On-Line NMR Spectroscopic Reaction Kinetic Study of Urea–Formaldehyde Resin Synthesis

Abstract

Quantitative on-line NMR spectroscopy is used to study the kinetics of the reaction of aqueous formaldehyde and urea. The investigation focuses on the formation of low molecular mass compounds during the methylolation step. The experiments were carried out at overall formaldehyde to urea molar ratios between 1:1 and 4:1, pH values between 6 and 8, and temperatures between 313 and 353 K. The experimental data were used to develop a kinetic model based on the true species concentrations. The model describes the experimental data well and can be used to predict the composition of the reacting mixture of aqueous formaldehyde and urea during the methylolation step as a function of time.

Characterization of the Low Molar Ratio Urea–Formaldehyde Resin with 13C NMR and ESI–MS: Negative Effects of the Post-Added Urea on the Urea–Formaldehyde Polymers

“…For each sample, a 400 µL liquid UF sample was directly mixed with 100 µL of dimethyl sulfoxide (DMSO-d6 ) for 13 C NMR determination. The spectra were recorded with a pulse angle of 90 degrees (12 µs).…”

“…The spectra were recorded at 150 MHz with 400-600 scans accumulated. The observed chemical shifts were assigned by referring to the assignments in the literature [3][4][5][6][7][8][9][10][11][12][13][14][15].…”

“…After the acidic condensation, the pH is adjusted back to alkaline and the additional urea (the second urea) is added to lower the F/U molar ratio to 0.9-1.2, aiming to capture the un-reacted formaldehyde. Although intensive studies have been completed to investigate the chemical structures of resin polymers by employing nuclear magnetic resonance d(NMR) analysis [3][4][5][6][7][8][9][10][11][12][13][14][15], our survey of the literature showed that the effects of post-added urea on the UF polymers were not focused or well-discussed. However, this issue is important for better understanding the relationship between low molar ratio and poor performance of the resin.…”

“…However, this issue is important for better understanding the relationship between low molar ratio and poor performance of the resin. In this study, quantitative 13 C NMR determinations were performed to track the typical three-step synthesis of UF resin, aiming to determine the changes in the chemical structures occurring after additional urea was added.…”

“…In this study, this technique was introduced to track the changes in the molecular mass distribution of UF resin components during synthesis and to investigate the influences of the urea added at the final alkaline stage. By combining the results of 13 C NMR and ESI-MS, the mechanism of the low F/U ratio that results in the poor performance of UF resin can be better understood.…”