The Influence of Extended Cure on Mechanical Performance of Aerospace-Grade Film Adhesives

Bonded adhesive joints have seen increased usage in aircraft structures as weight and cost reductions drive manufacturers to composite designs. As the use of epoxy film adhesives become more prevalent, it is important to characterize the thermal and mechanical properties of the adhesive after exposure to various manufacturing conditions. Prior studies suggest that thermal characteristics and mechanical performance of epoxy resin formulations may be considered a function of chemical structure, cure temperature, and cure duration. It was therefore hypothesized that by controlling cure temperature and duration, the Tg, degree of cure, and mechanical performance of aerospace-grade film adhesives may be controlled. Three epoxy film adhesives cured with three different cure cycles were thermally characterized using DSC, DMA, and rheological techniques. It was shown that increased cure duration provided increased degrees of cure and Tg of the adhesives as a result of a more developed polymer network. V-notch shear and flat-wise tensile testing at ambient dry and elevated temperature wet conditions were used to mechanically characterize the performance of the adhesives cured with three different cure cycles. Mechanical test results showed that adhesives with more aromatic chemical structures saw less degradation as a result of moisture exposure and elevated temperature testing, while the adhesive with simpler chemical structure exhibited significantly reduced performance at elevated temperature wet conditions. It was concluded that while extended cure exposure did not provide significant gains in mechanical performance, the increased polymer network development provided increased resistance to the effects of moisture absorption and elevated temperature environments