The driving force behind the increased usage of timber in construction is mainly sustainability of the raw materials and reduced carbon emission from the conventional construction sectors (Sciomenta et al., 2024). The limitations of solid timbers are removed by using engineered wood products (EWPs) in which the solid wood are glued together under optimum conditions to achieve robust structural components of desired sizes. However, achieving optimum conditions in daily production becomes challenging due to the ever-fluctuating climatic conditions. In Northern Europe, the temperatures during colder months fall below 10°C which results in a reduced surface temperature of the wood, and this directly influences the bonding quality.
The most common adhesive used for structural purposes is polyurethane (PUR) adhesives. Polyurethane adhesives perform better on wood in the wet state than normal RH conditions (Sterley and Gustafsson, 2012) since this type of adhesive have a residual isocyanate group that reacts with water (Properzi et al., 2003). However, very high moisture content in wood can result in weaker bond strength when using PUR adhesives (Favis et al., 2024). The adhesion strength of the wet bonded Pinus radiata wood (MC%: 101%) with PUR resulted in weaker shear strength compared to wood bonded at 30% MC. Properzi et al., 2003 studied that glulams prepared in wet conditions tend to achieve maximum tensile strength faster compared to glulams produced at 12% MC. Studies have showed that the application of primers on the glued wood surface before adhesive application also ensures expected adhesion properties (Hänsel et al., 2022). Recent experimental trials revealed that primers, including surfactants, are very efficient in 1C-PUR systems, especially when gluing problematic softwoods (e.g. larch, southern yellow pine) and hardwoods (e.g. ash, oak, and beech). These studies show the multivariate influence of temperature and moisture on the mechanical properties of the prepared joints. This research explores the impact of pressing pressure, pressing time, surface temperature, and moisture content on the glue line strength of bonded spruce elements, aiming to optimize the production of block-glued components, such as ribbed floors, alongside conventional products like glulam. Tensile shear tests were conducted on glued specimens, with adhesive bond performance evaluated through tensile shear strength and percentage wood failure. Furthermore, microscopic analysis was performed to assess the bondline thickness and mean penetrations of the adhesives under different processing conditions.
Keywords: PUR adhesive, Mechanical properties, Microscopy
Authors
Souvik Ray
Department of Forestry and Wood Technology, Linnaeus University, 35195 Växjö, Sweden
Noor Aho
Department of Building Technology, Linnaeus University, 35195 Växjö, Sweden
Roqeia Ahmed
Department of Building Technology, Linnaeus University, 35195 Växjö, Sweden
Michael Schweigler
Department of Building Technology, Linnaeus University, 35195 Växjö, Sweden
Mattias Brännström
Dalahusgruppen, Linghed, 79025 Dalarna County, Sweden
Reza Hosseinpourpia
Department of Forestry and Wood Technology, Linnaeus University, 35195 Växjö, Sweden
College of Forest Resources and Environmental Science, Michigan Technological University, Houghton, Michigan 49931, United States
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