Urea formaldehyde (UF) resins are widely used thermosetting polymers in adhesives, finishes, molded objects, etc. The existence of crystalline regions has been detected in UF resins. These crystalline regions are believed to be beneficial for the hydrolytic stability or water resistance and advanced mechanical properties of the resin. In this study, characterization was conducted on crystalline regions of a cured UF resin with a formaldehyde to urea (F/U) molar ratio of 1.2. A slow scanned X-ray diffraction (XRD) pattern was obtained to estimate the crystalline percentage, grain sizes, and interplanar spacing (d-spacing) of the resin crystalline structures. The results showed that the crystalline regions accounted for nearly 14.48% of the resin. From the XRD pattern, the estimated grain size of 4.1 nm was accounted for the peak (two theta degrees, 2θ) of 21.55°, 4.1 nm for 24.35°, 4.2 nm for 31.18°, and 4.8 nm for 40.43°, respectively. Furthermore, a selected area electron diffraction (SAD) pattern of the resin was obtained to confirm the results. The calculated d-spacing values were 2.2242 Å for the peak (2θ) of 21.55°, 1.2833 Å for 24.35°, and 1.0978 Å for 31.18°, respectively. The obtained SAD pattern matched the corresponding XRD pattern of a UF resin. This work provides information for studying the mechanism of formation and the application of crystalline regions of UF resins.

Reactions between CaCO ₃ and CH ₂ O ₂ during polycondensation of UF resin produce Ca ²⁺ . Ionic bond complexation binds Ca ²⁺ with UF resin. The UF resin crystalline percentage decreases from 26.86% to 22.71%. IB strength of resin bonded fiberboard increases from 0.75 to 0.94 MPa.