Cellulose fibers and nanofibrils for adhesive reinforcement

Introduction In wood industry, adhesive bonding is one of the key process steps. To ensure continuing technological progress in wood industry it is necessary to explore new possibilities of improvement in adhesive bonding. The potential routes for performance improvements suggested by a state of the art report issued by COST Action E13 Wood Adhesion and Glued Products (Dunky et al. 2002) focus very much on polymer chemistry, where tremendous improvements have been achieved in past decades. Based on the results achieved in own preliminary experiments, the authors of the present report think that also another well known component of adhesives in wood bonding, usually addressed under the term “filler”, deserves more attention. As is well known from the science of composite materials (e.g. Chawla 1987), the addition of fibrous fillers significantly changes the physical properties of a polymer. Consequently, a large number of studies dealing with the reinforcement of polymers by various types of fibers (e.g. glass fibers, carbon fibers or polymeric fibers) can be found in literature. In contrast, for adhesive bonding of wood, literature on true reinforcing fillers is very scarce. A recently published study by Richter et al. (2009) reports for the first time on wood adhesives reinforced with cellulose nanofibrils. Two different adhesives, based on polyvinyl acetate (PVAc) and polyurethane (PUR) respectively, were considered. Varying amounts of surface-modified fibrils were added to the adhesive and woodadhesive assemblies bonded with these modified adhesives were tested. In spite of significant changes observed for cellulose reinforced PVAc composite films characterised in a different experiment in the same study, no significant effects of cellulose nanofibrils on wood-adhesive shear strength were observed neither for PUR nor for PVAc.