Patent Publication Number: US-2013248062-A1

Title: Gas generating composition

Description:
BACKGROUND 
     The present invention relates to a gas generating composition, especially for use in safety devices for vehicles. 
     U.S. 2009/0308509 describes compositions of guanidine nitrate, basic copper nitrate and potassium perchlorate. The examples illustrate potassium perchlorate proportions of approximately 14% and it is outlined that amounts of 10 to 20% are preferred. The known compositions are intended to bring about high burn rates. It is known, however, that upon reducing the potassium perchlorate proportion the burn rate is continuously decreased. 
     From U.S.-2007/0119530 also compositions of guanidine nitrate and basic copper nitrate are known. Various compounds also including potassium perchlorate can be added as burn moderators. 
     EP 1 006 096 A discloses a gas generating composition comprising a fuel consisting of one or more components at a proportion of 20 to 60% by weight and an oxidant mixture consisting of at least three components at a proportion of 40 to 80% by weight each related to the total composition. The fuel consists of a guanidine compound at a proportion of at least 95% by weight and a further fuel component at a proportion of 0 to less than 5% by weight each related to the mass of the fuel components. The oxidant mixture consists of one or more transition metal oxides at a proportion of 20 to 80% by weight, basic copper nitrate at a proportion of 0 to 50% by weight, metal chlorate, metal perchlorate, ammonium perchlorate or mixtures thereof at a proportion of 1 to 15% by weight and alkali nitrate, earth alkali nitrate or mixtures thereof at a proportion of 1 to 15% by weight each related to the oxidant mixture. 
     Propellants based on guanidine compounds can have an increased combustion chamber pressure during burning in the inflator after thermal shock. Variations of the combustion chamber pressure during propellant burning in the inflator are undesired, however, for reasons of product quality. The thermal shock resistance of those propellants is usually controlled by observing very low moisture limits in the finished inflator. 
     Alternatively, molecular sieve can be put into the inflator. The molecular sieve absorbs the moisture from the propellant after closing the inflator and the propellant is stabilized against thermal shocks. 
     Mixtures of nitroguanidine nitrate and potassium nitrate can be stabilized by the use of a mixture of hydrophilic and hydrophobic silicon dioxide. 
     It is complicated and cost-intensive to observe low propellant moistures during the manufacture of inflators. Molecular sieve as well as silicon dioxide are themselves hygroscopic substances that can introduce moisture to the propellant. Filling the inflator with molecular sieve or with propellants containing silicon dioxide therefore requires dosing units encapsulated against moisture at the production line. Equally the storage, the transport and the open time of the molecular sieve and of the propellants have to be strongly regulated so as to minimize absorption of ambient moisture. 
     Therefore there is a further need for gas generating compositions&#39; or use in safety devices for automotive vehicles by which high temperature shock resistance can be achieved while the inflator output is substantially constant. 
     For this, the present invention provides a composition comprising the features of claim  1 . 
     SUMMARY 
     The gas generating composition according to the invention comprises
         a) 40 to 60% by weight of fuel consisting of one or more components selected from the group of guanidine compounds;   b) 40 to 60% by weight of basic copper nitrate;   c) 0 to 8% by weight of at least one transition metal oxide; and   d) 1 to 5% by weight of at least one stabilizer component;   e) 0 to 5% by weight of further additives; each related to the total weight of the components a) through e);       

     wherein at room temperature the stabilizer component has a solubility in water of not more than 1 g/100 ml and the solubility in water at T=90° C. amounts to a least seven times the solubility at room temperature. 
     A proportion of transition metal oxides of more than 8% by weight is undesired as the presence of transition metal oxides results in a reduced gas yield. Due to the higher oxygen proportion of basic copper nitrate available compared to transition metal oxides, in oxidant mixtures of basic copper nitrate and transition metal oxides as high proportions of basic copper nitrate as possible are desired to achieve maximum gas yield. 
     Suited stabilizer components are substances that have a high solubility in water at high temperature and a poor solubility in water at room temperature. By room temperature a temperature of 23° C. is understood. In accordance with the invention, the stabilizer components at room temperature show a solubility in water of not more than 1 g/100 ml and at T=90° C. a solubility in water higher by at least the factor 7 than at room temperature. 
     The proportion of the stabilizing additive is to be as low as possible and is to range from 1 to 5% by weight. At lower proportions no stabilizing effect occurs with respect to resistance of the propellant to thermal shocks. At higher proportions the propellant properties such as the gas yield, the burn rate and the combustion temperature can be affected in an undesired manner. 
     It was surprisingly found that the thermal shock resistance of propellants based on guanidine compounds can be positively influenced by the addition of the stabilizer component according to the invention. The combustion chamber pressure of the inflator does not significantly rise after thermal shock despite increased moisture content in the propellant. 
     Advantageous embodiments of the invention are stated in the subclaims which can be optionally combined. 
    
    
     DESCRIPTION 
     In accordance with an especially preferred embodiment, the solubility of the stabilizer component in water at room temperature ranges from 0.05 g/100 ml to 1.0 g/100 ml. 
     Preferably the solubility at 90° C. is higher by at least the factor 10 than at room temperature. 
     The stabilizer component can be both an organic and inorganic compound. Examples of suitable stabilizer components are cyanuric acid, melamine cyanurate, potassium bitartrate and 3,5-dinitrobenzoic acid. 
     The proportion of the stabilizer component should be as low as possible as both organic and inorganic compounds reduce the gas yield. The inorganic compounds usually are inert and have no or only a small share in the gas generation. The organic compounds usually exhibit a poorer oxygen balance than guanidine compounds and thus require more oxidant. Moreover, an undesired influence on the burn rate, the burn temperature or else the composition of the released gas mixture may occur. In contrast to organic compounds, inorganic compounds can result in an increased discharge of solid combustion products. 
     When producing the propellant the stabilizer component can be added to the layered propellant set prior to grinding. 
     As guanidine compounds preferably guanidine carbonate, guanidine nitrate, guanidine perchlorate, aminoguanidine nitrate, diaminoguanidine nitrate, triaminoguanidine nitrate, nitroguanidine or mixtures thereof are used. 
     According to an especially preferred embodiment the propellant consists of guanidine nitrate. 
     The gas generating composition preferably has an oxygen balance of −3 to +1, especially preferred from −2 to 0. The substantially balanced or slightly under-balanced compositions show good ballistic properties with low noxious gas concentrations. 
     By the “oxygen balance” the oxygen quantity in percent by weight is understood which is released upon complete conversion of a compound or a mixture to CO 2 , H 2 O, Al 2 O 3 , B 2 O 3  etc. (O 2  over-balancing). If the oxygen present is not sufficient for this purpose, the missing quantity required for the complete conversion is indicated by negative signs (O 2  under-balancing). 
     According to an especially preferred embodiment the proportion of transition metal oxides ranges from 0% by weight to 5% by weight. Small quantities of transition metal oxides can be effective as burn catalysts, igniting aids and coolants and serve for adjusting the desired burn characteristics. 
     The transition metal oxides are preferably selected from the group consisting of TiO 2 , Cr 2 O 3 , MnO 2 , Fe 2 O 3 , Fe 3 O 4 , CuO, Cu 2 O and mixtures thereof. 
     The composition according to the invention may finally also contain common further additives such as processing aids and burn moderators at a proportion of up to 5% by weight related to the total composition, the processing aids being selected from the group consisting of the anti-caking agents, pressing aids and/or anti-blocking agents. Examples of processing aids and burn moderators are polyethylene glycol, cellulose, methyl cellulose, soot, graphite, wax, calcium stearate, magnesium stearate, zinc stearate, boron nitride, talcum, bentonite, alumina, silica and molybdenum sulfide. The use of these agents is generally known. 
     According to an especially preferred embodiment, the composition according to the invention consists of:
         44 to 52% by weight of guanidine nitrate,   44 to 52% by weight of basic copper nitrate,   0-2.0% by weight of at least one transition metal oxide,   2 to 5% by weight of at least one stabilizer component; and   0 to 4% by weight of further additives; each related to the total composition.       

     The invention offers the advantage that the propellants to which the stabilizer component is added exhibit properties similar to the previously used propellants based on guanidine compounds. This permits an output-neutral design of the stabilized propellants compared to the previously used propellants. However, in addition the sensitivity to thermal shocks is also decreased with a higher moisture content of the propellant. 
     The invention shall be described hereinafter by Way of especially preferred embodiments which are not to be understood in a restricting manner. 
     Embodiments 1 to 4 and Comparative Example 5 
     47.2 g guanidine nitrate, 47.35 g basic copper nitrate, 2.25 g alumina, 0.25 g titanium dioxide, 0.36 g calcium stearate and 2.6 g of a stabilizer component according to the following table 1 are jointly weighed into a vibration mill, are ground for 20 min and are mixed. The mixture obtained is directly pressed into tablets without any further processing steps. The water content of the tablets is adjusted to 0.2% and 0.3%, respectively. 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Solubility in 
               
               
                   
                 water [g/100 ml] 
               
            
           
           
               
               
               
               
               
            
               
                 Example No. 
                 Stabilizer component 
                 @ RT 
                 @ 90° C. 
                 factor 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 1 
                 3,5 dinitrobenzoic acid 
                 0.14 
                 1.9 
                 14 
               
               
                 2 
                 potassium bitartrate 
                 0.6 
                 6.3 
                 11 
               
               
                 3 
                 cyanuric acid 
                 0.2 
                 2.6 
                 13 
               
               
                 4 
                 melamine cyanurate 
                 0.003 
                 0.02 
                 7 
               
               
                 5 
                 no stabilizer 
                 — 
                 — 
                 — 
               
               
                   
               
            
           
         
       
     
     The propellant compositions according to examples 1 to 5 are subjected to a thermal shock test pursuant to USCAR specification. Said “thermal shock test” is thermal shock storage of the propellant compositions in an inflator at the benchmark temperatures of −40° C. and +90° C. during 200 cycles. The exposure time at the benchmark temperatures is determined by way of the setting time until the benchmark temperature, multiplied by the factor 1.25, is reached. The setting time is the time required until the core temperature of the inflator, measured in the center of the propellant filling, has reached the benchmark temperature. The duration of relocating the inflators at the different benchmark temperatures is fixed to a maximum of 120 seconds. For the USCAR Thermal Shock Test usually temperature control cabinets including two temperature control chambers (−40° C. and +90° C.) are used, the test specimen being automatically relocated from one chamber to the other via a paternoster system. 
     For the compositions according to the examples 1 to 5 the results given in the following table 2 are obtained. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                 Change 
                   
               
               
                   
                 after Thermal 
               
               
                   
                 Shock Test [%] 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Example No. 
                 Relative moisture [%] 
                 Density 
                 RQ 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 1 
                 0.2 
                 −7 
                 +8 
               
               
                   
                 1 
                 0.3 
                 −9 
                 +3 
               
               
                   
                 2 
                 0.2 
                 −10 
                 +6 
               
               
                   
                 2 
                 0.3 
                 −12 
                 +5 
               
               
                   
                 3 
                 0.2 
                 −4 
                 0 
               
               
                   
                 3 
                 0.3 
                 −5 
                 −4 
               
               
                   
                 4 
                 0.2 
                 −9 
                 +6 
               
               
                   
                 4 
                 0.3 
                 −12 
                 +6 
               
               
                   
                 5 (comparative) 
                 0.2 
                 −22 
                 +92 
               
               
                   
                   
               
            
           
         
       
     
     In table 2 RQ means “relative quickness”. This is an evaluating parameter for the output of propellants from the burn within a closed test bomb. Usually the test composition is compared to a reference sample or to defined characteristic limits. In the tests according to the examples 1 to 5 the respective propellant composition without thermal shock is used as reference sample. The RQ value is based on the change of the burn rate at different pressures, each related to the maximum pressure. The mean value is determined from measurements at intervals of 10% over a range of from 20% of the maximum pressure to 80% of the maximum pressure. 
     A propellant composition according to example 3 including 2.6% cyanuric acid shows no significant increase in the combustion chamber pressure in the inflator after performing the USCAR Thermal Shock Test even with a moisture content of 0.30% in the propellant. The composition according to the comparative example 5 already shows an increase in the combustion chamber pressure by approx. 10% with a moisture content of 0.15% in the propellant.