MUF resin incorporating SiO2 and TiO2 nanoparticles: characterization and performance as a plywood adhesive

To overcome the defects of longer curing times and higher curing temperatures for melamine urea formaldehyde resins (MUF) used in wood panel products, nanofluid MUF resin adhesives were prepared by dispersing SiO₂ and TiO₂ nanoparticles into the MUF resin at six concentration levels based on the overall mass of the adhesive, 0%, 0.2%, 0.4%, 0.6%, 0.8% and 1%, with the assistance of ultrasound. The excellent thermal conductivities of the nanofluid adhesives eliminated the defects of the MUF resins. When the SiO₂ and TiO₂ nanoparticles were incorporated, the viscosity increased as the nanoparticle concentration increased, the maximum viscosity increased by 13.4% with SiO₂ nanoparticles (1.0% level) and 11.4% with TiO₂ nanoparticles (1.0% level), the pot life, curing time, and free formaldehyde content of the MUF resins declined to varying degrees as the nanoparticle loading level was increased, the pot life maximum decreased by 38.5% with SiO₂ nanoparticles (0.2% level) and 36.0% with TiO₂ nanoparticles (0.2% level), the maximum reduction in curing time was 13.3% with SiO₂ nanoparticles (0.4% level) and 13.6% with TiO₂ nanoparticles (0.6% level), and the maximum content of free formaldehyde was reduced by 32.3% with SiO₂ nanoparticles (1.0% level) and 41.9% with TiO₂ nanoparticles (1.0% level). The nanofluid MUF resins showed much higher bonding strengths and lower formaldehyde emissions than pure MUF resins after being treated with the same hot pressing method used during plywood manufacturing. The maximum bonding strength was increased by 28.8% with the SiO₂ nanoparticles (1.0% level) and 25.4% with the TiO₂ nanoparticles (1.0% level) at a hot-pressing temperature of 100 °C and a hot-pressing time of 60 s/mm. The maximum formaldehyde emissions were reduced by 46.3% with the SiO₂ nanoparticles (1.0% level) and 46.3% with the TiO₂ nanoparticles (1.0% level) at a hot-pressing temperature of 110 °C and a hot-pressing time of 40 s/mm. Furthermore, nanofluid MUF resins used in plywood manufacturing decreased the hot-pressing temperature or shortened the hot-pressing time required.