Urea‐formaldehyde resins. III. Constitutional characterization by ¹³C fourier transform NMR spectroscopy

Abstract

13C Fourier transform NMR has been used to characterize a random chemical structure of ureaformaldehyde resins. By comparison of 13C chemical shifts with synthesized standard derivatives from urea and formaldehyde the analysis of reacted formaldehyde was completed. In a 13C spectrum of resin each signal due to reacted formaldehyde (e.g., methylol group, methylene group, and dimethylene ether group) was isolated. Measurement of a 13C spectrum of resin by the gated decoupling of proton without nuclear Overhauser effect made a quantitative analysis of reacted formaldehyde possible. In this quantitative analysis a 13C quantity of carbonyl groups in urea residue can be directly compared with that of each combined formaldehyde.

The curing of UF resins studied by low-resolution1H-NMR

“…This can be seen quite clearly in Figure 3, which shows the 13 C CPMAS NMR spectrum of the UF8 resin with a mol ratio of 1.03 cured at 75°C. The main chemical groups present in the formaldehyde area are due to linear methylene bridges (47 ppm) and branched methylene bridges (54 ppm).…”

“…Using this, the correlation of chemical groups with curing behavior could be investigated. The results of the 13 C-NMR analysis for resins UF1-5 are given in Table I. Also included is the analysis for an "aged" resin, where aged means that the resin has been at room temperature for 6 weeks.…”

“…Five of these, denoted as UF1-5, were characterized by high-resolution 13 C-NMR to obtain the different chemical groups present in each resin. This method of characterisation has been described elsewhere, 12,13 and gives the distribution of the formaldehyde into the different chemical groups present using the peaks in the area 40 to 100 ppm. The percentage of free urea of the total urea present can also be calculated from the carbonyl area around 150 to 165 ppm.…”

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

“…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.…”

“…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].…”