NON-AMMONIUM NITRATE BASED GENERANTS

Abstract

This disclosure is directed to an airbag gas generant formulation optimized for hybrid airbag inflators, and an airbag inflator, an airbag, an airbag module comprising same, and a method of inflating an airbag using the airbag gas generant.

Claims

1. An airbag gas generant formulation optimized for hybrid airbag inflators, the formulation comprising: 40 wt % to 50 wt % strontium nitrate; 35 wt % to 45 wt % nitroguanidine; 3 wt % to 7 wt % potassium perchlorate; 3 wt % to 6 wt % w/w polyvinyl alcohol; and 2 wt % to 6 wt % w/w strontium oxalate.

2. The airbag gas generant formulation of claim 1 which has at least one property selected from the group consisting of: a gas yield of greater than 1.57 grams of per cubic centimeter (g/cc); a constant volume flame temperature of 2700 K to 2800 K; and an overall oxygen balance of the formulation 2% to +2%.

3. The airbag gas generant formulation of claim 1, wherein the formulation comprises 48.2 wt % strontium nitrate; 36.8 wt % nitroguanidine; 5 wt % potassium perchlorate; 5 wt % strontium oxalate; and 5 wt % polyvinyl alcohol.

4. The airbag gas generant formulation of claim 1, wherein the formulation further comprises 1 wt % to 5 wt % cupric oxide as a burning rate modifier.

5. The airbag gas generant formulation of claim 4 wherein the formulation is 44.1 wt % strontium nitrate; 39.9 wt % nitroguanidine; 5 wt % potassium perchlorate; 4 wt % strontium oxalate; 4 wt % polyvinyl alcohol; and 3 wt % cupric oxide.

6. The airbag gas generant formulation of claim 1 which further comprises 2 wt % to 6 wt % Kaolin for slag formation and as a coolant.

7. The airbag gas generant formulation of claim 1 which further comprises 2 wt % to 6 wt % aluminum oxide for slag formation and as a coolant.

8. The airbag gas generant formulation of claim 1 which further comprises 2 wt % to 6 wt % silicon dioxide for slag formation and as a coolant.

9. The airbag gas generant formulation of claim 1 which further comprises 2 wt % to 6% wt % of at least one selected from the group consisting of: kaolin; aluminum oxide; and silicon dioxide.

10. The airbag gas generant formulation of claim 1 which does not contain oxygen as a stored gas.

11. An inflator for an airbag comprising the airbag gas generant formulation of claim 1 wherein the inflator does not contain oxygen as a stored gas.

12. A method for inflating an airbag comprising the steps of igniting the airbag gas generant formulation of claim 1 to generate a gas; and inflating the airbag with the gas.

13. The method of claim 12 wherein the igniting step does not involve igniting oxygen as a stored gas.

Description

DETAILED DESCRIPTION

[0014] This disclosure describes non-ammonium nitrate based generants that are optimized as a replacement and improvement for ammonium nitrate based hybrid inflator generants.

[0015] Hybrid inflators contain both stored gas and pyrotechnic materials. In some hybrid inflator designs the stored gas vessel contains both high-pressure gas and pyrotechnic materials. In hybrid inflators the pyrotechnic materials are used for gas generation and heating of the stored gas. Ammonium nitrate based generants worked well due to their high gas yield and relatively high combustion temperature compared to that needed for a pyrotechnic inflator. Ammonium nitrate based hybrid generants were formulated near stoichiometric such that they did not generant unacceptable levels of carbon monoxide or nitrogen oxide compounds; they had oxygen balances near zero. In this type of hybrid inflator the stored gas is inert. Some hybrid inflator designs use a highly negative oxygen balance formulation that generate carbon monoxide (CO) and hydrogen (H.sub.2) requiring oxygen to be added to the stored gas to combust the CO and H.sub.2 to CO.sub.2 and H.sub.2O, respectively. Such a formulation is described in U.S. Pat. No. 7,942,990. The formulation described here requires no oxygen to be included in the stored gas.

[0016] Ammonium nitrate based generants also have a high gas yield to volume of solid generant. For example, the ammonium nitrate based generants described in U.S. Pat. Nos. 5,850,053 and 6,136,113 have a theoretical density of 1.66 g/cc with a gas yield of 1.57 grams of gas per cubic centimeter (cc) of solid generant. These formulations have a constant pressure flame temperature of 2240K and a constant volume flame temperature of 2700 K. For a replacement generant to work in the same hybrid inflator, it is preferred to have the same or equivalent gas yield per solid volume and flame temperatures.

[0017] Due to the preceding constraints of high gas yields per volume of solid gas generant and non-ammonium nitrate containing generants, this makes metal containing oxidizers and high-density fuels attractive. These types of generants have a lower gas yield per weight but due to having a high solid density can produce the same amount of gas per solid volume as the ammonium nitrate based generants. It is also desirable for a gas generant to use commonly available or lowest cost ingredients. For pyrotechnic inflators, the fuel of choice today is guanidine nitrate (GN). Example 1 shows GN with various oxidizers. As this example shows when GN is used as the fuel, the 1.57 grams of gas per cc solid generant cannot be met.

[0018] The following chemicals components recited in the claims are individually well-known to one of ordinary skill in the art. They include at least strontium nitrate (Sr(NO.sub.3).sub.2); potassium perchlorate (KClO.sub.4); polyvinyl alcohol; strontium oxalate (SrC.sub.2O.sub.4); cupric oxide (CuO); Kaolin; aluminum oxide (Al.sub.2O.sub.3); and silicon dioxide (SiO.sub.2). Nitroguanidine ((NH.sub.2).sub.2CNNO.sub.2) or NO.sub.2NHC(NH)NH.sub.2) is commercially available and also well-known. It exists in two tautomeric forms, as a nitroimine (left) or a nitroamine (right).

##STR00001##

In solution and in the solid state, the nitroimine form predominates (resonance stabilized).

INCORPORATION BY REFERENCE

[0019] All publications, patent applications, and patents mentioned anywhere in this disclosure are hereby incorporated by reference in their entirety as if each individual publication or patent was specifically and individually indicated to be incorporated by reference. In case of conflict, the present application, including any definitions herein, will control.

EXAMPLES

Example 1: Experiments on Various Formulations

[0020]

TABLE-US-00001 Example 1: Gas Mass per Volume of Solid Generant with Guanidine Nitrate, 1% Oxygen Balance Grams Gas % PVA per cc Solid Oxidizer % Oxidizer % GN Binder Generant Sr(NO.sub.3).sub.2 39.4 60.6 1.46 Sr(NO.sub.3).sub.2 49.1 46.9 4 1.45 BCN 44.9 55.1 1.48 KClO.sub.4 34.8 65.2 1.37 KClO.sub.4 44.8 55.2 4 1.35

[0021] Table 1 lists some fuels used or proposed to be used in airbag gas generants. In order to meet the criteria of 1.57 grams of gas per cc of solid volume low-density ingredients in general like guanidine nitrate can be rejected as a candidate. Also, fuels with a high oxygen demand like 5-aminotetrazole can be rejected due to the low gas yield of such a system with a metal-containing oxidizer.

[0022] The flame temperature of compositions tend to increase with fuel heat-of-formation so generants containing HMX and RDX tend to have high flame temperatures; besides HMX and RDX generants are sensitive and tend to fall under export control laws. So candidates like HMX and RDX are not considered as being viable; they also would require significant amounts of coolant (flame temperature reducing material) to be added to such a formulation. Of the two other ingredients listed in Table 1, AZODN while being the ideal fuel is not commercially available so the preferred fuel is nitroguanidine.

TABLE-US-00002 TABLE 1 Density Oxygen Heat of Formation Fuel (g/cc) Balance % (kcal/kg) HMX 1.91 21.61 60.54 RDX 1.816 21.61 75.64 Nitroguanidine 1.77 30.75 216.97 Azodicarbonamidine 1.70 13.33 366. Dinitrate (AZODN) 5-Aminotetrazole 1.65 65.83 587.77 Guanidine Nitrate 1.436 26.21 746.21
Table 2 lists oxidizers that have been used or proposed to be used in airbag gas generant formulations.

TABLE-US-00003 TABLE 2 Gas Den- Oxygen Percent Post Heat of Yield sity Balance Combustion Formation Density Oxidizer (g/cc) % Condensable (kcal/kg) (g/cc) Strontium Nitrate 2.986 37.8 48.96 1104.76 1.524 Basic Copper 3.394 29.98 52.93 866. 1.598 Nitrate (BCN) Potassium 2.52 46.19 53.81 742. 1.164 Perchlorate (KP) Potassium Nitrate 2.109 39.56 68.35 1169. 0.667 (KN) Sodium Nitrate + 2.069 39.51 28.86 901. 1.472 Ammonium Perchlorate (SNAP) Strontium Nitrate + 2.334 35.82 35.5 840. 1.505 Ammonium Perchlorate (SRAP)

Example 2: Measurements of Experimental Results

[0023] Example 2 lists 1% oxygen balance two-component systems with oxidizers from Tablet combined with nitroguanidine; PSAN-GN formulations are also listed for comparison purposes. Potassium nitrate-nitroguanidine formulations have a low gas yield making them unable to meet the 1.57 g/cc requirement. Potassium perchlorate and ammonium perchlorate containing formulations tend to have too high of a combustion temperature. Strontium nitrate and BCN are the remaining oxidizers to be considered.

TABLE-US-00004 Example 2: 1% O/B Constant Volume Flame Temperature at 0.08 loading volume (cc-solid/cc-total volume) Gas Yield Const. Vol. Oxidizer Density Flame Temp Oxidizer % Fuel Fuel % (g/cc) (K) PSAN (AN-KP-GN 50.8 GN 49.2 1.543 2693 Eutectic) PSAN AN-KN-GN 52.9 GN 47.1 1.542 2623 Eutectic) Strontium Nitrate 43.4 NQ 56.6 1.693 3057 BCN 49.0 NQ 51.0 1.712 2645 Potassium 38.7 NQ 61.3 1.584 3344 Perchlorate Potassium Nitrate 42.3 NQ 57.7 1.350 2776 SNAP 42.3 NQ 57.7 1.655 3223 SRAP 44.7 NQ 55.3 1.669 3236

[0024] Airbag generants have to be cost competitive so these formulations preferably use low-bulk-density (LBD) nitroguanidine which consists of long fibers which do not thermal cycle well. As mentioned in the literature LBD needs to be ground. U.S. Pat. No. 6,547,900 describes a method using vibratory ball-mills to break up the Nitroguanidine needles. It is preferred to have minimal grinding of the NQ to break up the needle bundles plus the addition of polyvinyl alcohol (PVA) as a binder allows NQ formulations to withstand thermal cycling and heat age environment conditioning. PVA is a preferred binder because it is water soluble. Binders that require organic solvents to dissolve them can add high production costs to the generant.

[0025] While a two oxidizer combination of strontium nitrate and BCN with NQ and PVA can achieve an ideal flame temperature, BCN and PVA do not heat age well together. Since PVA is the preferred binder, this eliminates BCN as a candidate. The general replacement generant formulation for an ammonium nitrate formulation is strontium nitrate, nitroguanidine, and polyvinyl alcohol as the binder plus a coolant to reduce the combustion temperature. In examples 3 through 8 various combinations of strontium nitrate and NQ with PVA as a binder are shown. These combinations all meet the minimum gas weight per volume of solid generant and flame temperature.

[0026] Because Sr(NO.sub.3).sub.2NQ formulations can have low burning rates potassium perchlorate (KP) and copper(II) oxide (CuO) or combinations thereof can be added to increase the burning rate. Examples 6 through 8 show cases where KP and CuO are included in the formulation. These cases also meet the gas yield and flame temperature requirements for a hybrid inflator application. Potassium perchlorate and CuO both act as burning rate catalysts.

Examples 3-8: Additional Formulations and their Experimental Results

[0027]

TABLE-US-00005 Const. Gas Vol. Stron- Stron- Yield Flame tium tium Density Temp Example Nitrate NQ Oxalate PVA KP CuO (g/cc) (K) 3 53.9% 36.6% 4.5% 5% 1.585 2716. 4 51.6% 39.4% 5.0% 4% 1.594 2700. 5 49.4% 42.6% 5.5% 3% 1.608 2700. 6 45.8% 39.7% 5.5% 4% 5% 1.575 2708. 7 43.7% 40.8% 5.5% 4% 6% 1.574 2720. 8 43.9% 39.6% 4.5% 4% 5% 3.0 1.587 2720.

[0028] While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.