Patent Publication Number: US-2013247790-A1

Title: Fuel tablet and propellan for a gas generator

Description:
RELATED APPLICATIONS 
     This application corresponds to PCT/EP2011/004625, filed Sep. 15, 2011, which claims the benefit of German Application No. 10 2010 049 765.7, filed Oct. 29, 2010, the subject matter of which are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to a propellant pellet for pyrotechnical safety devices in vehicles such as airbag modules, belt tensioners, micro-inflators, actuators and igniters as well as to a propellant charge, an igniter and an inflator each of which includes a plurality of said propellant pellets. 
     From WO 01/19760 A2 it is known to control the burn-off time of inflator propellants via the grain shape and size of the inflator propellant charges. For booster charges and inflator propellant charges requiring a short burn-off time usually propellant granules are used. 
     The production of propellant granules is known, for example, from DE 100 20 291 A1. Generally the relatively high production costs due to the complex processing steps constitute a drawback of the use of propellant granules. The production of granules can be performed according to different methods such as e.g. breaking of compacts, extruding or pressing of moist masses, build-up agglomeration or spray granulation. Basically a distinction can be made between dry or wet processes. 
     In the dry processes already manufactured compacts (e.g. pellets) are broken again and the fragments obtained are screened according to the desired granule size. Granules manufactured in this way usually have an inhomogeneous and irregular grain shape. In particular those granules are little abrasion-resistant, because the edges, corners and points are easily broken upon mechanical wear. 
     Granules manufactured by wet processes basically have the drawback of the use of solvents and binders. The former have to be removed again in a complicated manner in additional process steps, the latter are undesired formula ingredients which affect the burn-off (e.g. the gas composition) of the propellant. 
     Propellant granules further have a distribution of grains of different grain size and grain surface. Therefore, already during the transport of propellant granules a segregation of the larger grains from smaller granule grains can take place. Also when filling inflators with propellant granules such segregation and thus a scattering of the inflator output can occur, because the larger granule grains flow more rapidly than small grains. A segregation of the propellant granules finally can occur also in the mounted state of the inflator over the operating time of the vehicle so that the inflator output can vary over time. 
     Moreover adjusting the ballistic properties of propellant granules manufactured by breaking larger compacts is possible to a limited extent only, because the breaking operations resulting in granules are not exactly defined. Also by screening the grain surface and thus the ballistic behavior can only be adjusted in a very complex manner, as the granule grains usually have an irregular shape. Thus it is imaginable that ballistic results of individual inflators are broadly scattered also within a manufacturing batch. Moreover granules manufactured in this way are very sensitive to mechanical load due to the very irregular geometry and therefore exhibit low abrasion resistance. Thus the ballistic properties vary strongly upon loads as they occur during transport, for instance. 
     Although the granule grains manufactured by so called build-up agglomeration in a fluidized bed generator, for example, are rounder and thus better to screen, these propellant granules only have a low density and thus a lower volume-related gas yield than compacted granules, however. 
     Finally the grain sizes cannot be specifically adjusted during granule manufacture. Rather, the desired fractions have to be screened. Therefore during granule manufacture a higher proportion of waste always accumulates. Filling the inflators with propellant granules is difficult, too, and requires long cycle times because the flowability of granules is rather poor. 
     The manufacture of propellant compacts having a cylindrical base shape can take place by extruding suitable masses in wet process and cutting the extruded propellant strand. In this way propellant compacts having a diameter of 1 mm to approx. 5 mm can be obtained. The addition of a binder required to manufacture the extruded propellant compacts can be detrimental to the ballistic properties of the propellant composition. 
     When cutting the still soft extruded product, fraying of the cutting faces is always resulting that has strong and non-controllable effects on the ballistic properties of the thus manufactured propellant pellets mainly in the case of small diameters. When the extruded product is first dried and then cut, the propellant compacts are pre-damaged due to the strong shear forces occurring during cutting. Micro-fissures or bursts of the propellant occur. In some cases it can be necessary due to the propellant specifications to cut the extruded product to a height ranging from 0.4 mm to 1 mm. Those dimensions can hardly be manufactured in a reproducible manner by an extruded propellant strand. In these cases the ballistic properties of the propellant therefore either can no longer be controlled or else can be obtained only with great scattering in the pressure/time curves. 
     SUMMARY OF THE INVENTION 
     The present invention is based on the object to provide a propellant pellet for pyrotechnical applications in safety systems for vehicles which can be manufactured in an inexpensive and safe way and permits a reproducible control of the burn-off behavior. Furthermore the object underlying the invention is to provide a propellant charge, an igniter, an actuator and an inflator or actuator including a number of such propellant pellets. 
     To achieve these objects, concerning the propellant pellet in accordance with the invention a propellant pellet according to claim  1  is provided. Furthermore, concerning the propellant charge a propellant charge in accordance with the invention according to claim  8  is provided, concerning the igniter an igniter in accordance with the invention according to claim  9  is provided, concerning the inflator or the actuator an inflator or actuator in accordance with the invention according to claim  10  is provided. 
     The subject matter of the invention further is the use of the propellant pellets according to the invention in safety devices of vehicles such as an airbag module, a belt tensioner, a micro-inflator, an advanced ignition means and a pyrotechnical igniter. 
     Advantageous embodiments of the invention are stated in the subclaims. 
     The manufacture of propellant pellets by injecting propellant powders is known, for instance, from the afore-mentioned WO 01/19760 A2 and DE 100 58 934 A1. However, in applications for vehicle restraint means usually only propellant pellets having a relatively large diameter are used. 
     In accordance with the invention, however, a substantially cylindrical propellant pellet for use in a vehicle safety device is provided which can be obtained by dry injection of a powdered propellant composition and has a maximum diameter of 2 mm, the propellant pellet especially having a relative bulk density of at least 0.5, preferably 0.5 to 0.7. 
     The relative bulk density is defined to be the ratio of bulk density determined as mass of the propellant pellets related to the bulk volume to the maximum theoretical density of the propellant composition. The bulk density relates to values established in a 100 ml measuring glass having a diameter of 30 mm. Measuring takes place by filling the pellets to the filling line and subsequent weighing without shaking, blowing or other compacting of the bulk. The maximum theoretical density (tmd) is established from the crystal densities of the respective ingredients of the composition. Due to the reference to the maximum theoretical density, the amount of the relative bulk density is independent of the respective propellant composition and is substantially determined by the geometry of the propellant compact. 
     Especially preferred, the relative bulk density is within the range of from 0.54 to 0.6, preferably within the range of from 0.56 to 0.59. 
     The propellant pellet according to the invention has a maximum diameter of 2 mm. Preferably the diameter is within the range of from 0.8 mm to 2 mm, further preferably within the range of from 1 mm to 2 mm and especially preferred within the range of from 1.5 mm to 2 mm. 
     The ratio of height to diameter (hid) of the propellant pellets according to the invention preferably is within the range of from 0.2 to 0.8. 
     The dimensions (diameter×height) of the propellant pellets according to the invention preferably is within a range of from 1 mm×0.4 mm to 2 mm×1.5 mm, the ratio of height to diameter being not more than 0.8. 
     In these ranges the propellant pellets can be manufactured inexpensively having perfectly reproducible ballistic properties. Especially preferred are propellant pellets having dimensions in the range of from 1.5 mm×0.4-0.9 mm and 2 mm×0.4-1.5 mm. 
     The propellants used can be all propellants known in prior art for pyrotechnical restraint means as described, for example, in WO 01/19760 A2. 
     The invention also comprises a propellant charge and an igniter each of which comprises a plurality of afore-described propellant pellets according to the invention. 
     The propellant pellets according to the invention can be advantageously used as substitute for conventional propellant granules, especially for booster charges in pyrotechnical inflators and as propellants having a short burn-off time in micro-inflators, for example. Furthermore, the propellant pellets according to the invention can be used in igniters in a safety device, as for instance in a pyrotechnical inflator for inflating an airbag to protect a vehicle occupant in the case of accident. Equally the propellant pellets according to the invention can be incorporated in an actuator which props up the hood of a vehicle in the case of accident so as to provide an optimized crush zone for an impact of a pedestrian. 
     After complex tests applicant finally has succeeded in reducing the manufacturing costs of the propellant pellets according to the invention considerably below the costs of the known granule manufacture inter alia as the complicated screening and classifying processes and the additional drying step can be dispensed with for wet-manufactured granules. Moreover, also the working safety is increased because for the pellet manufacture no breaking operations have to be carried out and no highly inflammable fine dust accumulates. 
     The propellant pellets according to the invention have a reproducible geometry and thus an easily reproducible burn-off behavior. Therefore the pellets according to the invention can be easily and inexpensively adapted to the desired output requirements and customer specifications for inflators, for instance by variation of height and/or diameter. Even in comparison to the granules manufactured of compacts the bulk density of the propellant pellets according to the invention is higher. With equal propellant weight thus lower volumes of combustion chambers can be employed. For instance, for pellets having a diameter of 1.0 mm and a height of 0.5 mm made of a mixture of 60.8% by weight of guanidine nitrate, 36.7% by weight of potassium perchlorate and 2.5% by weight of additives a bulk weight of 1.01 g/cm 3  was measured, whereas from pellets injected before of the same composition which were subsequently broken and screened into granules having a grain distribution between 0.5 mm to 1.2 mm merely a bulk weight of 0.87 g/cm 3  was obtained. This corresponds to an increase in the bulk weight of approx. 16%. 
     The ballistic tests with the propellant pellets according to the invention result in a relatively narrow scattering, because the grain size distribution in the propellant charge with injected pellets of defined grain size can be adjusted within narrow limits and can be better reproduced compared to the irregular granule grains. Thus the ballistic properties of the propellant charges can be adjusted to be variable and reproducible also by a variation of the grain size distribution by the use of pellets of different size. 
     Tests have resulted in the fact that when using the propellant pellets according to the invention the inflator output remains stable over the operating time of the inflator (&gt;15 years), because the pellets are not segregated or irregularly reduced by vibration and thus enlarge the surface. Therefore the burn-off characteristic does not vary. Even during transport of the propellant pellets according to the invention to other locations of the inflator manufacture no segregation and thus no detrimental variation of the bulk composition does occur. 
     Finally the propellant pellets according to the invention can be dosed more easily and within a shorter cycle time into the inflator due to the better flowing behavior. The filling of the inflator with the propellant pellets according to the invention is carried out in a by far more homogenous manner than with granules, because even during dosing no segregation occurs so that ballistic results that can be better reproduced are obtained. Moreover, fine dust is prevented from forming, because the pellets used are by far more abrasion-resistant that the granules used so far. 
     Compared to extruded propellant compacts, the propellant pellets according to the invention also have a higher density while exhibiting the same composition. Therefore also the bulk density of the propellant pellets according to the invention is higher than the bulk density of the propellant compacts manufactured by extrusion while having equal dimensions and thus equal packing density, i.e. parts per volume unit. 
     For example, for a wet-extruded compact having geometry of 3.9×4 mm of 45% by weight of guanidine nitrate, 50% by weight of basic copper nitrate and 5% by weight of guar gum a density of the propellant compact of 1.7 g/cm 3  is found by experiment. The density of the extruded propellant compact thus only reaches 84% of the maximum theoretical density (tmd) of 2.03 g/cm 3 . The density of dry-injected propellant pellets having the same propellant composition, on the other hand, usually reaches up to 99% of the maximum theoretical density. Hence for the afore-mentioned composition a bulk density that is by 18.3% higher compared to the extruded compacts can be achieved with dry-injected propellant pellets. This allows for a reduction of the combustion chamber in the same magnitude. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features and advantages of the invention are obvious from the following description of preferred embodiments with reference to the enclosed drawings which are not meant to be limiting. In the drawings 
         FIG. 1  shows the schematic representation of an inflator; 
         FIG. 2  shows a diagram illustrating the relative bulk density of the pellets according to the invention against the pellet diameter; and 
         FIG. 3  shows the representation of pressure/time curves of a defined propellant for different pellet sizes. 
     
    
    
     DESCRIPTION 
     The components of the inflator  10  illustrated in  FIG. 1  are known per se. The inflator  10  has an external housing  12  in which an igniter  14  is inserted. In the case of accident the igniter is activated and ignites a propellant charge  16  accommodated in a combustion chamber  18  associated with the igniter  14 . 
     According to the invention, the propellant charge  16  disposed in the combustion chamber is provided in the form of a filling with the propellant pellets  20  according to the invention which are represented only schematically but not true-to-scale in  FIG. 1 . 
     The filling may contain the propellant pellets according to the invention at least partly, preferably at a proportion of at least 10% by weight, further preferably of at least 30% by weight, at least 50% by weight or at least 70% by weight, and especially preferred it may consist completely of the propellant pellets according to the invention. 
     Particularly preferred the propellant pellets of the invention are provided in the filling in a single-mode distribution. 
     The propellant pellets according to the invention have a substantially cylindrical shape and can be obtained by dry injection of a powdered propellant composition. The pellets have a maximum diameter of 2 mm. 
     The density of the dry-injected propellant pellets preferably is at least 95% of the maximum theoretical density (tmd), further preferred at least 97% and especially preferred between 97% and 99% of the maximum theoretical density. 
     The relative bulk density of the propellant pellets according to the invention is at least 0.5, preferably 0.5 to 0.7, and further preferred from 0.54 to 0.6. Especially preferred the relative bulk density is within the range of from 0.56 to 0.59. 
     In accordance with a further preferred embodiment, the propellant pellets of the invention have a diameter (d) of from 0.8 mm to 2 mm. Preferably the diameter ranges from 1 mm to 2 mm and in an especially preferred manner ranges from 1.5 mm to 2 mm. 
     The propellant pellets according to the invention preferably have a ratio of height (h) to diameter (d) of h/d=0.2 to 0.8. 
     It is understood to those skilled in the art that the afore-mentioned parameters can be freely combined with each other. 
     According to an especially preferred embodiment, the dimensions of the propellant pellets of the invention range from d×h=1 mm×0.4 mm to 2 mm×1.5 mm, the height-to-diameter (hid) ratio amounting to a maximum of 0.8. In this range the propellant pellets having perfectly reproducible ballistic properties can be manufactured at low cost. Especially preferred are propellant pellets having dimensions within the range of d×h=1.5 mm×0.4-0.9 mm and 2 mm×0.4-1.5 mm. 
     In the state of the art frequently cylindrical propellant pellets having a diameter of 3 mm to 8 mm are used. The height-to-diameter ratio for these pellets usually is within the range of from 0.2 to 0.5 for reasons of manufacture. 
       FIG. 2  shows a diagram illustrating the relative bulk density against the pellet diameter with a constant height-to-diameter ratio (hid). The measurements show that pellets of smaller diameter (with equal height-to-diameter ratio) have a higher relative bulk density. Furthermore it could be shown by experiment that the relative bulk density is increased with a constant pellet diameter when the pellet height and the pellet diameter are adapted to each other. 
     Since the relative bulk density is only dependent on the geometry of the propellant compacts, the described interactions are applicable to all propellant compositions. 
     It is directly resulting from the diagram illustrated in  FIG. 2  that by a reduction of the pellet diameter from e.g. 4 mm to 1.5 mm with an h/d ratio of 0.5 a change of the relative bulk density from approx. 0.51 to 0.54 is obtained. As a result, the volume of the combustion chamber can be reduced by about 6 percent so that with the same output, measured as mass of gas per time, the inflator can be manufactured to be cheaper, lighter and smaller. 
     The pellets according to the invention having a diameter of e.g. 1.5 mm can also be pressed more easily with a higher height-to-diameter ratio (hid). For instance, propellant pellets having a ratio of h/d=0.7 and a diameter of d&lt;2 mm can be easily realized. 
     Compared to a conventional pellet of 4×2.2 mm (h/d=0.55), for a pellet of 2×1.6 mm even an increase in the relative bulk density by 14 percent is resulting. Consequently, also the combustion chamber volume of the inflator can be reduced by 14 percent with the same output. 
     Further examples are listed in the following table showing a comparison of the propellant pellets according to the invention to the propellant pellets used according to the state of the art. Furthermore, the reduction of volume of the combustion chamber that is possible by the use of the propellant pellets according to the invention is indicated. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Comparison of different pellet geometries 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                   
                 Relative 
                 Volume saving (%)  
               
               
                 Diameter 
                 Height, 
                   
                 bulk  
                 compared to: 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 mm 
                 mm 
                 h/d 
                 density 
                 6 × 2.2 mm 
                 4 × 2.2 mm 
                 3 × 1.1 mm 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 3 
                 1.1 
                 0.37 
                 0.48 
                 23 
                 −9 
                 0 
               
               
                 4 
                 2.2 
                 0.55 
                 0.3 
                 36 
                 0 
                 10 
               
               
                 6 
                 2.2 
                 0.37 
                 0.39 
                 0 
                 −26 
                 −19 
               
               
                 2 
                 1 
                 0.50 
                 0.54 
                 37 
                 2 
                 12 
               
               
                 2 
                 1.2 
                 0.60 
                 0.56 
                 43 
                 5 
                 17 
               
               
                 2 
                 1.4 
                 0.70 
                 0.58 
                 49 
                 10 
                 22 
               
               
                 2 
                 1.6 
                 0.80 
                 0.60 
                 55 
                 14 
                 26 
               
               
                 1.5 
                 0.75 
                 0.50 
                 0.54 
                 39 
                 3 
                 14 
               
               
                 1.5 
                 0.85 
                 0.57 
                 0.55 
                 42 
                 5 
                 16 
               
               
                 1.5 
                 0.95 
                 0.63 
                 0.56 
                 45 
                 7 
                 18 
               
               
                 1.5 
                 1.05 
                 0.70 
                 0.58 
                 48 
                 9 
                 21 
               
               
                   
               
            
           
         
       
     
       FIG. 3  illustrates a diagram including an exemplary representation of the pressure/time curves of a defined propellant comprising 36.7% by weight of potassium perchlorate, 60.8%) by weight of guanidine nitrate and 2.5% by weight of additives for different pellet sizes. This is compared to pellets having dimensions of 3×1 mm and 2×1.55 mm. In this example the relative bulk density of the propellant pellets increases from about 0.46 to about 0.59. As a consequence, a volume saving of the combustion chamber of approx. 28 percent can be achieved. 
     The example shows that by using smaller pellets having a larger height-to-diameter ratio the changed ballistics can be compensated and the same inflator output, measured as gas mass per time, can be achieved. 
     Another possibility of reducing the combustion chamber volume is to use a propellant that burns more slowly to adapt the ballistic performance. In this case, also major changes of the pellet geometry can be made and for example pellets having a diameter of 6 mm can be replaced with pellets having a diameter of 2 mm. In this way great savings of the combustion chamber volume can be achieved while the ballistic performance remains constant, as is shown in the foregoing table. 
     The propellant pellets according to the invention can be manufactured in accordance with the methods known in the state of the art by dry injection of powdered propellant compositions, preferably in rotary pellet presses known per se. 
     The inventors now have found that by optimizing the process parameters also pellets having a diameter of &lt;2 mm to preferably about 0.8 mm can be manufactured. 
     For sufficient filling of the matrices at high extruding speed initially a propellant powder of appropriate grain size is provided. The exact grain size distribution depends on the matrix size and on the flow behavior of the respective propellant composition. 
     Furthermore the shaking parameters of the pellet press are configured so that a rapid flow of the propellant powder into the matrix is ensured. 
     The propellant pellets according to the invention can be used in all pyrotechnical restraint systems. Preferred is the use in inflators for airbag modules, belt tensioners, micro-inflators and igniters, especially in the form of a gas-generating propellant charge and/or booster charge. 
     Further preferred is the use in an advanced ignition device for inflators and/or as advanced ignition mixture directly integrated in the gas-generating propellant charge.