Interactions between F-111 Fuselage Fuel Tank Sealants. Part 2. Variation in Performance Properties of Polysulfides after Contact with Polyester Degradation Products.

摘要

Fuel leaks occur in F-111 aircraft from interactions between polysulfide sealants and the hydrolysis products of polyester sealants used to seal fuel cavities. Changes in properties of some polysulfides were examined following contact with degraded polyesters. Use of model degradation compounds indicated that ester groups cause swelling, alcohols suppress swelling when used with esters, and carboxylic acids both swell the polysulfides and harden exposed surfaces. Degraded polyesters cause swelling and embrittlement together with inner softening of the polysulfides. PR-1750, which has a high crosslink density, was the most resistant of the polysulfides examined. Studies with simulated fuel tank structures indicated that the polysulfides ruptured due to expansion pressures acting on a matrix weakened by swelling. Such pressures arise from swell of the opolyester sealants and thermal expansion of their degradation products under aerodynamic heating. For small contact areas between polyester and polysulfide, adhesion was relatively unaffected. Broad sealant fillets were shown to be essential for effective resealing over extended periods. (Author)