Influence of surface preparation methods on moisture-related performance of structural hardwood–adhesive bonds

Abstract

The aim of this study was to investigate the influence of three surfacing methods (peripheral planing, sanding and face milling) on the moisture-related performance of bonded ash assemblies (Fraxinus excelsior L.). The different surfaces were tested in combination with four adhesives: phenol-resorcinol-formaldehyde (PRF), melamine-urea-formaldehyde (MUF), polyurethane (PUR) and emulsion polymer isocyanate (EPI). For evaluation, the surface roughness was measured and surfaces and bonds were examined by means of scanning electron microscopy (SEM) and transmitted light microscopy, respectively. To analyze bond performance, tensile shear tests were carried out as per EN 302-1 and the resistance to delamination was determined according to EN 302-2. Microscopy and roughness measurements showed significant differences between the bonding surfaces, notably with regard to cell damage and the level of fibrillation. The surface texture had significant impact on shear and delamination results. While shear tests showed good bond performance when tested in dry condition, moisture treatment revealed differences between surfaces, in particular with regard to wood failure. Based on shear results, the most appropriate surfacing method to produce moisture-resistant bonds appeared to be face milling together with PRF. Delamination results varied strongly with the surfacing method and adhesive types. PRF and MUF showed highest resistance to delamination with sanded surfaces, possibly because damaged cells helped to dissipate strain. PUR and EPI provided lower moisture-related durability. For these adhesives, best results were obtained with face milled surfaces, probably because of a more homogenous strain dissipation in the glueline caused by fibrillation.