Using Kraft lignin, bio-based adhesives have been increasingly studied to replace those petrochemical-based solutions, due to low cost, easy availability and the potential for biodegradability of this biomaterial. In this study, lignin-based phenol-formaldehyde (LPF) resins were synthesized using commercial Eucalypt Kraft Lignin (EKL), purified at 95%, as a phenol substitute in different proportions of 10%, 20%, 30% and 50%. The properties of bio-based phenol formaldehyde (BPF) synthesized resin were compared with phenol-formaldehyde resin (PF) used for control sampling. The results indicated that viscosity, gel time and solid contents increased with the addition of pure EKL. The shear strength test of glue line was studied according to American Society for Testing and Materials (ASTM), and BPF-based results were superior to samples bonded with the PF as a control sample, being suitable for structural purposes. Changes in the curing behavior of different resins were analyzed by Differential Scanning Calorimetry (DSC), and sample comparison indicated that the curing of the LPF resin occurred at lower temperatures than the PF. The addition of EKL in PF reduced its thermal stability compared to traditional resin formulation, resulting in a lower decomposition temperature and a smaller amount of carbonaceous residues.

Wood industries use thermal and thermomechanical treatments as ecological approaches to increase the durability of wood products, avoiding the need for chemical additives. In this regard, the aim of this study was to compare the physical and mechanical properties of plywood made from veneers treated at different temperatures using thermal and thermomechanical processes, with untreated panels serving as a control. The treatment process involved Pinus taeda veneers submitted to treatment in a hot press at 1.0 MPa in a laboratory oven at temperatures of 160 °C, 180 °C, and 200 °C for 30 min. For bonding the veneers, a vegetable-based polyurethane resin derived from castor oil with a grammage of 395 g/m² was used, applying pressing conditions at 90 °C, 0.6 MPa, and 10 min. Our results indicate that temperature significantly influences plywood properties, playing a key role in the choice of equipment for the treatment process. Regardless of the method employed, the treatment resulted in an improvement in the hydrophobicity of the veneers due to the decrease in hemicellulose content. Notably, the reduction in strength and stiffness caused by the loss of cell wall polymers was not statistically significant. The treatment was successful in softening the wood material, reducing roughness, and increasing wettability. Despite a minimum of 20% reduction in glue line tension, the samples still surpassed the 1 MPa mark, showing satisfactory results. This demonstrates the feasibility of adjusting treatment variables to ensure the proper use of this adhesive.