A natural ageing of two gas generators (GG) used in seat belt restraint systems (pretensioner) was initiated by TRW. The gas producing materials of the GGs are of type single base nitrocellulose propellants. The natural ageing was performed by an environmental test center near Miami, Florida, USA. The climate at this site is characterized with warm to hot and humid. The situation in a car was simulated by placing the seat belt systems in a box. The GGs was not exposed to direct sun light but the humidity could reach them. The storage started in October 1987 and ended in October 2003. Several times samples have been taken out and analysed with regard to GG performance and propellent stability. After 16 years of natural ageing the GG performance expressed as pressure time curves is in the tolerance range which means that the GGs are still further usable. The results of the stability tests on the propelants lead to the assessment stable after this elevated environmental stress. A systematic change of the stability results was found and quantified. This allowed determining a further significant usetime for the GG of four to six years. A comparison is made with data from new type of propellants used in belt restraint systems. A certain amount of two belt restraint units with two different gas generator propellants have been exposed to natural ageing over 16 years in a weathering test center near Miami Florida, USA. The two propellants are of type single base nitrocellulose propellant as used in small arms cartridges. The ageing configuration simulated the situation in car. Elevated temperatures have been acted on the propellants. An estimation from climatic data results in temperatures up to 55 deg C. The regularly removed samples from the ageing storage were investigated by several tests and methods to reveal the evolution of performance, chemical stability and safety of the propellants: ballistic measurement of the pressure-time curves of the propellant gas production by ignition of the gas generator unit. Dutch mass loss test at 110 deg C, heat storage at 90 deg C up to the end of stabilization (means end of safe usetime, because so-named 'autocatalysis' starts and this may lead to autoignition but in any case to severe energy loss and in turn to malfunctioning), stabilizer consumption and heat generation rate. The monitoring and assessment of the chemical stability must insure that no triggering of the belt restraint units by autoignition or malfunctioning by energy loss occurs during in-service time period. The result of all stability and ageing tests is that both propellants are still stable and safe after 16 years of pronounced ageing. The ageing could be monitored and quantified by the evolution of all test and investigation data as function of ageing time. This enabled the extrapolation of the 'time to the end of active stabilization at 90 deg C' to the assigned limit value of 22 days as function of ageing time, and an additional usetime of 3 to 6 years could be assigned to both propellants for a time-temperature load at the weathering center. A comparison is made between the investigated propellants and two new types of propellants currently in use in belt restraint units. They are also single base nitrocellulose propellants but with another stabilizing system. Their stability and safety data are similar to the aged propellants and the expected use time of 15 years will be reached by them.


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    Investigation of Ageing Behaviour of Nitrocellulose-based Propellants for Modern Car Occupant Safety Systems

    Skriver, A. / Huber, A. / Vogelsanger, B. et al. | British Library Conference Proceedings | 2002



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    MAGEE, R. / MC ALEVY, III, R. / WRUBEL, J. | AIAA | 1964


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    Carter, G. T. | NTRS | 1965