Structurally durable epoxy bonds to aircraft woods

The lack of structural durability of epoxy bonds to wood has always been a problem for fabricators of adhesive-bonded wood products intended for service in exterior environments. Although epoxy adhesives develop dry shear strengths that exceed the strength of wood itself, the epoxy bonds fail in delamination once exposed to the severe stresses of water soaking and drying. Recent research at the Forest Products Laboratory (FPL) demonstrated that a hydroxymethylated resorcinol (HMR) coupling agent physi-cochemically couples to both epoxy adhesive and lignocellulosics of wood to produce bonds of extraordinary structural durability. The purpose of this report is to demonstrate how HMR enhanced adhesion of three epoxy adhesive formulations (based on diglycidylether of bisphenol-A resin) on two softwood and two hardwood species commonly used to construct aircraft components. A new FPL formulation met requirements for resistance to delamination, shear, and deformation in HMR-primed lumber joints on all four species of wood, in accordance with ASTM Specification D 2559. Two commercial formulations performed well in tests of resistance to delamination and shear on three woods of moderate density, but failed requirements on high-density yellow birch. Without an HMR coupling agent, none of the epoxy formulations had sufficient resistance to delamination to meet ASTM requirements on any of the species tested.

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