Improvement of ash (Fraxinus excelsior L.) bonding quality with one-component polyurethane adhesive and hydrophilic primer for load-bearing application

Abstract

The increasing quantity of hardwood growing in the Swiss forest represents an opportunity for the future of the wood industry. Glued laminated timbers made of hardwood offer higher strength and stiffness properties than the softwood equivalents, and they are often used in combination with softwood for local reinforcement. Additionally, their visual aspect can be an attractive selling argument. Until now, phenol resorcinol adhesive (PRF, Aerodux 185) has preferably been used for the bonding of ash wood. Despite its high bonding performance, this adhesive has several disadvantages, including a dark brown glue line and a strong emission of formaldehyde both during the gluing process and when in use. One-component polyurethane (1C-PUR) adhesives, widely used in load-bearing structures made of softwood, represent an interesting alternative for hardwood gluing. These formaldehyde-free adhesives require no mixing and present an invisible glue line without any emissions after full curing. Preliminary results have shown that the combination of a slow-reacting 1C-PUR adhesive with a water-based primer system obtained similar results as Aerodux 185 in laboratory conditions with respect to shear strength and delamination resistance. This paper focuses on the optimization of the gluing process of two different 1C-PUR adhesives and their combination with a special primer on ash wood to develop an economically attractive industrial application process. The influence of several factors, such as primer-water concentration, primer activation time and primer quantity, were investigated using a delamination test according to EN 302-2. It was shown that it is possible with 1C-PUR to reach the normative standard requirements according to EN 15425. The best results were obtained with a primer-water concentration of 10% and a primer-water applied quantity of 20 g/m2. The influence of the primer activation time was shown to have only a small effect. 1C-PUR with a long closed assembly time was shown to perform even better. Additionally, the reduction of the lamellar thickness led to a reduction of delamination. By considering these parameters, it was possible to develop an industrial process for producing high-performance glued laminated timber out of ash wood and glued with a certified and commercially available 1C-PUR adhesive.