Comparative study on the properties of cellulose nanofiber (CNF) and cellulose nanocrystals (CNC) reinforced 1C-PUR adhesive bonded wooden joints

Polyurethane (PUR) adhesive was reinforced with cellulose nanocrystal (CNC) and acetylated cellulose nanofiber (CNF) to enhance its bonding performance. The reinforced adhesive was used to prepare lap joints using spruce and beech wood. Cyclic moisture exposure was carried out to assess the effect of moisture variation on the bond performance. Elastic modulus and shear strength were determined to ascertain the bonding behavior of adhesives. Reinforcement of PUR with nanocellulose significantly improved (33%) the elastic modulus in spruce wood while the improvement was only marginal (12%) in beech wood. Cyclic moisture exposure reduced the elastic modulus of CNC reinforced PUR in both the species, while the value increased by 5% for 0.5% CNF reinforced PUR joints. Reinforcement of PUR with nanocellulose significantly improved the shear strength. The improvent was more in spruce (45%) as compared to beech wood (35%). Moisture exposure caused a significant improvement in the shear strength of pure and reinforced PUR bonded joints, however, the values decreased for spruce wood except for 0.5% CNF loading where it was marginally higher. SEM micrographs revealed improved bond line with nanocellulose reinforcement in both the wood species. The glass transition temperature of PUR adhesive slightly decreased due nanocellulose reinforcement.