Effects of acid hydrolysis on microstructure of cured urea-formaldehyde resins using atomic force microscopy

This study investigated the effect of acid hydrolysis on the microstructure of cured urea-formaldehyde (UF) resins using atomic force microscopy (AFM) to better understand its hydrolytic degradation process which has been known to be responsible for the formaldehyde emission of wood-based composite panels. The AFM was scanned on both outer surface and facture surfaces of the thin films of cured UF resins that had been exposed to the etching of dilute hydrochloric acid to simulate their hydrolysis process. The AFM images showed two distinctive parts, which were classified as the hard and soft phases in cured UF resins. For the first time, this study reports the presence of thin filament-like crystalline structures on the fracture surface of cured UF resin. The soft phase of cured UF resins by ammonium chloride was much more easily hydrolyzed than those cured by ammonium sulfate, indicating that hardener types had a great impact on the hydrolytic degradation behavior of cured UF resins. The surface roughness measurement results also supported this result. The results of this study suggested that the soft phase was much more susceptible to the hydrolysis of cured UF resin than the hard phase.