Crystalline Lamellar Structure of Thermosetting Urea–Formaldehyde Resins at a Low Molar Ratio

Abstract

Reducing formaldehyde emission from cured urea–formaldehyde (UF) resins forced us to synthesize UF resins at a low formaldehyde to urea (F/U) molar ratio (≤1.0), resulting in low emission and poor adhesion by forming crystalline domains instead of building a three-dimensional network in cured resins. For the first time, we report a crystalline lamellar structure of thermosetting UF resins at a low molar ratio and compare it with highly amorphous UF resins at a high F/U molar ratio (1.6). Films of crystalline and amorphous UF resins were prepared using a spin-coater and characterized using atomic force microscopy. In addition, transmission electron microscopy, gel permeation chromatography, and X-ray diffraction were also employed for their characterization. The results showed that 1.0 UF resins had a low molecular weight with a lamellar structure of the crystalline domain, whereas 1.6 UF resins exhibited a high molecular weight with an amorphous structure and had a lower surface roughness. Fourier transform infrared and carbon-13 nuclear magnetic resonance spectroscopy also proved that linear molecules and hydrogen bonding were responsible for the formation of crystalline lamellar structures in thermosetting UF resins at a low molar ratio.