Epoxy Adhesives Modified With Nano- and Microparticles for In Situ Timber Bonding: Effect of Environment on Mechanical Properties and Moisture Uptake

The environmental stability of three room temperature cure epoxy adhesives was evaluated following exposure to temperatures of $20°C$⁠, $30°C$⁠, and $50°C$ at 95%RH, to $50°C$ in air and soaked in water for up to 90 days. The adhesives contained nano- and microparticles and were especially formulated for bonded-in timber connections, and the properties of bulk adhesives and adhesively bonded block shear specimens were evaluated. After 90 days of aging the results demonstrate critical temperature effects controlled by the glass transition temperature. The apparent free volume for all the adhesives remains constant as moisture is absorbed but plasticization takes place at high temperature and relative humidity, evidenced by the increased elongation and yield observed by strain values and scanning electron microscope. Exposure at $50°C$ in air causes the adhesives to postcure enhancing strength but high humidity causes degradation. Nanofiller additions enhance environmental stability but the addition of microparticles provides better moisture resistance.