Improvement of tensile shear strength and wood failure percentage of 1C PUR bonded wooden joints at wet stage by means of DMF priming

Abstract

Tensile shear tests according to EN 302-1 for load-bearing timber structures were performed on European beech wood (Fagus sylvatica L.) and Douglas fir [Pseudotsuga menziesii (Mirb.) Franco] bonded by means of a one-component polyurethane adhesive (1C PUR). Results reveal a substantial loss of tensile shear strength (TSS) and wood failure percentage (WFP) at the wet stage compared to the dry stage. As can be seen from microscopic images, this is accompanied by a loss of adhesion at the boundary layer. Therefore, the aim of this work was to find a priming fluid that improves the load transmission between adhesive and adherend at the wet stage without introducing formaldehyde into the gluing process. A substantial improvement of TSS and WFP was achieved by means of the hygroscopic organic solvent N,N-dimethylformamide (DMF). In addition, contact angle measurements were carried out, revealing that DMF heavily enhances the wettability of the joining surface. Furthermore, it was attempted to integrate the outcomes into the swelling strain model stated by Frihart in 2009. By way of comparison a hydroxymethylated resorcinol coupling agent, a mixture of diphenylmethane-4,4 0-diisocyanate isomers and water were also tested as priming fluids. The data confirm that TSS and WFP of 1C PUR bonded wooden joints do not correlate, whilst WFP is mostly not normally (at wet stage often bimodally) distributed.

Shear Bond Strength of Two-Layered Hardwood Strips Bonded with Polyvinyl Acetate and Polyurethane Adhesives

“…Similar values were reported by Özkaya et al (2015), who found the SBS value of 8.1 MPa in beech LVL glued with PVAc glue. Slightly higher values were found by Kläusler et al (2014), who reported an SBS value of 12.5 MPa for beech wood glued with single-component PUR glue. Shukla and Kamden (2008), who studied LVL consisting of thin aspen veneer and PVAc glue, found SBS values in the range of 2.1 to 3 MPa.…”

Bondability of Black locust (Robinia pseudoacacia) and Beech wood (Fagus sylvatica) with polyvinyl acetate and polyurethane adhesives

“…The adhesives were applied to the joints of the wooden elements, following the adhesive bonding parameters and the manufacturer's instructions. Prior to the spreading of PUR adhesive on wood, moisturizing of both wood surfaces, prepared for the adhesion, had preceded for each specimen, in order to contribute to the success of element bonding (Klaüsler et al 2014), while in the case of PVAc moisturizing was not necessary (ISO 6238:2001). There is beneficial effect of water spray on the 1C PUR-gluing results, when the ambient conditions are very dry, leading to fast superficial drying of the wooden adherents (Klaüsler et al 2014).…”

“…Prior to the spreading of PUR adhesive on wood, moisturizing of both wood surfaces, prepared for the adhesion, had preceded for each specimen, in order to contribute to the success of element bonding (Klaüsler et al 2014), while in the case of PVAc moisturizing was not necessary (ISO 6238:2001). There is beneficial effect of water spray on the 1C PUR-gluing results, when the ambient conditions are very dry, leading to fast superficial drying of the wooden adherents (Klaüsler et al 2014). PUR, generally reacts with the functional hydroxyl groups of wood creating strong chemical bonds, therefore, increasing the initial moisture content could have a positive effect on shear strength and glue penetration (Sedlia and Smidriakova 2010).…”

“…As previous researches showed, PUR adhesives exhibit excellent joint strength, when tested in standard climate conditions, but seem not to be so reliable in the case of low wood moisture content or under high temperatures, therefore PUR is characterized by a controversially discussed behavior and its use in load bearing timber structures could be unsafe (Richter andSteiger 2005, Beaud et al 2006). Furthermore, Klaüsler et al (2014) implemented tensile shear tests on European beech and Douglas fir bonded by means of a one-component PUR, revealing a substantial loss of tensile shear strength and wood failure percentage at the wet stage compared to the dry stage, as well as the necessity of testing different priming fluids.…”

Determination of the Effect of Surface Roughness on the Bonding Strength of Wooden Materials

“…In addition, there are the difficulties of separating shallow wood failure, as well as adhesion and cohesion failure in the bond line from each other, especially when adhesive and wood have almost the same color. Particularly in such cases, the noted wood failure percentage (WFP) of one and the same sample can vary quite a bit, depending on the person evaluating it (Künniger 2008;Kläusler et al 2014;Hass et al 2014). According to Burdurlu et al (2006), the shear strength values of black pine specimens bonded with PVAc adhesive were higher (8.16 N/mm 2 ) than those bonded with PUR adhesive (7.95 N/mm 2 ).…”