Urea–Formaldehyde Synthesis with Kraft Lignin Nanoparticles and Effects on Formaldehyde Emissions

To seek improvements in the quality and sustainability of adhesives for bonding wood, the objective of this work was to use pine and eucalyptus kraft lignin nanoparticles in the synthesis of urea–formaldehyde adhesives. The rheological properties, shear strength of glued joints and formaldehyde emission reduction were evaluated. The lignin nanoparticles were produced from pine and eucalyptus kraft lignins, and two milling conditions were evaluated, in which the material was passed through the mill 5 and 10 times. The adhesives were synthesized by adding 0, 2, 4, 6, 8 and 10% lignin nanoparticles, replacing the total mass of urea. The treatments with better performances in relation to the control were submitted to Fourier transform infrared (FTIR) analysis and quantification of formaldehyde emissions. The lignin nanoparticle addition promoted an increase in the viscosity and pH of the adhesives. The treatments with 4% eucalyptus lignin nanoparticles (E5P 4%) and 6% pine lignin nanoparticles (P5P 6%) increased the shear strength by 10% and 13% in the dry and wet conditions, respectively. At higher replacement levels, there was a decrease in resistance. An increase in the intensity of the peaks at 1240 cm−1 (C-N stretching and angular deformation of NH2) and 1031 cm−1 (stretching of C-N bonds or N-C-N of methylene-ether bonds) was observed. The formaldehyde emissions of selected adhesives were reduced by up to 75%. The E5P 4% and P5P 6% adhesives had the best performances. Lignin nanoparticles in the urea–formaldehyde adhesive promoted improvements in the quality of the adhesive and replacement of urea by a renewable resource. Another highlight is the sustainability, due to the reduction in formaldehyde emissions obtained in adhesives produced with lignin nanoparticles.