Abstract:
A gas generator is provided, the gas generator having a propellant cushion that prevents movement of propellant tablets or grains by providing a bias thereagainst. Furthermore, the cushion is formed from a desiccating material thereby removing moisture and inhibiting moisture uptake by the propellant during manufacture of the gas generator. The elastomeric cushion is also able to manage the presence of chlorine-containing products during periods of inactivation of the gas generator. Accordingly, variables such as fractured propellant and/or moisture retained within the propellant are mitigated or eliminated, thereby enhancing repeatability of inflator performance.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims the benefit of U.S. Application Ser. No. 60/666,695 having a filing date of Mar. 31, 2005. The present application is a co-owned continuation-in-part application of U.S. application Ser. No. 11/395,477 having a filing date of Mar. 30, 2006, (abandoned on Sep. 29, 2008), and claims the benefit thereof. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to pyrotechnic gas generators for inflatable restraint devices, and more particularly to such a gas generator having a propellant cushion for biasing a resistance against the propellant bed to prevent fracture of propellant grains and/or tablets therein. 
     BACKGROUND OF THE INVENTION 
     Inflatable restraint systems or “airbag” systems have become a standard feature in many new vehicles. These systems have made significant contributions to automobile safety; however, as with the addition of any standard feature, they increase the cost, manufacturing complexity and weight of most vehicles. Technological advances addressing these concerns are therefore welcomed by the industry. In particular, the gas generator or inflator used in many occupant restraint systems tends to be the heaviest, most complex component. Thus, simplifying the design and manufacturing of airbag inflators, while retaining optimal function, has long been a goal of automotive engineers. 
     Typical inflators are constructed having an elongate metallic body. Because many inflators utilize pyrotechnic gas generant compounds to produce inflation gas for the associated airbag, the inflator structure is necessarily robust, making such inflators correspondingly heavy. An increasingly popular and useful inflator style uses multiple, selectively activated gas generant charges. In such systems, the multiple propellant beds disposed within the inflator body may be ignited either simultaneously or serially. Certain vehicle and occupant parameters may justify firing both propellant beds in the event of a crash. Other scenarios may be best addressed by firing only one of the propellant charges, or firing the charges sequentially, with a delay between the two events. In order to avoid sympathetic ignition of one charge during firing of the other, the combustion chambers must generally be fluidly isolated. The relatively large forces on the inflator generated by the combustion of pyrotechnics therein requires the internal partitions and other structural members of the inflator that fluidly isolate the charges to be relatively sturdy, further adding to the weight of the inflator. 
     Various schemes have developed for constructing sturdy, internally partitioned multi-chamber inflators. One approach involves inserting a partition into the interior of the inflator, then crimping or roll-forming the inflator body to retain the partition. This approach has proven effective; however, in many circumstances a heavier-duty/thicker inflator body must be used that will withstand the crimping and/or roll forming process. Such inflator bodies can be quite heavy, and the manufacturing process is relatively complicated given processing steps necessary to secure the internal partitions. 
     Yet another concern is repeatability of performance of the gas generator. Propellant springs or cushions are employed to prevent fracture of the propellant thereby maintaining a relatively constant propellant surface area of combustion. Additionally, certain propellants may be hygroscopic wherein the absorption of humidity and/or water may inhibit expected burn characteristics and therefore may result in performance variability of an associated airbag cushion during a crash event. Even though useful in preventing the fracture of propellant, propellant springs or cushions add to the manufacturing complexity and cost, and to the weight of the overall inflator. 
     Certain gas generating compositions or auto-ignition compositions contain constituents that contain chlorine, such as potassium perchlorate or potassium chlorate. These oxidizers may liberate chlorine-containing products over extended periods of time that are typically managed by constituents contained within each composition, such as clay or calcium oxide. The concern with utilizing clay or calcium oxide is that the relative amount of solids released after inflator activation is increased as compared to compositions that do not contain metal-containing constituents. It would be an improvement in the art to manage chlorine-containing products residing within the inflator without the use of metal-containing constituents in the respective composition, thereby increasing the relative mols of gas produced per gram of gas generant while continuing to manage the chlorine-containing products to optimize the performance of the inflator. 
     U.S. Pat. No. 6,779,812 to Ishida et al. describes an inflator containing silicone cushioning members made from silicon rubber and silicon foam. Ishida fails to recognize the problem of absorption of chlorine-containing products. In particular, Cole-Parmer® recognizes the general incompatibility of chlorine-containing products and silicone because of degradation of the silicone. Accordingly, in the presence of chlorine-containing products, silicon-based products are subject to chemical degradation thereby detracting from their cushioning ability in an inflator for example. See the chemical resistance charts of Cole-Parmer® on the web. Accordingly, Ishida fails to recognize the general incompatibility of silicon-containing cushions within a gas generator containing chlorine-containing products. In automotive gas generators, the shelf-life of the gas generator must meet customer specification and safety requirements. As such, vibration control relative to the propellant and management of the chlorine-containing decomposition products is desirable, while yet minimizing the metal-containing constituents in the various compositions. 
     WO 97/29151 to Frampton describes various pharmaceutical stoppers for capping vials of pharmaceutical products. The stoppers are made of elastomeric materials containing a desiccant. Frampton does not recognize the problem of chlorine-containing products affecting degradation of an elastomeric or silicone-based stopper. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a gas generator having a propellant cushion that prevents movement of the propellant tablets or grains by providing a bias thereagainst. Furthermore, the cushion is formed from a desiccating material thereby removing moisture and inhibiting moisture uptake by the propellant both during manufacture, and during its normal shelf-life within a vehicle interior. In yet another advantage, gas generators containing “smokeless” or minimal metal-containing auto-ignition or gas generating compositions typically contain perchlorate-based or chlorate-based constituents. The present invention permits the use of silicon-based or silicone-based cushions, or more generally elastomeric cushions, within the gas generator without degradation in the presence of chlorine-containing products. As a result, management of the chlorine-containing products using clay or other metal-containing constituents within the various compositions is not necessary, thereby resulting in filtration and performance improvements. 
     In accordance with the foregoing and other objects of the invention, an exemplary inflator having a lightweight propellant cushion formed from a desiccating material within an inflatable restraint system, is provided. An exemplary inflator preferably includes an elongate inflator body having a first and a second end and a plurality of inflation apertures. The inflator body defines a first combustion chamber wherein a first propellant charge is positioned. A partitioning assembly is nested within the inflator body, and positioned proximate the second end, the partitioning assembly defining a second combustion chamber wherein a second propellant charge is positioned. The exemplary inflator further includes a first and a second initiator, the initiators operably associated with the first and second propellant charges, respectively. The initiators are selectively operable to ignite the propellant charges, thereby supplying an inflation gas via the first chamber to an inflatable restraint cushion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial side view of an inflator according to a preferred constructed embodiment of the present invention. 
         FIG. 2  is a schematic view of an exemplary gas generating system, a vehicle occupant protection system, in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , there is shown an exemplary gas generator or inflator  10  according to an embodiment of the present invention. Inflator  10  is designed for use with an inflatable restraint system in an automobile, supplying inflation gas for inflation of a conventional airbag cushion, a function well known in the art. Inflator  10  utilizes two propellant charges, described herein, that are ignited in separate combustion chambers, and discharge inflation gas via a common plenum  21 . Exemplary inflator  10  further provides independently operable initiators for igniting the respective propellant charges, imparting significant variation to the available operating schemes for the inflator. For instance, both sequential and serial firing of the two charges is possible, depending on the optimal deployment of the associated airbag. It is contemplated that inflator  10  will find greatest utility in passenger-side airbag systems; however, other applications are possible without departing from the scope of the present invention. All the components of the present invention are formed from known materials that are readily available commercially, and are made by known processes. 
     Inflator  10  includes an elongate pressure vessel or inflator body  11 , preferably a hollow steel cylinder. Inflator body  11  is characterized by a first end  15  and a second end  17 , and defines a plurality of inflation apertures  40  that allow fluid communication between the exterior of the inflator body and plenum  21 . A first end closure  13  is positioned at first end  15  of inflator body  11 , preferably creating a fluid seal therewith. A second end closure  34  is preferably positioned at second end  17 , also preferably creating a fluid seal with inflator body  11 . Closures  13  and  34  are preferably metallic; however, they might be made from another suitable material such as a plastic, a ceramic, or a composite material. First end  15  and second end  17  are preferably crimped inwardly to hold first and second closures  13  and  34  in place; however, some other suitable method such as welding or mating threads on inflator body  11  and the respective closures might be used. In addition, rubber O-rings may be snap-fit around closures  13  and  34 , creating or enhancing seals with inflator body  11 . 
     Inflator  10  includes a first combustion chamber  25 , within which a quantity of gas generant material or first propellant charge  28  is placed. In a preferred embodiment, chamber  25  comprises a significant proportion of the interior of inflator body  11 , defined in part by longitudinal walls of inflator body  11 , and in part by first end closure  13 . Plenum  21  is the region of inflator body  11  whereby inflation gas is passed to apertures  40 . Thus, chamber  25  and plenum  21  are at least partially coextensive. Stated another way, plenum  21  may be loosely defined as the portion of chamber  25  that occupies the middle region of the interior of inflator body  11 . The phrase “at least partially coextensive” should be understood to include designs wherein chamber  25  is subdivided by foils, burst shims, etc., as described herein, as well as designs wherein chamber  25  is uninterrupted by such features. First end closure  13  preferably includes a cylindrical extension  16  wherein a perforated disk  18  is positioned, separating chamber  25  into two sub-chambers  25   a  and  25   b . An initiator assembly  12 , preferably including a conventional igniter or squib, is positioned at first end  15 , and preferably mounted in first end closure  13  such that it can ignite compositions in chamber  25 . A second initiator assembly  9 , also preferably including a conventional igniter or squib, is positioned at second end  17 . 
     Propellant charge  28  may be any suitable gas generant composition known in the art, preferably a non-azide composition containing phase stabilized ammonium nitrate. Other gas generating compositions or auto-ignition compositions contained within the gas generator may contain perchlorate and chlorate containing oxidizers as known in the art. Exemplary, but not limiting formulations are described in U.S. Pat. Nos. 5,872,329, 5,756,929, and 5,386,775, and are herein incorporated by reference. In a preferred embodiment, propellant charge  28  is provided in both tablet  28   a  and wafer  28   b  forms, both of which are illustrated in  FIG. 1 . The tablets  28   a  and wafers  28   b  may be different compositions, but are preferably the same material in different, commercially available forms. In a preferred embodiment, a retainer disk  32  separates tablets  28   a  from wafers  28   b . Disk  32  may be made from a relatively porous material such that a flame front or heat from ignition of tablets  28   a  can ignite wafers  28   b , or it may be made from a known material that allows ignition of wafers  28   b  by heat convection from the burning of tablets  28   a . A quantity of booster propellant  14  is preferably placed in sub-chamber  25   a , and is ignitable via initiator  12  in a conventional manner to ignite and enhance the burn characteristics of the first propellant charge  28   a  and  28   b.    
     In accordance with the present invention, a cushion  33  is positioned between propellant tablets  28   b  and a cap  29 , thereby inhibiting fracture of the tablets  28   b . In further accordance of the present invention, the cushion  33  is formed from a composition containing silicone and a desiccating material such as synthetic zeolites or molecular sieves, calcium oxide, and/or calcium sulfate. The composition of cushion  33  has a silicone to desiccating material ratio ranging from 10/90 to 90/10, and more preferably has a silicone to desiccating material ratio ranging from 20/80 to 50/50. It will be appreciated that cushion  33  may also be positioned anywhere within the inflator  10 , and may provide a resilient support wherever required therein. Accordingly, the shape of the cushion  33  is not limited to the exemplary structure shown. In yet another advantage, the cushion also absorbs other undesirable gases thereby improving the quality of the gaseous effluent upon gas generator activation. Another advantage is that the adsorption of the desiccant is slowed by being mixed within the silicone matrix, thereby preventing excess adsorption of moisture during the assembly of the gas generator. Yet another advantage is that the adsorption of undesirable gases mitigates the likelihood of auto-catalyzed decomposition of the “smokeless” main gas generant due to excess buildup of chlorine-containing products, for example. In still a further advantage, the cushion is made from a lightweight material rather than a typical wire mesh material, thereby reducing the overall weight of the gas generator  10  or gas generating system  10  associated therewith. 
     FTIR analysis confirms the efficacy of molecular sieve in absorbing chlorine-containing products. It has been unexpectedly discovered that the incorporation of the desiccant within the cushion  33  inhibits degradation of an elastomeric cushion, or a silicon-based cushion, in the presence of chlorine-containing products, thereby enhancing gas generator performance. The elastomeric cushion is thus able to manage the presence of chlorine-containing products during periods of inactivation of the gas generator, notwithstanding the general incompatibility of the elastomer or silicon in the presence of chlorine-containing products. A preferred gas generator includes a gas generating composition, a chlorine-containing auto-ignition composition, and a cushion  33 , the cushion  33  includes a silicon-based or silicone-based elastomer containing molecular sieve, manufactured as described below. In yet another embodiment, the gas generating composition and the chlorine-containing auto-ignition composition may be the same composition. 
     The cushion  33  may be formed by mixing a desired amount of the desiccant, synthetic zeolite for example, provided by companies such as Johnson Matthey identified at jmgpt on the web or Grace Davison identified at gracedavison on the web, into a desired amount of uncured silicone. In one embodiment, the gas generant retainer  33  may be formed from silicone and zeolite, the silicone and zeolite in weight percent ratio of 10/90 to 90/10 of silicone to zeolite. In et another embodiment, the gas generant retainer  33  may be formed from silicone and zeolite, the silicone and zeolite provided in weight percent ratios of 20/80 to silicone to zeolite. Other desiccants may be provided by known suppliers such as Aldrich or Fischer. Zeolite has been found to be particularly desirable in view of favorable results with regard to heat aging for 400 hours at 107 C. The silicone may then be finally mixed to a substantially homogeneous mixture, and cured according to manufacturer instructions. Silicone is readily available and may for example be provided by companies such as Shin-Etsu of Japan. 
     A partitioning assembly  26  is positioned proximate second end  17 , and preferably comprises a substantially cylindrical base member  27  and a cap  29 . Base member  27  and cap  29  define a second combustion chamber  35 , that at least partially encases a second quantity of propellant  38 , preferably in both tablet and wafer form. Base member  27  and second end closure  34  may be the same piece, as in one preferred embodiment, or a plurality of separate, attached pieces might be used. In a preferred embodiment, partitioning assembly  26  is formed structurally independent from inflator body  11 . Partitioning assembly  26  is an independent piece having no physical attachment with the longitudinal sidewall of inflator body  11 . During assembly of inflator  10 , partitioning assembly  26  is slid into position in inflator body  11 , and second end  17  is crimped inwardly to secure assembly  26  therein. Thus, other than securing second end closure  34 , no modifications are made to inflator body  11  to accommodate or otherwise secure the components defining second combustion chamber  35 . 
     Cap  29  preferably includes a plurality of apertures  30  that can connect second chamber  35  with plenum  21  (as well as first chamber  25 , since plenum  21  and chamber  25  are fluidly connected and partially coextensive). In a preferred embodiment, a foil or burst shim (not shown) is placed across apertures  30  to block fluid communications between the two chambers. It should be appreciated, however, that the foil or burst shim is positioned and/or manufactured such that it will not burst inwardly, i.e. in the direction of second end  17  during combustion of propellant in chamber  25 . Combustion of propellant in second chamber  35 , on the other hand, is capable of bursting the foil or shim outwardly, allowing the combustion products in chamber  35  to escape to plenum  21 /first chamber  25 , and thereby discharge from inflator body  11 . The preferred foils and shims, and the described methods of mounting them are all known in the art. By fluidly isolating first and second chambers  25  and  35 , sympathetic ignition of the propellant in chamber  35  during combustion of the propellant in chamber  25  can be avoided, as described herein. The outer diameter of base member  27  is preferably substantially equal to the inner diameter of inflator body  11 , such that base member  27  is nested therein, i.e. fits relatively snugly. Because both second end closure  34  and inflator body  11  are preferably substantially cylindrical, the two components are preferably axially aligned. One or more autoignition tablets  50  may be placed in inflator  10 , allowing ignition of the gas generant materials upon external heating in a manner well known in the art. 
     In one embodiment, wafers  28   b  are positioned in a stack in plenum  21 . Again, the cushion  33 , is positioned adjacent the stack  28   b , and biases the entire stack  28   b  toward first end  15 . Wafers  28   b , in turn, preferably bias disk  32  against tablets  28   a , preventing tablets  28   a  from being jostled while the inflator is idle for long periods, helping avoid mechanical degradation of tablets  28   a.    
     The inflator  10  described herein may be altered in design depending on application requirements. Nevertheless, the cushion or propellant restraint  33 , in accordance with the present invention is provided in any inflator design, and biased against at least one propellant thereby providing a cushioning effect as formally realized by metallic cushions for example. 
     In a typical inflatable restraint system design, inflator  10  is connected to an electrical activation system that includes a crash sensor, of which there are many well-known suitable types. In addition, various sensing systems may be incorporated into the vehicle electronics, including seat weight sensors, occupant detection systems, etc. During a typical deployment scenario, an impact or a sudden vehicle deceleration, an activation signal is sent from an onboard vehicle computer to inflator  10 . The signal may be sent to either or both of the initiator assemblies housed with inflator  10 . Because chamber  25  preferably contains the larger, main charge, the activation signal is typically directed initially to the initiator assembly operably associated with first chamber  25 . In certain scenarios, for example with larger occupants, or where occupants are out of a normal seated position in the vehicle, it may be desirable to activate both propellant charges simultaneously. Other scenarios may call for different activation schemes. For instance, certain conditions may make it desirable to fire only the first propellant charge, or sequentially fire both charges, with varying time delays between the two events. Once an electrical activation signal is sent to the initiator associated with first chamber  25 , combustion of booster propellant  14  in sub-chamber  25   a  is initiated. The flame front and/or hot combustion gases from booster  14  subsequently traverse disk  18 , initiating combustion of propellant tablets  28   a  in chamber  25   b . The burning of tablets  28   a  produces inflation gas that flows rapidly out inflation apertures  40 , initiating filling of an associated airbag. A cylindrical, metallic mesh filter  16  is preferably positioned in inflator body  11 , and filters slag produced by the combustion of the compounds therein, also serving as a heat sink to reduce the temperature of the inflation gas. Combustion of tablets  28   a  initiates combustion of wafers  28   b , preferably made from the same or similar material as tablets  28   a , providing a sustained burn that delivers a relatively constant supply of gas to the associated airbag via plenum  21  and apertures  40 . When desired, an electrical activation signal is sent to the initiator operably associated with second chamber  35 , containing a gas generant composition  38  that is preferably similar to the composition in chamber  25 . Rapid creation of gas in chamber  35  causes a rapid rise in the gas pressure therein, outwardly bursting the foil or shim (not shown) that covers apertures  30 , in cap  29 . The gas is subsequently discharged from inflator  10  via plenum  21  and apertures  40 . Activation of the gas generant in chamber  35  can take place before, during, or after an activation signal is sent to initiator assembly  12 , operably associated with chamber  25 . 
     Because both chambers  25  and  35  discharge inflation gas through plenum  21 , the present invention provides different operating advantages over many earlier designs wherein separate plenums are used for each combustion chamber. By discharging inflation gases from both combustion chambers  25  and  35  through plenum  21 , the inflation profile characteristics across the length and width of an associated airbag can be improved as compared to earlier multi-chamber designs wherein the combustion chambers discharge via separate plenums. In addition, the use of a partitioning assembly structurally independent from the inflator body sidewalls allows the inflator to be constructed without crimping or otherwise modifying the inflator body itself. Moreover, because inflator  10  utilizes a plenum that is coextensive with a first of the combustion chambers, inflator  10  has a simpler design than multi-chamber inflators utilizing combustion chambers that are both partitioned from a common plenum. Inflator body  11  utilizes no attached internal partitions, and can therefore be manufactured without the need for strengthening to compensate for weakening caused by partition attachment. These and other advantages reduce the cost, manufacturing complexity, size and weight of the inflator. 
     Referring now to  FIG. 2 , the exemplary inflator  10  described above may also be incorporated into an airbag system  200 . Airbag system  200  includes at least one airbag  202  and an inflator  10  containing a gas generant composition  12  in accordance with the present invention, coupled to airbag  202  so as to enable fluid communication with an interior of the airbag. Airbag system  200  may also include (or be in communication with) a crash event sensor  210 . Crash event sensor  210  includes a known crash sensor algorithm that signals actuation of airbag system  200  via, for example, activation of airbag inflator  10  in the event of a collision. 
     Referring again to  FIG. 2 , airbag system  200  may also be incorporated into a broader, more comprehensive vehicle occupant restraint system  180  including additional elements such as a safety belt assembly  150 .  FIG. 2  shows a schematic diagram of one exemplary embodiment of such a restraint system. Safety belt assembly  150  includes a safety belt housing  152  and a safety belt  100  extending from housing  152 . A safety belt retractor mechanism  154  (for example, a spring-loaded mechanism) may be coupled to an end portion of the belt. In addition, a safety belt pretensioner  156  containing propellant  12  may be coupled to belt retractor mechanism  154  to actuate the retractor mechanism in the event of a collision. Typical seat belt retractor mechanisms which may be used in conjunction with the safety belt embodiments of the present invention are described in U.S. Pat. Nos. 5,743,480, 5,553,803, 5,667,161, 5,451,008, 4,558,832 and 4,597,546, incorporated herein by reference. Illustrative examples of typical pretensioners with which the safety belt embodiments of the present invention may be combined are described in U.S. Pat. Nos. 6,505,790 and 6,419,177, incorporated herein by reference. 
     Safety belt assembly  150  may also include (or be in communication with) a crash event sensor  158  (for example, an inertia sensor or an accelerometer) including a known crash sensor algorithm that signals actuation of belt pretensioner  156  via, for example, activation of a pyrotechnic igniter (not shown) incorporated into the pretensioner. U.S. Pat. Nos. 6,505,790 and 6,419,177, previously incorporated herein by reference, provide illustrative examples of pretensioners actuated in such a manner. 
     It should be appreciated that safety belt assembly  150 , airbag system  200 , and more broadly, vehicle occupant protection system  180  exemplify but do not limit gas generating systems contemplated in accordance with the present invention. 
     The present description is for illustrative purposes only, and should not be construed to limit the breadth of the present invention in any way. Thus, those skilled in the art will appreciate that various modifications could be made to the presently disclosed embodiments without departing from the intended spirit and scope of the present invention. Other aspects, features and advantages will be apparent upon an examination of the attached drawing figures and appended claims.