Low temperature cure adhesive for honeycomb repair applications

An epoxy adhesive suitable for field repair applications was developed at the Naval Air Development Center (NADC). This material is a two part adhesive which consists of diglycidyl ether of bisphenol-A epoxy, carboxy terminated butadiene nitrile rubber, and diamino-dicyclohexyl methane. Previous work has shown that the adhesive is ambient temperature storable, that has viscosity and cure characteristics which prevent moisture induced voiding in bonding monolithic laminates, and that it is processable with field repair equipment. Currently adhesive properties have been demonstrated with a 149°C (300°F) 1 hour cure. An adhesive with similar characteristics has been commercialized as Dexter Hysol EA 9391. The upper use temperature of the adhesive under wet conditions is greater than 104°C (220°F). A number of current and emerging applications use honeycomb sandwich structures. The repair of these structures requires lengthy drying cycles to remove entrapped moisture in the core. A program to investigate the effect of alternate (low temperature) processing cycles on the mechanical properties of currently qualified adhesive (Dexter Hysol EA 9391) was performed. Low temperature cure cycles were identified which produced adhesive material properties comparable to those found with the standard cure cycle. It was found that a cure temperature of 93°C (200°F) provided acceptable mechanical strength at 82°C (180°F)/wet and may eliminate the long repair processing times associated with honeycomb structures.