Improving the understanding of wood bonding: Behavior of different adhesives on the surface of eucalyptus and pine wood

Abstract This work aimed to determine the contact angle of adhesives in eucalyptus and pine wood and the influence on bonding, besides evaluating the wood-adhesive interface of glued joints with different adhesives and weights. Wood samples of two different species (Eucalyptus spp. and Pinus spp.) and five types of adhesives (polyvinyl acetate - PVA; sodium silicate - SS; resorcinol-formaldehyde - RF; bio-content polyurethane based on castor oil - BP; and modified silicate - MS) were used. For the contact angle determination, approximately 35 μL of each adhesive was inserted on the surface of the wood and the drops were photographed at five intervals after contact with the substrate. For the glue line determination, wood sheets with dimensions of 40 cm long x 10 cm wide x 0.6 cm thick were glued using three weights. Then slides were made and the images were captured using the Axio-Vision software. For the evaluation of the wood-adhesive interface, a scanning electron microscope was used. The mean values of contact angles ranged from 28.2 to 87.9° for eucalyptus wood and from 26.0 to 86.9° for pine wood. The smallest contact angles were obtained with the BP adhesive and the largest with the PVA adhesive. For eucalyptus wood, the lowest thickness of the glue line was 25.9 μm (SS adhesive) and the largest was equal to 149 μm (MS adhesive). For pine wood, the smallest thickness was 3.0 μm (BP) and the largest was 128 μm (PVA). Regardless of the wood used, the contact angle of the adhesives decreased with increasing time. The viscosities of the adhesives influenced the contact angle and the wettability of the wood. BP, PVA, and RF adhesives penetrated the wood, filling cell cavities adjacent to the main glue line.