Morphological, chemical and crystalline features of urea-formaldehyde resin cured in contact with wood

The issue whether urea-formaldehyde (UF) resins possess crystallinity when cured in contact with wood has not been clearly resolved, although according to some published work low mole UF resins do not exhibit crystallinity in contact with wood. Here we provide evidence using field emission scanning electron microscopy (FE-SEM), Fourier transform infrared (FTIR) spectroscopy and wide angle X-ray diffraction (WAXD) that UF resin with a low formaldehyde/urea (F/U) mole ratio does crystallize when cured in contact with wood and show that crystals have various morphologies and sizes and thus can be regarded as polycrystals. Morphological characterization by FE-SEM revealed the presence of globular/nodular particles and crystalline structures. The particles occurred in two distinct populations with respect to their size, and the fractured faces of particles and their associated plate-like structures revealed presence of sub-particles of extremely small size (26.3-56.1 nm size range). Some crystalline regions of the resin contained expanded fan-shaped crystals, representing mature crystals. Other crystalline regions displayed features suggestive of emergence of crystals from globular particles. Transmission electron microscopy (TEM) made it possible to obtain electron diffraction (ED) pattern by analyzing directly the resin contained within cell lumens, which also suggested resin crystallization into polycrystals. The micromorphological information presented, particularly on nano-substructures and crystalline features, is novel and provides greater understanding of the architecture and crystalline characteristics of UF resin cured in contact with wood. Furthermore, information included provides evidence of a close relationship of emerging crystals with globular particles. FTIR spectra demonstrated that a quantitative calculation of the methylene linkages formed by cross-linking of UF resins in contact with wood has been advanced in its cure and crystalline structures formed in the cured UF resin. WAXD results also confirmed a polycrystalline nature of UF resin cured in contact with wood. These results corroborate that a low formaldehyde thermosetting UF resin cured in the presence of wood possesses distinct crystalline structures, an aspect of UF resin behavior that remained poorly understood and needed clarification. However, presence of crystallinity, albeit not widespread, may negatively impact cohesive strength of the resin, leading to a poor adhesion performance.