Composite Technologies

Composite Repair Primers

Aging and durability of aircraft in both the military and civilian sectors have always been major issues as the existing fleet continues to age. The increased use of composite structures in the civilian fleet, such as in the Boeing 787 Dreamliner and the Airbus A380, make the understanding and/or improvement of composite durability, particularly durability of repairs, even more critical. Several areas have been identified as targets for improvement in composite aircraft repair. These include the development of rapid, low-temperature repair methods and associated materials as well as improvement of the quality of repairs when they are made. Adhesion of bonded repairs to the original substrate is a critical area that needs to be addressed. Adhesion of the repair to the aircraft can be an issue both initially and especially over time as exposure to moisture and thermal cycles can weaken the interface between the repair and the aircraft substrate.

Adherent Technologies, Inc. (ATI) has developed a novel moisture-resistant primer system for use in repairs of standard carbon/epoxy composites used in many subsonic aircraft. Our proprietary chemistry comprises a reactive coupling agent and a carrier resin compatible with standard aerospace epoxy resins that bonds directly to the prepared aircraft composite surface while retaining residual functionality that can be cured directly into the matrix of the repair. This leads to a covalently bound repair, thereby strengthening the repair interface. Proper selection of the coupling agent structure and carrier resin can serve to further enhance the moisture resistance resulting in greater durability of the composite repair.

The graph below shows the effect of the repair primer in a lap shear configuration. The effect of the primer on bond strength after moisture aging (noted as POST on the graph) is striking.

Lap shear bond strength

Lap shear bond strength

Scarf joints bonded using either of ATI’s M-series primers showed approximately 30% higher bond strengths than those bonded without primer (see table below). Furthermore, the bond strengths observed for the primed, moisture-aged samples were higher than those observed for dry-bonded samples with no primer.

Plate IDAvg % H2OPrimerBond Strength (psi)
JK22590.93None69159 (477 MPa)
JK22601.09M82893284 (643 MPa)
JK22631.11M116393926 (648 MPa)
JK2229DryNone78770 (543 MPa)
JK2234DryM828104271 (719 MPa)

While the development and testing of these repair primers is ongoing, these results clearly show the opportunity for significant improvements in composite aircraft repair systems.