Use of Weibull distribution statistics in assessing long term compatibility of propellants with polymeric materials

The long term compatibility of propellants with polymers may be evaluated by statistical means. The Weibull distribution function was used in an effort to provid a more accurate means of estimating the effect of aging of the inert material in contact with the propellant.This method was applied to several systems exhibiting different degrees of compatibility. Exposure at 140°F of the RTV 680 rubber with M30 propellant in the XM735 projectile displayed no apparent adverse effects. Distribution analysis supported the conclusion that the RTV 680/M30 system is compatible at ambient temperatures. Another silicone displayed an initial decline in mechanical properties followed by a period of stability when stored with M30 at elevated temperatures over a period of 12 months.A fair amount of incompatibility was displayed by 30% glass filled nylon which had been exposed to a double base propellant WC870 for an extended time. The Weibull distribution plots indicate that the propellant exerts a damaging effect on both Nylon 12 materials, particularly at elevated temperatures.Thermal aging may contribute to the ultimate failure of a polymer, as well as direct contact with an energetic. This is illustrated in the case of the M205 non-metallic cartridge case used in the projectile of a 152mm round, with M26E1 propellant. Although it is difficult to separate the effect of these variables upon the lifetime of the system, Weibull statistics provide some measure of prediction.