The effect of curing temperature on thermal, physical and mechanical characteristics of two types of adhesives for aerospace structures

Abstract

The effects of cure temperatures on the thermal, physical and mechanical characteristics of two types of thermosetting structural epoxy film adhesives were determined in detail. The aim of this paper is to assess the effect of cure temperatures (82-121 degrees C) on the degree of cure of the two adhesives and the relevant void formations that need to be addressed in bonded part production and repair. Two thermal parameters were used to characterize the advancement of the reaction, such as degree of cure and glass transition temperature. The joint properties with respect to the cure temperatures were characterized by void content and bond-line thickness measurements and lap shear strength tests. Experimental results presented that all lap shear strengths were well within minimum shear strength (29MPa) required by the specification of the film-type adhesive. However, the lap shear strength testing after aging at 82 degrees C and 95%R.H for 1000h showed that the improved durability when the adhesive is cured at 121 degrees C did not occur for the 82 degrees C cure. Low curing conversion (75-77% degree of cure) combined with high voids (over 2 areal%) has a catastrophic effect on the bonding qualities at the metal-adhesive interface and due to lack of cohesion in the adhesive. The changes in the interface caused by the low temperature curing may contribute to an increased susceptibility of the bonded joint to moisture and consequent bond-line degradation.