Evaluation of Adhesion Promotion Techniques for Structural Bonded Timber Joints

Abstract

Long-term durability of a structural adhesive joint is an important requirement, because it has to be able to support the required design loads, under service conditions, for the planned lifetime of the structure. Epoxy adhesives, whilst not ideal, are currently the best family of adhesives for in situ repair operations. As long as the bonded joint remains dry and unexposed to high service temperatures, epoxy adhesives produce strong bonds to timber. However, once they are exposed to severe stresses as a result of repeated water soaking and drying cycles, the bonded joint delaminates and does not fulfill the requirements for structural timber adhesives intended for exterior exposure. One way of improving bond durability is through the use of surface treatments prior to bonding. In this study, the effects of corona discharge surface treatment, hydroxymethylated resorcinol (HMR) and γ-glycidoxypropyltrimethoxysilane (GPMS) adhesion promoters on the durability enhancement of pine, iroko, and oak bonded joints were evaluated. The results proved that surface modification methods for adhesion promotion can be adapted to cellulosic substrates with significant improvements in bonded joint durability.

Keywords:

• Adhesion promoters
• Durability
• Epoxy
• Solid timber
• Structural bonded joints
• Surface treatments

ACKNOWLEDGMENTS

This work was carried out within the scope of a PhD research project (SFRH/BD/17210/2004/DGW8) at Oxford Brookes University, Oxford, UK, financed by “Fundação para a Ciência e a Tecnologia, Portugal”. The authors wish to acknowledge this financial support.

Notes

Note: *Values taken from the manufacturer material data sheet.

Notes: Stress = shear strength at failure; Strain = displacement at maximum load (in percentage); AFW = adhesion failure to wood (when there are no wood fibres on the adhesive surface after bonded joint failure); CWF = cohesive wood failure; CAF = cohesive adhesive failure; AFWAI = adhesion failure at wood adhesive interface (when the failed specimens exhibit shallow timber on the adhesive surface and/or shallow adhesive on the timber surface). The standard deviation is presented between brackets.

Notes: Percentage of specimens that failed during the artificial accelerated weathering – *20%, ^§40%, †50%, ‡40%; Stress = shear strength at failure; Strain = displacement at maximum load (in percentage); AFW = adhesion failure to wood (when there are no wood fibres on the adhesive surface after bonded joint failure); CWF = cohesive wood failure; CAF = cohesive adhesive failure; AFWAI = adhesion failure at wood adhesive interface (when the failed specimens exhibit shallow timber on the adhesive surface and/or shallow adhesive on the timber surface). The standard deviation is presented between brackets.

¹For comparison purposes it was considered that for each timber/treatment/adhesive combination 100% shear strength corresponds to the respective timber/adhesive combination not weathered and not treated, so that the percentages shown are calculated in respect to that value.