(449a) Applications of 2D and On-Line NMR Spectroscopy in Elucidation and Reaction Kinetic Studies of the Synthesis of Urea-Formaldehyde Resins

The reaction of formaldehyde with urea provides urea-formaldehyde resins (UF resins) which are essential binding agents for the production of wood composite products like plywood and fiber boards. Despite the economic importance of UF resins, the complex reaction network occurring upon their synthesis is still far from being fully understood. The aim of this work was to obtain new insight and quantitative information on that reaction network by using 2D and on-line NMR spectroscopic techniques.

The synthesis of UF resins is usually carried out in a two-step process: first formaldehyde is added to urea at high pH, secondly, the condensation is carried out, at lower pH. Condensation pathways involving formation of methylene bridges are predominant in acidic medium but it is conventionally believed that competing condensation reactions occurring via the formation of ether bridges also take place in small amount at this pH-range. Ether-bridged condensation products have not yet been clearly proved and their existence is a controversial issue in the literature. Knowledge on the reaction mechanisms is essential for the product quality, as cleavage of ether bridges will release free formaldehyde. In the present study, a proof of the existence of ether bridges was given for the first time. This was achieved by combining 2D NMR spectroscopy techniques (¹H-{¹⁵N} gHSQC, ¹H-{¹³C} gHSQC and ¹H-{¹³C} gHMBC) on UF resin samples with 2D NMR spectroscopy (¹³C,¹³C) gCOSY on ¹³C labeled urea-formaldehyde samples. An ether-bridged condensation product was characterized and the ¹⁵N, ¹³C and ¹H chemical shifts associated to the ether bridge were assigned.

On-line NMR spectroscopy is a powerful analytical tool allowing qualitative and quan­tita­tive analysis of complex reaction mixtures. Non-invasive measurements can be carried out without changing temperature, pressure or sample composition. No dilution or addition of deuterated solvent is required. On-line NMR spectroscopy is suited both for elucidation and monitoring of reactions. The typical on-line set-up includes an external reactor directly coupled to an NMR spectrometer equipped with a flow cell. The temperature and the pH-value inside the reactor were controlled and recorded. Without any sample preparation the reaction mixture is pumped through the NMR flow cell, NMR spectra are recorded on flow and the mixture flows back to the reactor. This set-up is suited for studies of reactions with time constants down to about 5-10 minutes [1]. Reaction kinetics of the urea-formaldehyde system was extensively studied by recording on-line ¹H NMR spectra using the set-up presented above. Based on these data a model describing the complex reaction network was developed. The implementation of this model in Matlab resulted in a simulation tool which enables the optimization of the industrial process.

[1] Maiwald M., Steinhof O., Sleigh C., Bernstein M., Hasse H., Quantitative High-resolution Online NMR Spectroscopy in Pharmaceutical Process Monitoring; in: Wawer I., Diehl B., Holzgrabe G. (Eds.), NMR Spectroscopy in Pharmaceutical Analysis, Elsevier Science, Amsterdam, Netherlands, 471-490 (2008), ISBN 13-978-0444531735.