase the combustion rate. Some examples of burn rate catalysts include metallic aluminum, copper II oxide, and metallic silicon. The burn rate for a gas generant containing nitroguanidine is a little less than the burn rate for a gas generant containing 5-amino-tetrazole. In order for the nitroguanidine based gas generant to have similar ballistic properties to 5-amino-tetrazole, the combustion pressure of the inflator needs to be about 6897 kPa (1000 psi) greater. The preferred burn rate catalyst in the present invention is copper II oxide.EXAMPLE 1 Preparation of Gas Generant
 TABLE 1 provides the compositions for three samples. Sample 1 is directed to a 5 amino tetrazole (5-ATZ) formulation discussed in co-owned patent U.S. Pat. No. 6,071,364, and the process of mixing the gas generant is discussed thereto and is incorporated herein by reference.
 TABLE 1 also provides samples 2 and 3 which are directed to a gas generant having nitroguanidine (NQ) as the fuel. The difference between samples 2 and 3 is the choice of burn rate catalyst. The process of mixing sample 2 is virtually identical to mixing sample 3 except for the type of burn rate catalyst added.
 Samples 2 and 3 were prepared by individually grinding all of the components, except for the mica and respective burn rate catalysts. The nitroguanidine was ground to amorphous crumbs through a Palla-mill. The strontium and potassium nitrates were ground in a fluid energy mill. The mica used was Micro Mica 3000 (muscovite) obtained from the Charles B. Chrystal Co., Inc. of New York, N.Y., U.S.A. It was a finely divided mica having a bulk density of about 12.4 lbs./cubic foot and a specific gravity of about 2.8.
 The ground NQ and ground strontium and potassium nitrates were mixed with the mica and respective burn rate catalyst to homogenize the formulation. The mixture was then placed in a plough-type mixer and about 15% by wt. water was add to form an agglomerated material that was then passed through a granulator with an 8 mesh screen.
 The granules were placed on a tray and dried at 120° C. in an explosion proof oven for about 3 hours. The water content after drying was between 0.5 and 1% by wt. The dried granules were then passed through the granulator using a 20 mesh screen. The gas generant was then pelletized with a rotary 1 TABLE 1 Sample # 5-ATZ NQ KNO3 Sr(NO3)2 Mica BN Si CuO 1 32 8 44 16 1 2 50 6 35 8 1 1 3 50 6 35 7 1 2EXAMPLE 2 Tests on the Gas Generant
 For the burn rate tests, the samples were prepared in a similar fashion to the procedure set forth in Example 1 except the components were ground separately, dry blended, and pressed into strands for testing. The strands had a rectangular shape with about 10.16 cm in length and about 0.63 cm on each side. The sides of each strand were coated with an epoxy-based adhesive. Strands were placed in a strand burner bomb. The bomb was equipped with a pressure transducer, acoustic devices, and mechanical wire burn through recorders. The strands were ignited at 7585 kPa (1100 psi), and pressure versus time was recorded. The acoustic and mechanical devices calculated burning time. Burning rate was determined by dividing the length of each strand by its burning time. The average burn rate for six strands for each sample is presented in TABLE 2. The burn rates for samples 2 and 3 are a little less than the burn rate for sample 1.
 The moisture absorption test was utilized to determine the amount of moisture the samples absorbed. The samples were pre-weighed and then exposed to the following conditions: 50% relative humidity and 22.2° C. The samples were weighed again after 6 hours to determine moisture gain. The results from this experiment are displayed in TABLE 2. Samples 2 and 3 each had less than half the moisture sensitivity of sample 1.
 To arrive at the conversion efficiency, the thermodynamic program “NEWPEP” is employed. “NEWPEP” is based on the PEP program described in a Naval Weapons Center Report entitled, “Theoretical Computations of Equilibrium Composition, Thermodynamic Composition, Thermodynamic Properties, and Performance Characteristics of Propellant Systems,” published in 1960, 1979, and 1990. This program is in the public domain and is readily available to those in the industry.
 After entering into said program the weight of the ingredients in the gas generant and the possible species that can be formed by the burning of the gas generant, the program can calculate the number of moles for each theoretical combustion product. Conversion efficiency is calculated by dividing the total number of moles of the gaseous reaction products by the total number of moles of reaction products, and then this quotient is multiplied by 100 to give the conversion efficiency value as a percentage. The percentage equivalence to the conversion efficiencies for the three samples is presented in TABLE 2. Samples 2 and 3 had a larger conversion efficiency than sample 1. 2 TABLE 2 Moisture Conversion Sample Burn rate sensitivity efficiency 1 2.84 cm./sec. 0.30% 64% (1.12 in/sec) 2 2.09 cm./sec. 0.12% 71% (0.823 in./sec.) 3 2.21 cm./sec. 0.12% 68% (0.869 in./sec.)
 Airborne particulates and toxic gas data were obtained for the various samples. In order to obtain said data, the gas generants were placed in identical single stage inflators, which were added to identical airbag modules. The tests were conducted in a 100 cubic foot test chamber. This test is designed to simulate the interior volume of the standard automobile. The test equipment consisted of a 100 cubic foot steel chamber containing a steering wheel simulator. To the chamber was attached a vacuum pump, a bubble flow meter, filters, and a FT/IR (Fourier Transform Infrared Spectroscopy) gas analyzer. The inflator was attached to the simulated steering wheel assembly within the chamber, the chamber was sealed and the gas generant ignited. Airborne particulate production was measured by filtering post-ignition air from the chamber through a fine filter and measuring the weight gained by the filter. The CO and NOx levels of the gases produced were analyzed by using FTIR at intervals of before deployment (background), 1, 5, 10, 15, and 20 minutes after deployment. Samples were transferred directly to the FTIR gas cell from the 100 cubic foot chamber via six feet of ¼ inch OD fluoropolymer tubing.
 The amount of toxic gas and total airborne particulates for sample 2 was significantly lower than sample 1. 3 TABLE 3 Total airborne Sample CO (ppm) NOx (ppm) particulates (mg/m3) 1 172 15 53 2 84 8 32 3 173 7 50
1. A gas generant comprising:
- (a) 15-70 wt. % of nitroguanidine;
- (b) 20-80 wt. % of an oxidizer selected from transition metal oxides; alkali metal nitrates, chlorates and perchlorates; alkaline earth metal nitrates, chlorates and perchlorates; and mixtures thereof; and
- (c) greater than 5 and less than 25 wt. % mica.
2. The gas generant of claim 1 further comprising greater than 0 to about 3 wt. % of a burn rate catalyst wherein the burn rate catalyst is a chemical selected from the group consisting of copper 11 oxide, metal silicon, or metal aluminum.
3. The gas generant of claim 2 wherein the preferred burn rate catalyst is copper II oxide and the preferred amount is 1 wt. %.
4. The gas generant of claim 1 wherein the preferred wt. % for nitroguanidine is 50.
5. The gas generant of claim 1 wherein the preferred oxidizer is a mixture of potassium nitrate and strontium nitrate.
6. The gas generant of claim 5 wherein the preferred wt. % for potassium nitrate is 6 and the preferred wt. % for strontium nitrate is 35.
7. The gas generant of claim 1 wherein the preferred wt. % for mica is 8.
International Classification: C06B033/08;