Abstract:
A pyrotechnic composition, in particular for use in safety arrangements for motor vehicles, has an auto-ignition temperature between 140 and 180 degrees C. and contains hydroxybenzotriazole in an amount of about 25 to 95% by weight of the composition.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates to a pyrotechnic composition, in particular for use in safety arrangements for motor vehicles, having an auto-ignition temperature of between about 140 and 180 degrees C.  
       BACKGROUND OF THE INVENTION  
       [0002]     The gas-generating mixtures used in safety arrangements for motor vehicles, for example in gas generators for gas bag modules, are generally thermally stable. In order to activate the gas-generating mixtures in a controlled manner at high ambient temperature, e.g. under vehicle fire conditions, so-called thermal fuses or auto-ignition compositions are used. Such an auto-ignition composition is necessary in order to prevent the gas-generating mixtures from self-igniting in an uncontrolled manner at extraordinarily high temperatures. At these high temperatures, the gas-generating mixtures would in fact not react normally, but rather, owing to the increased temperature, would react in an accelerated and violent manner. In unfavourable cases, even explosions may occur. A gas generator housing is not designed for this accelerated violent reaction and can be destroyed. The auto-ignition composition guarantees that the reaction of the gas-generating mixture is triggered thermally below a critical ambient temperature. In such a case, through the controlled activation and reaction of the gas-generating mixture, the auto-ignition composition prevents the destruction of the gas generator housing and dangers connected therewith.  
         [0003]     In the prior art, hitherto stabilized nitrocellulose powders were used as pyrotechnic auto-ignition compositions. These compositions have an auto-ignition temperature of about 180 to 200 degrees C. The nitrocellulose powders do not, however, satisfy the stability requirements which are set by the automobile industry. Nitrocellulose tends to already decompose slowly at low temperatures and therefore does not ensure the capability of functioning as an auto-ignition mixture over a lengthy period of time, as is necessary, however, in motor vehicles.  
         [0004]     Auto-ignition compositions for gas-generating mixtures are known from EP 0 914 305. The auto-ignition temperatures of these compositions extend from 150 to 185 degrees C. The known compositions contain for example 3-nitro-1,2,4-triazol-5-one (NTO) and an oxidizing agent, such as sodium nitrate. A commercially available composition consists of 40% by weight of guanidine nitrate, 40% by weight of 3-nitro-1,2,4-triazol-5-one (NTO) and 20% by weight of sodium nitrate. After hot storage for 408 hours at 110 degrees C., this composition shows a weight loss of up to 2% and therefore fulfils the product specifications accepted by the automobile industry, according to which auto-ignition mixtures must withstand a hot storage for 400 hours at 107 degrees C., maintaining full functional capability. A disadvantage, however, are the very high costs and the low reaction intensity of the known composition. In order to ensure that the activated auto-ignition mixture reliably ignites the gas-generating composition, the auto-ignition composition must be combined with the ignition composition for the gas generator, which can lead to problems in the compatibility of the auto-ignition mixture and ignition composition.  
       SUMMARY OF THE INVENTION  
       [0005]     An object of the present invention is therefore to provide pyrotechnic auto-ignition compositions having an improved reaction intensity, which are sufficiently stable to storage and reliable in handling, can be produced at a favourable cost and are suitable for use in safety arrangements for motor vehicles.  
         [0006]     According to the invention, a pyrotechnic composition having an auto-ignition temperature of between about 140 and 180 degrees C. is provided, the composition containing hydroxybenzotriazole in an amount of about 25 to 95% by weight of the composition.  
         [0007]     The use of hydroxybenzotriazole in the auto-ignition mixture according to the invention is particularly advantageous, because it is easily available, stable in storage and is able to be handled readily. In addition, hydroxybenzotriazole can be processed well in a dry or wet process, as is used for the production of auto-ignition mixtures.  
         [0008]     The composition according to the invention can, moreover, contain at least one additive which further reduces the auto-ignition temperature of the composition. The additive preferably comprises at least one of sulphur and an inorganic sulphur compound, the proportion of the sulphur and/or of the inorganic sulphur compound in the composition amounting to 1 to 20% by weight. The inorganic sulphur compound is preferably selected from the group consisting of sulphates, sulphites, thiosulphates, thiosulphites, disulphites, disulphates, dithionites, dithionates, peroxosulphates, peroxodisulphates, thiocyanates, isothiocyanates, sulphides and polysulphides and mixtures thereof.  
         [0009]     The advantages of sulphur and inorganic sulphur compounds consist in that they are easily available and are favourably priced. In addition, mixtures which comprise hydroxybenzotriazole and at least one of sulphur and inorganic sulphur compounds, or consist substantially thereof, are able to be handled simply and fulfil all the requirements which must be fulfilled for use in safety arrangements for motor vehicles. They are, in particular, stable in storage and have a sufficient reaction intensity which ensures a reliable over-ignition in the case of activation. The auto-ignition mixtures according to the invention can therefore also be arranged separately from the ignition composition in the gas generator, whereby problems owing to chemical incompatibility of the auto-ignition mixture on the one hand and the ignition composition or propellant charge on the other hand, are avoided.  
         [0010]     Depending on requirements, the composition can additionally contain a fuel, an oxidizer or both, which can be contained in the mixture respectively in a proportion of up to 74% by weight, besides the hydroxybenzotriazole and the at least one of the sulphur and inorganic sulphur compound. Alternatively, the pyrotechnical composition according to the invention can comprise, or consist essentially of, hydroxybenzotriazole in an amount of about 25 to 95 weight percent of the composition, and the remainder being at least one of the fuel and the oxidizer.  
         [0011]     The fuel is preferably selected from the group consisting of guanidine compounds, triazoles, tetrazoles, bitetrazoles, dinitramides, such as e.g. guanylureadinitramide, and of metals, such as e.g. boron or aluminium, and mixtures thereof.  
         [0012]     Particularly preferably, the fuel is guanidine nitrate, guanylureadinitramide or nitroguanidine.  
         [0013]     The oxidizer is preferably selected from the group consisting of the nitrates, nitrides, perchlorates, chlorates and peroxides of alkali metals, alkaline earth metals and transition metals, ammonium nitrate, ammonium perchlorate and transition metal oxides, as well as mixtures thereof. Potassium nitrate and ammonium perchlorate are particularly preferred.  
         [0014]     In a preferred embodiment, the inventive auto-ignition mixtures comprise hydroxybenzotriazole in a proportion of about 30 to 90% by weight and also one of sulphur and an inorganic sulphur compound in a proportion of about 1 to 10% by weight. In another preferred embodiment, the auto-ignition mixtures consist essentially of about 30 to 90% by weight of hydroxybenzotriazole, 1 to 15% by weight of at least one of sulphur and an inorganic sulphur compound, 0 to 60% by weight of the oxidizer and 0 to 20% by weight of the fuel. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0015]     The invention is further described below by means of some preferred embodiments. These examples are, however, intended merely to illustrate the invention but are not to be understood in a limiting sense.  
       EXAMPLES 1 to 9 AND COMPARATIVE EXAMPLE 10  
       [0016]     The ground, solid mixture components indicated in Table 1 were mixed with each other in dry state in the indicated weight ratios and were compressed into tablets.  
                                                                                                                     TABLE I                           Composition of the Auto-Ignition Mixtures            Component   Example No.            [% by weight]   1   2   3   4   5   6   7   8   9   10                    hydroxybenzotriazole   90   70   70   70   70   70   40   70   33           sulphur   10   10   10   10   10   5   5       12.5       sodium disulphite                               10       guanidine nitrate           20           10   5   20   4   40       guanylureadintramide                   20       nitroguanidine               20       NTO                                       40       aluminum                                   8.5       ammonium perchlorate       20       potassium nitrate                       15   50       42       sodium nitrate                                       20                  
 
         [0017]     The auto-ignition temperature of the various auto-ignition mixtures was determined by means of differential scanning calorimetry both before storage in a drying chamber and also after hot storage for 408 hours at 110 degrees C. The reaction intensity of the auto-ignition mixtures was determined by comparative subjective assessment.  
         [0018]     Table II gives the results for the auto-ignition temperatures determined for the auto-ignition mixtures, the reaction intensity and also the weight change, as separation after hot storage.  
                                                   TABLE II                           Characteristics of the Auto-Ignition Mixtures                Auto-ignition                   Temperature   Weight loss after           [degrees C.]   408 h/110 degrees C.   Reaction       Example   Before/after storage   storage   Intensity                    1   174/174   0.29%   strong       2   173/175   0.28%   strong       3   156/159   0.89%   medium       4   174/174   0.33%   medium       5   166/170   0.61%   strong       6   160/165   1.47%   medium       7   157/157   0.2%   strong       8       169/—  [2]     —  [2]         9   146/149   0.23   strong       10   160/160   up to 2%   weak                   [2]  Values were not determined             
 
         [0019]     A comparison of the percentage values for the weight loss after storage of the auto-ignition mixtures 1 to 7 and 9 according to the invention and the comparative mixture 10 shows that through the use of hydroxybenzotriazole a low weight loss occurs. The commercially available composition of Example 10 (Comparison) shows a weight loss of up to 2%, whereas the compositions according to the invention after 408 hours storage at 110 degrees C. merely have a weight loss between 0.2% and 1.47%. Consequently, the auto-ignition mixtures according to the invention show a lower tendency to gas separation than the commercially available composition, which suggests an increased stability of these auto-ignition mixtures.  
         [0020]     The auto-ignition temperatures of the auto-ignition mixtures according to the invention of Examples 1 to 7 and 9 show after 408 hours storage at 110 degrees C. merely differences of 0 to 5 degrees C. and are therefore in the required range of 140 to 180 degrees C. both before and also after thermal stress.  
         [0021]     The reaction intensity of the auto-ignition mixtures according to the invention is higher than that of the commercially available composition according to Example 10. From the higher reaction intensity, conclusions can be drawn as to the over-ignition capability of the individual auto-ignition mixtures. The results show that the compositions according to the invention react more violent and therefore in the case of activation have a better capability to prime the ignition mixture or activate the propellant charge.  
         [0022]     For the application of the auto-ignition mixtures according to the invention, this means that they do not necessarily have to be combined with an energy-rich ignition mixture for a gas generator, but rather can be mounted separately. Thereby, problems resulting from the incompatibility of the auto-ignition mixture with other chemicals in the gas generator are largely avoided.