Novel hybrid polymer adhesives for laminated materials based on hardwood

Gluing hardwood with standard adhesives often appears problematic; therefore, their broader use in practice is limited. On the other hand, using hardwood in laminated beams would undoubtedly benefit because mechanical properties would reduce construction dimensions compared to commonly used materials. The hardwood surfaces are highly heterogeneous and porous structure, and the high tannin content in heartwood restrict the use of standard structural PUR adhesives. Using modified adhesives is one of the possibilities to overcome these problems. This work deals with PUR adhesive based on polymeric methylene diphenyl diisocyanate (C15H10N2O2). For this specific PUR adhesive, three different application methods were used (pure polyurethane adhesive as a reference, the adhesive system with application of deep penetration, and the adhesive system with application of deep penetration with an admixture of thermoset particles (C7H6O) and polyurethane adhesive). Such a combination of a particular adhesive with a thermoset increases adhesive parameters, especially for hardwood. Above glass transitions point Tg of the thermoset resin, a 3D network between the thermoset and PUR is formed as a novel species which appears highly resistant to moisture. This protocol has been verified on glued surfaces of several difficult-to-glue types of wood, such as oak (Quercus robur), beech (Fagus sylvatica), and black locust (Robinia pseudoacacia). A lap joint shear strength test using a joint test according to EN 302-1 [1] was used to determine the mechanical properties of the adhesives. The results of lap joint shear strength tests are determined based on the classification standard EN 15425 [2]. Based on the collected results, it is possible to confirm the positive effect of adhesives modified by thermosets on the overall strength of the glued joints in the longitudinal direction of the laminates.