As a part of abating formaldehyde emission of wood-based composite panels bonded with urea-formaldehyde (UF) resin adhesives, this study investigated hydrolytic stability and microstructure of cured UF resins, using acid hydrolysis, x-ray diffraction (XRD), field-emission-scanning electron microscopy (FE-SEM), energy dispersive x-ray analysis (EDXAj, and atomic force microscopy (AFM). As the formaldehyde/urea (F/U) molar ratio decreased, the hydrolytic stability of cured UF resins increased. UF resins with lower F/U mole ratios (1.2 or 1.0) showed additional crystalline peaks and spherical structures. The EDXA result showed that as ammonium chloride content increased, the amount of carbon and oxygen decreased, but that of the chloride increased. The EDXA also revealed the presence of three different types of chlorides in cured UF resins, and indicated the distribution of chloride as hydrochloric acid in cured UF resin. The analysis of cured UF resins using AFM also showed hard and soft phases. The AFM evaluation of the film surface of cured UF resins showed that as the etching time increased, the surface roughness and hard-phase area decreased and then increased. The fracture surface showed different trends depending on hardener type. These results partially explain the processes of hydrolytic degradation of cured UF resins, which is responsible for the formaldehyde emission of UF resin-bonded wood panel products.
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