¹³C‐NMR, ¹³C‐¹³C gCOSY, and ESI‐MS characterization of ether‐bridged condensation products in N,N′‐dimethylurea‐formaldehyde systems

Abstract

The reaction of urea with formaldehyde is the basis for the production of urea‐formaldehyde (UF) resins which are widely applied in the wood industry. The presence of ether‐bridged condensation products in the UF resin reaction system is an open question in the literature. It is addressed in the present work. The N,N′‐dimethylurea‐formaldehyde model system was studied since it is chemically similar to the UF resin reaction system but allows for a simple elucidation of all reaction products. It was analyzed by 13C‐NMR spectroscopy and ESI‐MS. In corresponding NMR and MS spectra, peaks due to methoxymethylenebis(dimethyl)urea and its hemiformal were observed. 13C‐13C gCOSY analysis was conducted using labeled 13C‐formaldehyde. The correlation spectra showed evidence for an ether‐bridged compound and mass spectra exhibited peaks agreeing with labeled methoxymethylenebis(dimethyl)urea and its hemiformal. Methoxymethylenebis(dimethyl)urea was characterized in N,N′‐dimethylurea‐formaldehyde systems in acidic and slightly basic media. As urea is very similar to N,N′‐dimethylurea, the results of this work strengthen the assumption that ether‐bridged condensation products are likely to form in UF resins. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Investigation of the reaction of 1,3‐dimethylurea with formaldehyde by quantitative on‐line NMR spectroscopy: a model for the urea–formaldehyde system

“…Chemical shifts of the species identified in the reaction system 1,3-dimethylurea-formaldehyde, Part I (hydroxymethylation products). The chemical shifts of formaldehyde FA and the methyleneglycols MG 1:::10 can be found elsewhere [4,14,15] [23,24] Because of this evidence, the formation of ether bridges has been taken into account in the present study in the reaction kinetic model. More details on the indications supporting the formation of ether bridges as found during the course of the present work are provided in the Supplementary Information.…”

“…The reaction kinetic constants k C 5:::14 of the forward reactions and the equilibrium constants K 5:::14 describing the formaldehyde-water section of the model were taken from the work of Hahnenstein. [15,36] For the hydration of formaldehyde, Hahnenstein used the empirical approach of Schecker and Schulz [37] to estimate k C 5 in dependence of temperature and pH value, as given by Eqn (23). The parameters C 3 , C 4 , C 5 , A 2 , and B 2 are listed in Table 6.…”

“…R is the number of Table 5. Parameters to estimate the rate constant k C 5 of reaction R5 using Eqn (23) to describe the hydration of formaldehyde. From Schecker and Schulz [37] Parameter …”

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].…”