Evaluation of Two Ultrasonic Methods for Preparing Urea-Formaldehyde Adhesives with Nano-SiO2

This paper evaluates two methods for ultrasonic mixing of nano-SiO2 into urea-formaldehyde (UF) adhesives. Nano-silica (SiO2) was used in the form of 30% colloidal suspension, with the nanoparticles having a diameter of 7 nm. Its addition into the UF adhesive was 15%, based on the dry weight. The mixing of these components was performed with a probe-type ultrasonic homogenizer and with an ultrasonic bath, during 25 minutes of mixing time. The effects of each method were evaluated by SEM microscopy, and the results showed that the ultrasonic probe achieved much higher and almost complete homogenization of the UF / nano-SiO2 mixture, in contrast to the ultrasonic bath method. Therefore, the ultrasonic probe was subsequently used to test the influence of the mixing time on the homogenization level and on the curing kinetics of the UF adhesive with nano-SiO2. The homogenization level was not significantly altered when decreasing the mixing time from 25 to 5 minutes. For the same time range, the differential scanning calorimetry analysis also showed no significant influence of the mixing time on the curing behavior of the UF adhesive with nano-SiO2. The results of this research provide the basis for a more effective and efficient mixing procedure when using similar nanomaterials with UF adhesives, without significantly decreasing its performances.