Ageing of Structural Film Adhesives — Changes in Chemical and Physical Properties and the Effect on Joint Strength

Epoxy-based film adhesives are extensively used in structural aircraft applications but although the one part nature of these materials has many advantages in terms of ease of use, the short shelf-life can be a serious disadvantage, especially when the material spends lengthy times in transit between manufacturer and user. Studies on a number of epoxy and nitrileepoxy adhesives have shown that slow cure, hydrolysis of the resin and specific interactions between components can occur during storage which result in modification of various chemical and physical properties of the uncured adhesives. The relative importance of these reactions depends on the adhesive composition. These modifications are reflected in changes in the strength of joints made with aged adhesives. This paper presents examples of these effects drawn from the results of a number of ageing studies.