Patent Publication Number: US-2011057429-A1

Title: Gas generating system and composition

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to and is a continuation-in-part of co-pending and co-owned U.S. application Ser. No. 11/497,149 having a filing date of Jul. 31, 2006 (originally claiming the benefit of U.S. Provisional Application Ser. No. 60/703,998 filed on Jul. 29, 2005), and, this application also claims priority to and is a continuation-in-part of co-pending and co-owned U.S. application Ser. No. 11/906,348 filed on Oct. 1, 2007 (originally claiming the benefit of U.S. Provisional Application Ser. No. 60/848,682 filed on Sep. 30, 2006). 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to gas generating systems, and to autoignition compositions employed in gas generator devices for automotive restraint systems, for example. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to autoignition compositions that upon ignition provide the flame front and pressure front necessary to safely ignite gas generant compositions in combustible communication therewith. As known in the art, gas generators are typically provided with an autoignition composition that in the event of a fire ignites responsive to a desired threshold temperature. As a result, the gas generant is ignited prior to melting for example, thereby safely igniting the main gas generant composition to inhibit or prevent the likelihood of an explosive event once the gas generant begins to combust. 
     An ongoing challenge is to simplify the manufacture of a gas generator by reducing the constituents required in the production thereof. For example, in many gas generators used in vehicle occupant protection systems, several discrete compositions are provided to serve correspondingly discrete functions. These compositions often include a primary gas generating composition that when combusted is employed to provide sufficient quantities of gaseous products to operate the associated restraint device, such as an airbag or seatbelt pretensioner. A booster composition is utilized to elevate the pressure and heat within the gas generator prior to combustion of the primary gas generant, thereby creating favorable conditions within the inflator for acceptable combustion of the primary gas generant. Of course, still yet another composition is the auto-ignition composition employed to provide safe combustion of the other compositions in the event of a fire. The auto-ignition composition is designed to ignite at temperatures below the melting point of the primary gas generant for example, thereby ensuring the controlled combustion of the primary gas generant, as opposed to an explosive reaction perhaps. 
     The use of potassium chlorate within an autoignition composition has been considered given the autoignition properties of this oxidizer. Furthermore, carboxylic acid in combination with potassium chlorate typically provides a desired autoignition temperature of 200 degrees Celsius or less. Nevertheless, these types of compositions typically do not provide anything but auto-ignition function when employed in gas generators used in vehicle occupant protection systems, for example. As known in the art, gas generating systems may be used for providing a supply of inflation or actuation gas to a gas-actuated element of a vehicle occupant protection system. An ongoing challenge is to simplify the manufacture of a gas generating system by reducing the size, weight, and number of constituents required in the production thereof. For example, in many gas generators used in vehicle occupant protection systems, several discrete compositions are provided to serve correspondingly discrete functions. These compositions often include a primary gas generating composition that when combusted provides sufficient quantities of gaseous products to operate an associated restraint device, such as an airbag or seatbelt pretensioner. 
     While each separate composition contributes to efficient and effective operation of the gas generating system, each composition also adds weight, cost (in materials and assembly time), and volume to the system. For example, to facilitate operation of each composition and to prevent mixing between the various compositions, the booster composition, gas generant, and autoignition compositions are typically stored in separate tubes or chambers. Provision of a separate storage chamber for each composition generally adds to the weight, cost, and assembly time needed to construct the gas generating system. In addition, if a relatively greater the amount of combustible material is burned during operation of the system, a correspondingly greater amount of effluent and heat will be usually generated by the burning of the material. Therefore, it would be advantageous to reduce the number of gas generating system components and the number of compositions used in the operation of the system. 
     SUMMARY OF THE INVENTION 
     The above-referenced concerns and others may be resolved by gas generating systems including an autoignition composition containing a first oxidizer selected from metal chlorates, such as potassium chlorate, a carboxylic acid or dicarboxylic acid as a primary fuel, a secondary oxidizer selected from metal and nonmetal nitrates, nitrites, oxides, basic metal nitrates, and other known oxidizers, and a secondary fuel selected from azoles including tetrazoles, triazoles, and furazans, and salts thereof. Other constituents including extrusion aids, such as fumed silica and/or graphite, may be included in relatively small amounts. 
     In further accordance with the present invention, a gas generator devoid of a booster composition tube, and a vehicle occupant protection system incorporating the gas generator are also included. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional side view showing the general structure of an inflator in accordance with the present invention; 
         FIG. 2  is a schematic representation of an exemplary vehicle occupant restraint system containing a gas generant composition in accordance with the present invention; 
         FIG. 3  is a cross-sectional side view showing the general structure of a conventional gas generating system incorporating separate booster and gas generating chambers; 
         FIG. 4  is a cross-sectional side view showing the general structure of a gas generating system in accordance with the present invention, in which the booster chamber has been eliminated; and 
         FIG. 5  is a cross-sectional side view showing the general structure of a gas generating system in accordance with an alternative embodiment of the present invention, in which the booster chamber has been eliminated. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
     Compositions useful in the present invention contain a first oxidizer selected from alkali, alkaline earth, and transitional metal chlorates, and mixtures thereof, such as potassium chlorate, at about 10-60 weight %; a primary fuel selected from carboxylic acids and dicarboxylic acids, such as DL-tartaric acid, at about 15-45 weight %; a secondary oxidizer selected from metal and nonmetal nitrates, nitrites, oxides, and other known oxidizers at about 30-50%; and a secondary fuel selected from tetrazoles, triazoles, furazans, and salts thereof at about 0-30 weight %, said weight percent calculated with regard to the weight of the total composition. Extrusion aids or processing additives such as graphite or fumed silica may be added in relatively smaller amounts, such as 0.1-2% by weight of the total composition for example. 
     The present compositions contain a metal chlorate such as potassium chlorate; a primary fuel selected from carboxylic acids and dicarboxylic including DL-tartaric acid, L-tartaric acid, D-tartaric acid, succinic acid, glutamic acid, adipic acid, mucic acid, fumaric acid, oxalic acid, galactaric acid, citric acid, glycolic acid, L-malic acid, and compounds having at least one —COOH— group, and mixtures thereof; a second fuel selected from an azole including tetrazoles, triazoles, furazans, salts thereof, and mixtures thereof; a secondary oxidizer selected from metal and nonmetal nitrates or other known oxidizers not containing a perchlorate. The carboxylic acid or dicarboxylic acid will preferably have a primary hydrogen or PKA less than or equal to 3. Nevertheless, it has been found that with certain fuels/salts, the pKa of the base acid may range up to 5.0 or less. 
     In one embodiment, the total fuel constituent including the carboxylic fuel and the second fuel is provided at about 20-45% by weight of the total composition; the oxidizer constituent is provided at about 20-50% by weight of the total composition; and the potassium chlorate or metal chlorate is provided at about 10-60% by weight of the total composition wherein the weight percent of the chlorate is separately calculated from that of the oxidizer. The composition may be formed by wet or dry mixing the constituents in a granulated form in a known manner, and then pelletizing or otherwise forming the composition for further use. The constituents may be provided by Fisher Chemical, Aldrich Chemical, GFS, and other known suppliers. 
     Compositions that may be employed in the present inventive inflators are exemplified by the following Examples: 
     Comparative Example 1 
     A known autoignition composition was prepared by homogeneously mixing dried and granulated D-glucose at about 26.875 wt % and potassium chlorate at about 73.125 wt %, the percents stated by weight of the total composition. The composition autoignited at about 144 C as measured by DSC analysis. The propellant formed from the constituents resulted in an approximate 55.5% gas yield. The impact sensitivity of this formulation had an HD50 of 2.0 inches as conducted in conformance with the Bruceton Test. 
     Example 2 
     An exemplary formulation was provided that functions as a booster, an autoignition, and a gas generant composition. The formulation contains 5-aminotetrazole at about 19.0 wt %, DL-tartaric acid at about 20.0 wt %, strontium nitrate at about 35.0 wt %, and potassium chlorate at about 26.0 wt %. The constituents were previously and separately ground to a relatively small size in a known manner. They were then dry-mixed to form a substantially homogeneous composition. The composition autoignited at about 140 C as measured by DSC analysis. The propellant formed from the constituents resulted in an approximate 67% gas yield. The impact sensitivity of this formulation had an HD50 of 11.5 inches as conducted in conformance with the Bruceton Test. The composition was aged for about 480 hours at 107 C and still autoignited at about 145.1 C as determined by DSC analysis. 
     Example 3 
     An exemplary formulation was provided that functions as a booster, an autoignition, and a gas generant composition. The formulation contains 5-aminotetrazole at about 19.0 wt %, DL-tartaric acid at about 19.0 wt %, strontium nitrate at about 50.0 wt %, and potassium chlorate at about 12.0 wt %. The constituents were granulated and dry-mixed to form a substantially homogeneous composition. The composition autoignited at about 141 C as measured by DSC analysis. The propellant formed from the constituents resulted in an approximate 68.2% gas yield. The impact sensitivity of this formulation had an HD50 of 8.8 inches as conducted in conformance with the Bruceton Test. As shown in  FIG. 3 , the composition reflected a relatively strong burn rate across several pressure regimes, and in particular indicated burn rates of over 0.8 inches per second (ips). Again referring to  FIG. 3 , it can be seen that the composition exhibited a burn rate of about 0.2 ips at about 200 psig, about 0.35 ips at about 550 psig, about 0.5 ips at about 1000 psig, about 0.55 ips at about 1500 psig, about 0.85 ips at about 2000 psig, about 0.9 ips at about 2500 psig, about 0.85 ips at about 3000 psig; and about 1.2 ips at about 3900 psig. It can therefore be seen that a composition in accordance with the present invention exhibits a satisfactory burn rate (typically 0.4 ips or more at about 2500-3000 psig) thereby ensuring satisfactory functionality as a primary gas generant. The composition was aged for about 480 hours at 107 C and still autoignited at about 174.7 C as determined by DSC analysis. 
     Example 4 
     An exemplary formulation was provided that functions as a booster, an autoignition, and a gas generant composition. The formulation contains DL-tartaric acid at about 28.0 wt %, strontium nitrate at about 32.0 wt %, and potassium chlorate at about 30.0 wt %. The constituents were previously and separately ground to a relatively small size in a known manner. They were then dry-mixed to form a substantially homogeneous composition. The composition autoignited at about 153 C as measured by DSC analysis. The propellant formed from the constituents resulted in an approximate 66.1% gas yield. The impact sensitivity of this formulation had an HD50 of 8.1 inches as conducted in conformance with the Bruceton Test. 
     As indicated in Examples 1-4, compositions formed in accordance with the present invention (Examples 2-4) preferably autoignite at or below about 180 C and provide a booster function as well. The compositions of the present invention may also produce substantial quantities of gas, and exhibit sufficient burn rates thereby producing sufficient amounts of gas when activated. Compositions employing a secondary oxidizer, such as strontium nitrate, provide relative increased quantities of gas and an improved sensitivity. A Bruceton sensitivity result wherein H 50 =3.9 or more relaxes the packaging requirements as per U.S.D.O.T regulations. Accordingly, compositions having a sensitivity result of 3.9 or greater provide substantial packaging advantages. It will further be appreciated that the use of a secondary fuel, such as 5-aminotetrazole, in conjunction with the carboxylic or dicarboxylic acid, the secondary oxidizer, and the potassium chlorate produces greater amounts of gas, acceptable autoignition temperatures, and booster functionality. As such, compositions formed in this manner may be provided to singularly replace the three discrete booster, autoignition, and primary gas generant compositions normally found in a gas generator. 
     Examples 5-16 
     As shown in Table 1 below, the various acids shown, when converted to salts and mixed with potassium chlorate in stoichiometric amounts exhibit acceptable autoignition temperatures for a variety of uses. Certain autoignition temperatures exceed 180 C but may still be useful in selected applications such as hybrid inflators and seatbelt pretensioners for example. It will be appreciated that these Examples reflect the autoignition character imparted by the resulting salts and the potassium chlorate. As further shown, acids exhibiting a pKa of about 3.05 or less generally provide autoignition temperatures generally less than 170-180 C. However, acids exhibiting a pKa of about 5.0 or less may still be acceptable wherein autoignition temperatures of 250 or so are acceptable, for example. It will be appreciated that certain acids such as citric acid and malonic acid when stoichiometrically combined with potassium chlorate may not satisfy the autoignition function, but still when combined with at least a second oxidizer function as a booster oxidizer and a primary gas generant. It has further been determined that the use of a desiccant as described in co-owned and co-pending U.S. Ser. No. 11/479,493, herein incorporated by reference, may in certain circumstances maintain optimum environmental conditions within the gas generator thereby facilitating the tri-functionality of the composition when used as an autoignition, booster, and primary gas generating composition. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                   
                   
                   
                 Stoichiometric Mixture w KC 
               
            
           
           
               
               
               
               
               
               
            
               
                 Name 
                 Structure 
                 Lit. mp 
                 DSC/TGA 
                 Hot Plate 
                 PKa 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 L-Tartaric Acid 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 168-170 
                 Al 142 
                 154 
                 3.02 
               
               
                   
               
               
                 D-Tartaric Acid 
                   
                 168-170 
                   
                   
                 2.98 
               
               
                 DL-Tartaric Acid 
                   
                 206 
                 Al 171 
                 185 
                   
               
               
                 Meso-Tartaric Acid 
                   
                 140 
                   
                   
                 3.22 
               
               
                   
               
               
                 Succinic Acid 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 188-190 
                 mp 184 followed by small exo; no TGA step function 
                 210 
                 4.16 
               
               
                   
               
               
                 Diglycolic Acid 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 142-145 
                 mp 130 followed by small exo; TGA slow dec. 
                 155 
                 3.28 
               
               
                   
               
               
                 Malonic Acid 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 135-137 
                 mp 124 followed by small exo; TGA slow dec. 
                 &gt;250 
                 2.83 
               
               
                   
               
               
                 Trans-Glutaconic Acid 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 137-139 
                 mp 136; Al 166 
                 188 
                   
               
               
                   
               
               
                 D-Glutamic Acid 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 200-202 
                 mp 206; Al 213 
                 235 
                 2.13 
               
               
                   
               
               
                 Adipic Acid 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 152-154 
                 mp 153; Al 222 
                 237 
                 4.43 
               
               
                   
               
               
                 Mucic Acid 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 215 
                 Al 200 
                 223 
                 3.08 
               
               
                   
               
               
                 Citric Acid 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 152-154 
                 mp 141 followed by small exo; no TGA step function 
                 &gt;250 
                 3.12 
               
               
                   
               
            
           
         
       
     
     It will be appreciated that in further accordance with the present invention, gas generators made as known in the art and also vehicle occupant protection systems manufactured as known in the art are also contemplated. As such, autoignition compositions of the present invention are employed in gas generators, seat belt assemblies, and/or vehicle occupant protection systems, all manufactured as known in the art. 
     In yet another aspect of the invention, the present compositions may be employed within a gas generating system. For example, as schematically shown in  FIG. 2 , a vehicle occupant protection system made in a known way contains crash sensors in electrical communication with an airbag inflator in the steering wheel, and also with a seatbelt assembly. The gas generating compositions of the present invention may be employed in both subassemblies within the broader vehicle occupant protection system or gas generating system. More specifically, each gas generator employed in the automotive gas generating system may contain a gas generating composition as described herein. 
     Extrusion aides may be selected from the group including talc, graphite, borazine [(BN) 3 ], boron nitride, fumed silica, and fumed alumina. The extrusion aid preferably constitutes 0-10% and more preferably constitutes 0-5% of the total composition. 
     The compositions may be dry or wet mixed using methods known in the art. The various constituents are generally provided in particulate form and mixed to form a uniform mixture with the other gas generant constituents. 
     It should be noted that all percents given herein are weight percents based on the total weight of the gas generant composition. The chemicals described herein may be supplied by companies such as Aldrich Chemical Company for example. 
     As shown in  FIG. 1 , an exemplary inflator incorporates a dual chamber design to tailor the force of deployment of an associated airbag. In general, an inflator containing a primary gas generant  12 , also functioning as an autoignition/booster composition and formulated as described herein, may be manufactured by dry and/or wet methods known as useful in the art. U.S. Pat. Nos. 6,422,601, 6,805,377, 6,659,500, 6,749,219, and 6,752,421 exemplify typical airbag inflator designs and are each incorporated herein by reference in their entirety. 
     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 multifunctional 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, each 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. 
       FIG. 3  shows a cross-section of a conventional gas generating system  10  incorporating separate booster and gas generating chambers therein. The embodiments of the gas generating system shown in  FIGS. 3 ,  4 , and  5  are in the form of inflators usable for inflating associated elements of a vehicle occupant protection system, for example. However, such gas generating systems may also be used in other applications. The structure and operation of the basic system components described herein is known in the art. In addition, the materials and techniques used in manufacturing the structural components of the gas generating system are known in the art. 
     System  10  includes an outer housing  12  and an inner housing  14  positioned within the outer housing and containing a quantity of gas generant material  16  therein. Inner housing  14  defines a combustion chamber for the gas generant. Inner housing orifices  18  provide fluid communication between the interior and exterior of inner housing  14 . A fluid flow path is provided within housing  12  and between orifices  18  and gas exit openings  20  formed in an end or other portion of housing  12 . A booster chamber  22  is formed by a booster cup  23  and a divider  28 . Chamber  22  houses a booster composition  24  therein. Divider  28  separates booster composition  24  from gas generant  16  and enables fluid communication (via opening  28   a ) between the booster chamber and the combustion chamber upon activation of the gas generating system and combustion of the booster composition. As used herein, the term “booster chamber” is understood to designate any structure and/or components which perform the function of separating the booster composition from the gas generant composition. An initiator  32  is provided for initiating combustion of booster composition  24  upon receipt of an activation signal, in a manner known in the art. An autoignition material  30  is positioned so as to provide or enable fluid communication with the booster composition  24  upon exposure of the system to an elevated external temperature (such as that produced by a fire, for example) sufficient to cause ignition of the autoignition material. 
       FIG. 4  shows a cross-sectional side view showing the general structure of a gas generating system  100  in accordance with the present invention. Components common to the systems shown in  FIGS. 3 ,  4 , and  5  have been given similar element numbers for simplicity and clarity. It may be seen from a comparison of  FIGS. 3 and 4  that the separate booster composition  24  and booster chamber  22  shown in  FIG. 3  have been eliminated in the system shown in  FIG. 4 . This is accomplished through the use of a gas generant composition  116  in accordance with one of the embodiments described herein. Such compositions perform the functions of both gas generant and booster compositions, or the functions of gas generant, booster, and autoignition compositions, thereby eliminating the need for separate compositions and the structure (such as the booster cup and divider) needed to separate and support the separate compositions. This reduces system weight and enables the length of the system envelope to be shortened. Where a gas generant is formulated which serves the functions of only gas generant and booster compositions, a separate autoignition composition may still be provided in an appropriate location within the gas generating system to ensure safe actuation of the system in case of fire, as previously described. 
     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 scope of the present invention as defined in the appended claims.