Hydrofluorocarbon High-Temperature Integral Fuel Tank Sealants
Abstract Supersonic aircraft in the Mach 3 range have increased temperature requirements for many materials of construction. One of the most critical problem areas is the integral fuel tank where state-of-the-art elastomeric sealants no longer meet those requirements. New sealants resistant to hydrocarbon fuels at temperatures in excess of 500° F are urgently needed. A high temperature integral fuel tank filleting sealant has been developed based on a mixture of high and low molecular weight hydrofluoroearbon polymers. This sealant has a solids content of 85 per cent and cures at modest temperatures. It is a one component system stable for several weeks at ambient temperatures and for longer periods if refrigerated. The sealant exhibits good room temperature and high temperature mechanical properties both before and after aging in hydrocarbon fuel and fuel vapor at temperatures up to 500° F. A typical formulation showed an increase in tensile strength and better than 50 per cent retention of elongation after 1000 continuous hours exposure to fuel vapor at 500° F. Adhesion to metal substrates is good and samples have resisted rupture and loss of adhesion when pressurized at 500° F for 1000 hours. This is only an interim solution to the critical fuel tank sealant problem in Mach 3 aircraft. Research is continuing to improve its useable life and to eliminate a possibly serious corrosion problem with titanium substrates. It must be emphasized, however, that sealants based on hydrofluorocarbons, fluorosilicones, and other available elastomers can at best provide interim solutions to the high temperature sealant problems and that new base polymers, such as the triazine and polyether elastomers currently being developed by the Air Force Materials Laboratory, are needed to provide sealants which completely meet requirements.