Patent Publication Number: US-6709537-B2

Title: Low firing energy initiator pyrotechnic mixture

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of U.S. application Ser. No. 09/972,463 filed Oct. 5, 2001, which application is hereby incorporated by reference herein in its entirety and is made a part hereof, including but not limited to those portions which specifically appear hereinafter. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to pyrotechnic mixtures and, more particularly, to pyrotechnic mixtures for use in initiator devices having a low firing energy. 
     Initiator devices or assemblies, such as composed of an initiator transducer and a supply of energetic and rapid burning primer material, have been developed for use in various assemblies. For example, inflatable restraint systems such as used in the protection of vehicle occupants in the event of a collision commonly include or incorporate one or more gas generating or supplying devices each commonly referred to as an “inflator.” Inflators, used for inflating one or more inflatable airbag cushion or a similar inflatable element, typically include an initiator device or assembly for igniting a supply of gas generant reactant housed or contained within the inflator housing. 
     Common primer materials used in such initiator devices or assemblies include lead styphnate and zirconium potassium perchlorate (commonly referred to as “ZPP”), as well as other pyrotechnic mixtures or formulations. Unfortunately, various of such common or conventional primer materials, while energetic and rapidly burning, are typically difficult to handle in a desirably safe manner. Further, at least some such common or conventional primer materials may require exposure to a higher than desired firing energy in order to effect initiation or reaction thereof. 
     In addition, where such primer materials are used in or as a part of a gas generating or supplying device, primer materials which, upon reaction, additionally generate, produce or otherwise provide additional gaseous products are generally highly desirable as such additional gaseous products can supplement the gas otherwise provided or supplied thereby. For example, in various inflatable restraint system inflator applications, it may be desirable to employ an initiator device primer pyrotechnic formulation that itself forms gaseous products upon the reaction thereof. 
     In view of the above, there is a need and a demand for improved pyrotechnic compositions. In particular, there is a need and a demand for a pyrotechnic composition such as may be used as a primer material in initiator devices and such as may provide or result in increased or improved safety in handling, as compared to conventional primer materials. Further, there is a need and a demand for a pyrotechnic composition such as may be used as a primer material in initiator devices and such as may require exposure to a suitably lower or reduced firing energy, as compared to conventional primer materials, in order to effect initiation or reaction thereof. Still further, there is a need and a demand for a pyrotechnic composition such as may be used as a primer material in initiator devices and such as may produce, provide or otherwise desirably result in the formation of additional gaseous reaction products. 
     SUMMARY OF THE INVENTION 
     A general object of the invention is to provide an improved pyrotechnic composition. 
     Another general object of the invention is to provide an improved initiator device having a low firing energy. 
     A more specific objective of the invention is to overcome one or more of the problems described above. 
     The general object of the invention can be attained, at least in part, through a gas generant pyrotechnic composition which includes a mixture containing: 
     a fuel component comprising a quantity of at least one Iron Blue Pigment and 
     an oxidizer component comprising at least one oxidizer effective, upon reaction initiation with at least a portion of the quantity of the at least one Iron Blue Pigment, to produce gaseous reaction products. 
     The prior art generally fails to provide an initiator pyrotechnic mixture having either or both as low as may be desired firing energy and which facilitates or otherwise more easily permits handling in a desirably safe manner. 
     The invention further comprehends, in accordance with another preferred embodiment of the invention, an initiator device which includes a charge holder housing containing a gas generant pyrotechnic composition, such as described above, and an initiator transducer in reaction initiating communication with at least a portion of the gas generant pyrotechnic composition contents of the housing. 
     In accordance with another particular embodiment, the invention further comprehends a gas generant pyrotechnic composition which includes a mixture containing: 
     a fuel component comprising a quantity of at least one Iron Blue Pigment, 
     an oxidizer component comprising at least one oxidizer effective, upon reaction initiation with at least a portion of the quantity of the at least one Iron Blue Pigment, to produce gaseous reaction products, and 
     a burn rate enhancing additive comprising a quantity of zirconium metal powder, the burn rate enhancing additive effective wherein, upon ignition of the composition, the composition burns at an increased rate as compared to a similar composition without the inclusion of the burn rate enhancing additive, 
     wherein the composition contains about 15 to about 35 composition weight percent of the at least one Iron Blue Pigment, about 45 to about 70 composition weight percent of the at least one oxidizer and about 12 to about 25 composition weight percent of the burn rate enhancing additive. 
     The invention also comprehends corresponding initiator devices such as which include a charge holder housing containing a gas generant pyrotechnic composition, such as described above, and an initiator transducer in reaction initiating communication with at least a portion of the gas generant pyrotechnic composition contents of the housing. 
     In yet another preferred embodiment, the invention comprehends an initiator device which includes a charge holder housing containing a gas generant pyrotechnic composition and an initiator transducer in reaction initiating communication with at least a portion of the gas generant pyrotechnic composition contents of the housing. In such embodiment, the gas generant pyrotechnic composition contains about 15 to about 35 composition weight percent Iron Blue Pigment, about 45 to about 70 composition weight percent potassium perchlorate and about 12 to about 25 composition weight percent zirconium metal powder. 
     As used herein, references to “Iron Blue Pigment” are to be understood to generally refer to that class, family or variety of pigment materials based on microcrystalline Fe(II)Fe(III) cyano complexes. According to results obtained by X-ray and infra-red spectroscopy, the basic general chemical formula for the Iron Blue Pigments is believed to be: 
     
       
         Me(I)Fe(II)Fe(III)(CN) 6 ·H 2 O 
       
     
     In this formula, Me(I) stands for potassium, sodium or ammonium, with the alkali ion being believed to play a role in the color properties of Iron Blue. Iron Blue Pigments, also sometimes referred to as “iron ferricyanides”, have been produced or sold under a variety of different names related to either the place where the compound was made or to represent particular optical properties. Examples of such different names include: “Berlin Blue”, “Bronze Blue”, “Chinese Blue”, “Milori Blue”, “Non-bronze Blue”, “Paris Blue”, “Prussian Blue”, “Toning Blue” and “Turnbulls Blue”, for example. 
     Other objects and advantages will be apparent to those skilled in the art from the following detailed description taken in conjunction with the appended claims and drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The FIGURE is a simplified sectional schematic of an initiator device in accordance with one embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides improved pyrotechnic compositions such as used in initiator devices and improved initiator devices having a low firing energy. The compositions of the invention are high energy compositions which are particularly suited as ignition or primer materials. Further, the compositions of the invention, as described in greater detail below, are relatively safe to handle in either a slurry or dry form, as compared to currently used initiator primer materials such as lead styphnate and ZPP, for example. Still further, the compositions of the invention advantageously produce, provide or otherwise desirably result in the formation of additional gaseous reaction products. 
     Such gas generant pyrotechnic compositions typically include a mixture containing a fuel component comprising a quantity of at least one Iron Blue Pigment and an oxidizer component comprising at least one oxidizer effective, upon reaction initiation with at least a portion of the quantity of the at least one Iron Blue Pigment, to produce gaseous reaction products. 
     Iron Blue Pigment is a non-toxic material widely used in printing inks of various types. Iron Blue Pigment is generally characterized by very fine inherent particle size (typically in the order of about 0.025 microns). Further, Iron Blue Pigment is thermally stable over the temperature range of interest (e.g., temperatures between about −40° C. and 107° C.), non-hygroscopic (insoluble) and non-corrosive in the environment of an airbag inflator. 
     Those skilled in the art and guided by the teachings herein provided will appreciate that, as identified above, a wide variety of specific or particular Iron Blue Pigment iron ferrocyanide materials are available. MANOX-Blue 4050 Iron Blue Pigment iron ferrocyanide produced or sold by Degussa Corp. is a currently preferred Iron Blue Pigment material for use in the practice of the invention as this material has been found to generally exhibit a relatively high sensitivity to bridgewire activation in initiator device applications. 
     According to Degussa product literature, MANOX-Blue Iron Blue Pigment manufacture results by precipitation from complex alkali iron (II) cyanides and iron (II) salts in aqueous solutions. In this two-step method, a whitish precipitate is stated as being formed of alkali iron(II) hexacyanoferrate(II) of formula Me(I) 2 Fe(II)[Fe(II)(CN) 6 ]. The Degussa product literature further states that by means of special precipitation parameters such as concentration of the solutions, temperature, and pH level during the precipitation, important pigment properties are determined during this step of the process. The production process continues with an aging period having a length and temperature which vary depending on the type of pigment and then by oxidation to the finished blue pigment material. 
     A variety of oxidizer materials, such as known in the art, can be used or included in the pyrotechnic compositions of the invention. Examples of such oxidizer materials useable in the practice of the invention include potassium chlorate, strontium nitrate and ammonium perchlorate. Potassium perchlorate is a currently preferred oxidizer material for use in the practice of the invention as such material has been found to generally provide or result in desired reactivity and reaction sensitivity. 
     In general, preferred pyrotechnic compositions in accordance with the invention will contain or include about 15 to about 55 wt. % Iron Blue Pigment and about 45 to about 85 wt. % potassium perchlorate, wherein the weight percentages of Iron Blue Pigment and potassium perchlorate are calculated relative to the total of Iron Blue Pigment and potassium perchlorate equaling 100 wt. %. 
     In accordance with certain preferred embodiments of the invention, it has been found generally desirable that pyrotechnic compositions in accordance with the invention also contain or include a quantity of a burn rate enhancing additive effective wherein, upon ignition of the composition, the composition burns at an increased rate as compared to a similar composition without the inclusion of the burn rate enhancing additive. While a variety of burn rate enhancing additives are generally available, burn rate enhancing additives such as in the form of a metal powder, such as of zirconium, titanium or a mixture thereof, are believed to be particularly effective and useful, with zirconium metal powder such as of a particle size in a range of at least about 0.5 microns to no more than about 4 microns in diameter being particularly preferred. In particular, it has been found that through the composition inclusion of such burn rate enhancing additive, the function time for the composition in typical inflatable restraint gas generation applications can be desirably reduced, as compared to a similar composition without such additive inclusion, while maintaining the desirable bridgewire sensitivity of the Iron Blue material. 
     In view thereof, one particular preferred gas generant pyrotechnic composition in accordance with the invention contains about 15 to about 35 composition weight percent Iron Blue Pigment, about 45 to about 70 composition weight percent potassium perchlorate and about 12 to about 25 composition weight percent zirconium metal powder. 
     In addition, pyrotechnic compositions in accordance with the invention may additionally contain or include a small amount, typically between about 0.25 wt. % and no more than about 6 wt. %, of one or more additives such as selected from the group consisting of binders, rheology enhancers and wetting agents. Examples of suitable such additive materials include materials such as hydroxypropyl cellulose, ethyl cellulose, acetyl triethyl citrate, polyacrylate emulsions (such as the polyacrylate emulsion sold under the trade designation MC-1834, by Rohm and Hass Company, for example), and fluoroelastomers based on the copolymer of vinylidene fluoride and hexafluoropropylene and sold under the name VITON by Du Pont de Nemours, E. I. &amp; Company, for example, which can serve primarily as binders. As will be appreciated by those skilled in the art and guided by the teachings herein provided, the presence or inclusion of a suitable such binder material can beneficially serve to hold the composition in the desired form both prior and after drying. 
     For example, in accordance with one particular preferred embodiment of the invention, it has been found useful to include a binder mixture of hydroxypropyl cellulose, ethyl cellulose and acetyl triethyl citrate in n-propyl alcohol in the subject pyrotechnic composition. In particular, such a binder mixture has been found to provide or otherwise result in a composition having a proper or desired viscosity to permit and facilitate fluid dispensing of the binder mixture onto an initiator body without excessive shrinkage or cavitation upon drying, while also being generally stable over the range of required environmental conditions. 
     In accordance with another preferred embodiment of the invention, a binder mixture of polyacrylate emulsion, e.g., MC-1834 emulsion of Rohm and Hass Company, and alcohol, e.g., n-propyl alcohol, can desirably be used. 
     Further, as iron ferricyanide typically has a very small and fine crystal structure, it has been found generally advantageous to include a quantity of a wetting agent additive to the mixture to avoid or reduce the undesirable formation of clumps by the iron ferricyanide. Thus, a gas generant pyrotechnic composition in accordance with a preferred embodiment of the invention will desirably include or contain a wetting agent such as the maleic anhydride-styrene copolymer wetting agent TEGO, produced by Goldschmidt Chemical Corporation, Hopewell, Va., to prevent or avoid the undesirable formation of clumps. In practice, the compositional inclusion of between about 1 and 2 wt. % of the maleic anhydride-styrene copolymer wetting agent TEGO 750 in an alcohol-water mixture has been found generally effective in preventing or avoiding undesirable clump formation. 
     The pyrotechnic mixture of the invention desirably yields or otherwise results in a lower all-fire result with conventional bridgewire firing signal devices, as compared to standard initiator pyrotechnic compositions. Those skilled in the art and guided by the teachings herein provided will appreciate that the achieving or realizing of a lower all-fire result is an important factor or criterion in meeting industry demands for new initiator developments. 
     As will be appreciated, the compositions in accordance with the invention can be incorporated, utilized or practiced in conjunction with a variety of different structures, assemblies and systems. As representative, the FIGURE illustrates an initiator device or assembly, generally designated by the reference numeral  10 . The initiator device  10  includes a charge holder housing  12 , such as formed of stainless steel or other suitable material. The charge holder housing  12  forms or otherwise provides a charge-containing volume, designated by the reference numeral  14 , within which is contained a supply or charge of the subject pyrotechnic composition described above and here designated by the reference numeral  16 . 
     As shown, the charge holder housing  12  can desirably be sufficiently sized such as to permit the housing or containment therewithin of variously sized igniter pyrotechnic charges such as to satisfy the requirements for various particular applications. As such, a void volume  20  may remain in the housing  12  after the loading of the charge  16  therewithin. 
     Alternatively, the charge holder housing  12  may be specifically sized and tailored to meet the requirements of a specific application. In such situations, the charge holder can, if desired, be sized such as to avoid the inclusion of significant void volume. 
     The initiator device also includes a body  22  such as adapted or suited for the secure positioning and placement of the initiator device  10  in a larger assembly such as an inflatable restraint system inflator, as described above, for example. The initiator device body  22 , as described in greater detail below, can desirably be formed at least in part by an insulating material. For example and as will be appreciated by those skilled in the art and guided by the teachings herein provided, various insulating materials including various plastic or polymer materials are well-suited for electrical insulation and injection molding. In accordance with one embodiment of the invention the initiator device body  22  can desirably be formed of molded nylon. 
     The initiator device  10  further includes a metal eyelet  24  such as formed of stainless steel. The eyelet has a bore  26  formed therein. A first conductive pin  30  is positioned through the bore  26  and held in place by a glass or other suitable insulator  32 . The first conductive pin  30  has an outer connection end  34  and an opposed inner terminal end  36 . A cap or cup member  40 , such as formed of stainless steel, surrounds, covers or otherwise encloses the charge-containing volume  14  and the charge  16  contained therewithin. The cap member  40  is suitably joined or connected to the eyelet  24 , such by welding. The initiator device  10  also includes a second conductive pin  42 . The second conductive pin  42  has an outer connection end  44  and an opposed inner terminal end  46 . 
     An initiator transducer  50  couples the first and second conductive pins  30  and  42  either directly or, as shown, via the eyelet  24 . The initiator transducer  50  serves to ignite the charge  16  when properly actuated. As will be appreciated by those skilled in the art and guided by the teaching herein provided, initiator devices in accordance with the invention may incorporate or include various initiator transducer elements or components such as including but not necessarily limited to bridgewire, semiconductor bridge (SCB), thin film bridge, fiber optic coupled laser and direct laser diode firing signal devices. 
     The initiator device body  22  provides electrical insulation between the first and second conductive pins  30  and  42 , respectively. The initiator device  10  may also include an insulating cover  54 , such as formed of an insulating plastic or polymer material, provided about the exterior of the cup member  40 . 
     The pyrotechnic composition mixture of the invention can be made as a slurry and loaded onto a bridgewire or other firing signal device and then dried and optionally pressed, such as described in Hamilton et al., U.S. Pat. No. 5,686,691, issued Nov. 11, 1997 and the disclosure of which is hereby incorporated herein in its entirety. Alternatively, the pyrotechnic composition mixture of the invention can be made or formed as a dry granule and loaded into an appropriate initiator device in a conventional manner as is known in the art. 
     The present invention is described in further detail in connection with the following examples which illustrate or simulate various aspects involved in the practice of the invention. It is to be understood that all changes that come within the spirit of the invention are desired to be protected and thus the invention is not to be construed as limited by these examples. 
     EXAMPLES 
     Example 1 
     In this example, a pyrotechnic mixture in accordance with the invention was prepared. As a first step, each of the ingredients identified in TABLE 1, below, were weighed to the respective specified mass. 
     
       
         
           
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Ingredient 
                 Mass (grams) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                 dispersant (75 wt. % n-propyl alcohol, 25 wt. % H 2 O) 
                 300.0 
               
               
                 wetting agent (TEGO 750) 
                 1.5 
               
               
                 Iron Blue Pigment (MANOX 4050, CAS-25869-00-5) 
                 70.0 
               
               
                 potassium perchlorate (CAS 7778-74-7) 
                 120.0 
               
               
                 hydroxypropyl cellulose (CAS-90004-64-2) 
                 0.5 
               
               
                   
               
            
           
         
       
     
     The TEGO 750 wetting agent was added to the dispersant liquid to form a dispersant mixture. The dispersant mixture was then placed in a table-top blender having rheostat speed control. The blender was then set on low speed (30 on the rheostat control, blender control on high). The MANOX 4050 Iron Blue Pigment was then slowly added to the blender while the blending speed was appropriately increased so as to maintain a constant vortex. 
     After all of the Iron Blue Pigment had been added to the blender, the blender was set at maximum blending speed and the contents were so blended for 10 minutes. 
     At this point, the potassium perchlorate was added to the blender contents and the contents were blended for 1 minute. 
     At this point, the hydroxypropyl cellulose was added to the blender contents and the contents were blended for 5 minutes. 
     The blend mixture was then placed in a vacuum oven and treated by removing solvent to adjust the volatile content to 32 wt. %, as confirmed via an assay of approximately 1 gram. 
     Example 2 
     In this example, fifty (50) test initiator devices each containing an 80-gram sample of the pyrotechnic mixture of Example 1 and having a bridgewire A-7 firing signal device initiator transducer were prepared and tested in a conventional Bruceton method to determine sensitivity. 
     Results 
     These tests showed an all-fire level of 0.7338 amps at 0.999999 reliability at 95% confidence level. An all-fire level of less than 0.8 amps was the target. 
     While lead styphnate has been commonly used in an effort to meet such initiator device primer material requirement, there is mounting interest and desire to minimize, avoid and/or eliminate the presence or inclusion of lead, lead-based and lead-containing materials in such or similar applications. These results highlight the utility of the subject compositions in such applications. 
     Thus, the invention provides improved pyrotechnic compositions such as used in initiator devices and improved initiator devices having a low firing energy. In particular, the compositions of the invention are high energy compositions which are well suited as ignition or primer materials. Further, the compositions of the invention are relatively safe to handle in either a slurry or dry form, as compared to currently used initiator primer materials such as lead styphnate and ZPP, for example. Still further, the compositions of the invention advantageously produce, provide or otherwise desirably result in the formation of additional gaseous reaction products. 
     The invention illustratively disclosed herein suitably may be practiced in the absence of any element, part, step, component, or ingredient which is not specifically disclosed herein. 
     While in the foregoing detailed description this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for purposes of illustration, it will be apparent to those skilled in the art that the invention is susceptible to additional embodiments and that certain of the details described herein can be varied considerably without departing from the basic principles of the invention.