Gas bag inflator containing inhibited generant

Abstract: An inflator, preferably for an automotive gas bag restraint system, containing a plurality of gas generant bodies (1) which have a relatively inert burn inhibitor coating material thereon. The inhibitor material is preferably a plastic resin. The inhibitor material may also be alumina, titania, silica or a silicate compound, e.g. bentonite. The inhibitors are preferably applied to the generant bodies as a spray. The gas generant may be an azide or a non-azide, preferably an azide and most preferably sodium azide. The inhibited bodies may be any shape, preferably washer-shaped discs in side by side array.

Description: The present invention pertains to a gas bag inflator or generator containing a plurality of pyrotechnic grains or bodies made of a conventional gas generant or propellant formulation, preferably an azide-based composition, which bodies have a coating of an inert ignition or burn inhibitor or retardant material thereon. The inhibited generant bodies are disposed in the combustion chamber of the gas generator or inflator which produce a gas upon combustion which, after typically filtering out condensed phase products, is preferably used to inflate a gas bag which serves as a vehicle occupant restraint cushion during a collision. More particularly these generant bodies are coated with burn inhibitor materials comprising a plastics resin (e.g. acrylic resin), preferably applied as a spray. Even though the generant bodies of this invention are especially designed and suited for creating gas for inflating passive restraint vehicle crash bags as indicated, they would also be useful in other less severe inflation applications, such as aircraft slides, inflatable boats and inflatable lifesaving buoy devices where retarded or slower initial expansion is desirable. Automobile gas bag systems have been developed to protect the occupant of a vehicle, in the event of a collision, by rapidly inflating a cushion or bag between the vehicle occupant and the interior of the vehicle. The inflated gas bag absorbs the occupant's energy to provide a gradual, controlled deceleration, and provides a cushion to distribute body loads and keep the occupant from impacting the hard surfaces of the vehicle interior. The use of such protective gas-inflated bags to cushion vehicle occupants in crash situations is now widely known and well documented. The requirements of a gas generant suitable for use in an automobile gas bag device are very demanding. The gas generant must have a burning rate such that the gas bag is inflated rapidly (within approximately 30 to 100 milliseconds). The burning rate must not vary with long term storage (aging) or as a result of shock and vibration during normal deployment. The burning rate must also be relatively insensitive to changes in humidity and temperature. When pressed into pellets, wafers, cylinders, discs or whatever shape, the hardness and mechanical strength of the bodies must be adequate to withstand the mechanical environment to which they may be exposed over the expected inflator system lifetime of at least ten years without any fragmentation or change of exposed surface area. Excessive breakage of the bodies could potentially lead to system failure where, for example, an undesirable high pressure condition might be created within the gas generator device, possibly resulting in rupture of the pressure housing. The gas generant must efficiently produce relatively cool, non-toxic, non-corrosive gas which is easily filtered to remove solid and liquid combustion by-products, and thus preclude damage to the inflatable bag or to the occupant of the automobile. The requirements as discussed in the preceding paragraphs limit the applicability of many otherwise suitable compositions, shapes and configurations thereof from being used in automotive air bag gas generators. Both azide and non-azide based generant formulations which generate nitrogen-containing or nitrogen-rich gas to expand an inflatable occupant restraint are well known. Exemplary azide-based generants include, for example, at least one alkali or alkaline earth metal azide as the base fuel constituent. See, for example U.S. Pat. Nos. 3,741,585; 3,895,098; 3,931,040; 4,062,708 and 4,203,787, as well as US Patent application Serial No. 07/749,032 (and EP-A-0531031). Exemplary non-azide generants are disclosed in commonly assigned U.S. Pat. Nos. 4,931,112 and 5,015,309 as well as US Patents US-A-5197758 (and EP-A-0536916) and US-A-5160386. filed November 4, 1991, and additional art cited therein. Particulate ingredients of such generant compositions are typically mixed and consolidated, with or without a small amount of a suitable binder and other auxiliary ingredients, by press molding into tablets, wafers, etc., as is conventional. When the gas generant bodies are ignited and burned, nitrogen-containing gas is produced which, after filtering, is used to inflate the gas bag. It has been proposed in U.S. Pat. Nos. 3,901,530 and 4,131,300 to form the pyrotechnic combustible material for inflators in the form of separate discs arranged side by side with inert separator means disposed between adjacent discs to facilitate quick and uniform combustion of the material as well as achieving slower inflation onset. Inflators such as shown in commonly assigned U.S. Pat. Nos. 4,005,876; 4,296,084 and 4,547,342 contain gas generant in the form of pressed pellets or tablets (similar in shape to aspirin tablets) which are randomly packed into the inflator combustion chamber. More recently it has been proposed in commonly assigned U.S. Pat. Nos. 4,890,860 and 4,998,751 to fabricate an inflator grain by assembling a plurality of washer-shaped propellant wafers or discs in alternating relationship with a plurality of similarly shaped, meshed inert cushion members which are held in compression in the inflator combustion chamber to achieve improved performance. While such arrangements may be satisfactory for some purposes, the propellant bodies in general present a high initial surface area for burning and thus do not provide as soft an inflation onset as is desired. It has also been proposed to provide gas bag inflator wafers or grains with a combustion booster or enhancer coatings. See, for example, U.S. Pat. Nos. 4,200,615; 4,244,758; 4,246,051; 4,696,705; 4,698,107; 4,806,180; 4,817,828; 5,034,070 and 5,051,143. The chief purpose of these booster or enhancer coatings is to speed up, rather than inhibit or slow down, the onset of propellant combustion. It is also known that inhibitor or restrictor, i.e. slower burning, coatings have been applied to ammunition or firearm type base propellants, as illustrated in U.S. Pat. Nos. 1,074,809; 1,308,343; 3,194,851 and 3,396,661 and solid rocket motor propellant grains, as illustrated in U.S. Pat. Nos. 3,493,446 and 5,000,885. US-A-4,106,960 discloses the spray coating of an acrylic resin onto nitrocellulose propellant grains. DE-C-3,933,554 discloses the coating of gas generant grains for an air bag inflator with an inorganic burn inhibitor such as sodium silicate. The present invention consists in a gas bag inflator comprising a combustion chamber having a gas outlet; a plurality of gas generant bodies disposed within said chamber, said generant bodies having first and second faces with peripheral walls between them and having thereon a coating of a relatively inert burn inhibitor material; means for igniting the inhibited bodies; and means for routing gas generated from said chamber through the gas outlet, characterised in that the inhibitor material comprises a plastics resin and is applied only to said first faces and peripheral walls. The gas generant bodies used according to the invention have a configuration that can be inhibited to restrict or retard the combustion of a portion of the base generant for a predetermined time period. The generant bodies have a dual gas output rate wherein the initial burn is at a low rate of gas output followed by a higher rate of gas output, and are so configured that on combustion a delay in bag onset will occur thereby improving the loading on the gas bag components as well as lessening out-of-position occupant concerns during bag deployment, particularly on the passenger side of an automobile. The burn inhibitor coating is preferably an acrylic resin. The generant body on which the inert inhibitor is applied may be made of any conventional aside or non-azide based generated formulation, most preferably sodium azide. The inhibited generant body may take any of the aforementioned conventional pellet, tablet, wafer, etc. forms, most preferably a washer-shaped disc. Another important feature relates to an inflator including stack or side by side assembly of a plurality of the inhibited generant bodies according to the present invention. Another important feature pertains to a method of inflating a gas bag by generating nitrogen-containing or nitrogen-rich gas by igniting the composite inhibited generant bodies according to the present invention and using the gas generated to inflate the gas bag. The above and other objectives, advantages and features of this invention will be apparent in the following detailed descriptions of the preferred embodiments thereof which is to be read in conjunction with the accompanying drawings. FIG. 1A is a space view of a washer-shaped generant wafer disc having a burn inhibitor coating on one face as well as the inner and outer peripheral walls. FIG. 1B is a space view of the flip-side of the FIG. 1A wafer showing the uninhibited other face. FIG. 2 is a space view of a pellet or tablet shaped generant body having a burn inhibitor coating on one face and peripheral wall. FIG. 3 is an S-curve graph showing the inflator burn traces of inhibited generant bodies as compared with uninhibited generant bodies. In some inflatable gas bag restraint applications, it is desirable to have an inflation system and procedure which demonstrates a dual gas output rate effect; that is, one which starts with a low rate of gas output during the first about 5 to 25 milliseconds, followed by a higher rate of gas output for the remainder of the inflation cycle whereby the loading on the gas bag system components are reduced as well as lessening the potential for harmful effects on an occupant (e.g. a small child) that is "out of position" (i.e. not properly positioned in the path of a deploying gas bag). Figs. 1A, 1B and 2 show two exemplary embodiments according to the invention of coated bodies 1 and 6, respectively, each having a main gas generant or propellant body part having a relatively inert burn inhibitor or restrictor layers 2, 3, and 4, and 7 and 8, respectively, thereon whereby the above objectives are realized. Fig 1A shows a washer-shaped generant disc 1 having an inhibitor coating on one face or side 2 and on the outer and inner peripheral walls 3 and 4, respectively, of the disc. Fig. 1B shows the flip-side of the Fig. 1A disc showing the uncoated and uninhibited other face or side 5 as well as inhibited peripheral walls 3 and 4. Fig. 2 shows a generant pellet or tablet 6 having an inhibitor coating on one face or side 7 and peripheral wall 8; whereas face or side 9 is uncoated and uninhibited. The composition of the generant part of the coated bodies 1 and 6 is not critical; thus any known gas generant composition, for example, any azide or non-azide based fuel formulation can be used, especially those used for automotive gas bag inflators meeting such well known requirements as burning rate, non-toxicity and flame temperature. The generant is preferably an azide-based fuel which produces nitrogen-containing or nitrogen-rich gas, more preferably an alkali metal azide, and most preferably sodium azide. Exemplary azide-based generant compositions are disclosed in aforementioned U.S. patents, preferably formulations containing sodium azide, iron oxide, molybdenum disulfide and optionally sulfur according to aforementioned U.S. Pat. No. 4,203,787; compositions containing sodium azide, iron oxide, sodium nitrate, silica, alumina and optionally bentonite according to aforementioned most preferably formulations containing sodium azide, molybdenum disulfide and sulfur according to aforementioned U.S. Pat. No. 3,741,585. Exemplary non-azide based formulations are disclosed in aforementioned U.S. Pat. Nos. 4,931,112 and 5,015,309 as well as aforementioned application EP-A-0536916 and US Serial No 07/787,500 and additional art cited therein. The burn inhibitor or retardant material coated on the generant wafers tested and represented by curve 1 in the Fig.3 graph was KRYLON® acrylic resin; and though this resin is the preferred inhibitor, other organic plastic resins may be utilized instead. The inhibitor coating may be applied by any of such well known painting techniques as spraying, rolling, brushing, buttering, etc. However, spray coating is preferred since it is better suited for continuous, automated application. Multiple coatings may be applied to achieve any desired inhibitor thickness. The crux of the present invention centers on the relatively inert nature and composition of the burn inhibitor, as above described, together with the characteristics and properties imparted to the generant due to the configuration and manner in which the inhibitor coating(s) is applied or combined with the baseline generant body. During the combustion process the inhibiting coating burns and/or attrites away progressively exposing the additional surface of baseline generant. This newly exposed generant burning surface proportionally increases the rate of gas output creating the desired dual rate effect. The timing of the rate change is a function of the rate of loss or erosion of the inhibitor. Varying the thickness of the baseline generant may be used to obtain a steeper or shallower pressure slope angle and a shorter or longer burnout time. The dual rate effect is graphically illustrated by the exemplary curves of Fig. 3 wherein Tank Pressures (kPa/psi x 10 1 ) vs. Time (milliseconds x 10 1 ) data is plotted for two sets of test samples carried out in a 100 liter closed tank comparing inflators with and without inhibited wafers. S-curve 1 represents a series of data points for a mass of burn inhibited generant wafers (similar to the wafers of Fig. 1A and 1B) in accordance with the invention. S-curve 2 represents a series of data points for a mass of uninhibited control or standard generant wafers. The inhibited wafers tested were comprised of approximately 6.5g of generant and 0.1g of KRYLON® burn inhibitor. The composition of both type generant wafers comprised (all percents by weight) about 68% NaN 3 , about 30% MoS 2 and about 2%S. As can be seen from comparing curves 1 and 2 the KRYLON® resin coating significantly reduces or suppresses the burn of the generant, for example, at 20 ms the burn would be suppressed by approximately 30%. The inhibited generant bodies according to the invention preferably have a wafer shape, more preferably a cylinder or disc, and most preferably a washer-shaped disc as shown in Figs. 1A and 1B. To particularly illustrate, the outside diameter of disc 1 as shown in Figs. 1A and 1B may vary from 34.93 to 38.1 mm (1.375 to 1.500 in), the inside diameter (i.e. diameter of opening) may vary from 10.16 to 14.27 mm (0.400 to 0.562 in), the thickness of the generant body may vary from 2.54 to 7.11 mm (0.100 to 0.280 in), and the thickness of the inhibitor layers 2-4 may vary from 0.254 to 0.635 mm (0.010 to 0.025 in). A somewhat less preferred body form is a pellet or tablet (similar in shape to an aspirin tablet) as depicted in Fig. 2. To particularly illustrate, the outside diameter of pellet or tablet 6 as shown in Fig. 2 may vary from 6.35 to 9.525 mm (0.250 to 0.375 in), the thickness of the generant body may vary from 1.78 to 7.11 mm (0.070 to 0.280 in) and the thickness of the inhibitor layers 7 and 8 may vary from 0.254 to 0.635 mm (0.010 to 0.025 in). However, the overall shape of the generant body is not critical and can be virtually any shape, such as elliptical, rectangular (preferably a square) or the like. Although central holes or openings as shown in Figs. 1A and 1B are preferred in the wafer disc design such openings may be omitted for certain applications, e.g. a solid multi-wafer grain as is known in the art. The shape of the opening in the wafer is not critical and may take a variety of shapes, such as elliptical, triangular, rectangular, etc., even though circular openings as shown in Figs. 1A and 1B are preferred. The shape of the opening is typically governed by the shape of the igniter chamber (which is normally circular) on which the wafers are preferably arranged. Also the perimeter wall 3 of the generant body 1, as well as the inner wall 4 defining the opening as shown in Figs. 1A and 1B, may have a saw-tooth or serrated design so as to increase the generant surface area presented for combustion, facilitate grain assembly, etc. Although, as previously indicated, the potential utility of such generant bodies may be quite varied, according to the invention the preferred application is to form the generant mass in conventional inflators or gas generators therefrom, most preferably that utilized in the combustion chamber of a conventional automotive gas bag crash protection restraint system. Although a plurality of the composite generant bodies of the invention (e.g. the pellet or tablet 6 of Fig. 2) may be randomly packed into an inflator combustion chamber (e.g. as shown in aforementioned U.S. Pats. 4,005,876 and 4,547,342), the preferred configuration and arrangement comprises a plurality of side by side (or stack of) composite wafer-shaped bodies (e.g. the washer-shaped disc 1 of Figs. 1A and 1B) having alternating inert spacer screens or discs forming a propellant grain or array (e.g. as illustrated in aforementioned U.S. Pats. 4,890,860 and 4,998,751). These documents also show the well known basic component parts of such exemplary gas bag inflators; namely, a combustion chamber with a gas outlet, a generant mass or grain disposed within the combustion chamber, an igniter for the generant and gas passage means for routing gas generated to the gas outlet from the chamber, which gas is typically filtered to remove condensed phase combustion products. When the generant in these systems is ignited and burned, nitrogen-containing gas is produced which is used to inflate the gas bag. Though it is preferred that the generant bodies be coated with burn inhibitor as depicted in the Figures, either more or less area of the bodies may be coated. For example, parts of both faces or side may be coated, peripheral walls 3, 4 and 8 may be left uncoated (e.g. by masking), or an annular band or strip of inhibitor may be applied on either the outer, inner or intermediate the periphery of a wafer disc such as shown in Figs. 1A and 1B. In addition, the inhibitor coating on one or both sides of the generant may be suitably built up to consist of a series of equally spaced, raised projections or pads which, for example, may have the configuration as disclosed in US Patent US-A-5551343 FILED March 10, 1992. If necessary, conventional binders (such as polypropylene carbonate, molybdenum disulfide and the like) and other auxiliary agents may be added in small amounts (typically 1-6 wt.%) to the particulate generant formulation prior to consolidation.

Claims: A gas bag inflator comprising: a combustion chamber having a gas outlet; a plurality of gas generant bodies (1, 6) disposed within said chamber, said generant bodies having first and second faces (2, 5, 7, 9) with peripheral walls (3, 4, 8) between them and having thereon a coating of a relatively inert burn inhibitor material; means for igniting the inhibited bodies; and means for routing gas generated from said chamber through the gas outlet, characterised in that the inhibitor material comprises a plastics resin and is applied only to said first faces (2, 7) and peripheral walls (3, 4, 8). A gas bag inflator according to claim 1 wherein the inhibitor coating is applied to the generant bodies by spraying. A gas bag inflator according to claim 1 or claim 2 wherein the inhibited bodies (1) are wafer-shaped and arranged in side by side relationship. A gas bag inflator according to claim 3 wherein the inhibited wafers (1) are individually separated by spacer means. A gas bag inflator according to claim 3 or claim 4 wherein the wafers are washer-shaped and the inhibitor coating substantially covers the first face (2) and the inner (4) and outer (3) peripheral walls of each wafer. A gas bag inflator according to any preceding claim wherein the gas generant bodies comprise an azide-based composition. A gas bag inflator according to claim 6 wherein the azide is sodium azide. A gas bag inflator according to any preceding claim including a gas bag deployment means connected to the gas outlet. A method of inflating a gas bag comprising igniting a gas generant body (1, 6) having first and second faces (2, 5, 7, 9) with peripheral walls (3, 4, 8) between them and having thereon a coating of a relatively inert burn inhibitor material coating, and using the gas generated to inflate the gas bag, characterised in that the inhibitor material comprises a plastics resin and is applied only to said first faces (2, 7) and peripheral walls (3, 4, 8). A method according to claim 9 wherein the inhibitor coating is applied to the generant body as a spray. A method according to claim 9 or claim 10 wherein the inhibited body (1) is wafer-shaped. A method according to any one of claims 9 to 11 wherein the gas generant body (1, 6) comprises an azide-based composition. A method according to claim 12 wherein the azide is sodium azide.

Inventors: CROWELL BRYAN P, LAURITZEN DONALD R, CROWELL, BRYAN P., LAURITZEN, DONALD R.
Applicants: MORTON INT INC, MORTON INTERNATIONAL, INC.
Application year: 1993
Main classification: B60R21
IPC codes: B60R21, B01J7, C06D5