Proof of ether-bridged condensation products in UF resins by 2D NMR spectroscopy

Kibrik, Éléonore J.; Steinhof, Oliver; Scherr, Günter; Thiel, Werner R.; Hasse, Hans

doi:10.1007/s10965-013-0079-7

Cited by 11 publications

(18 citation statements)

References 52 publications

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“…Recently, Kibrik et al . provided evidence that these intermediates do exist in UF reaction mixtures. Literature is ambiguous on whether the formation of these components is favored under basic conditions during hydroxymethylation or under neutral to acidic conditions during condensation and curing.…”

Section: Chemical Reaction Systemmentioning

confidence: 99%

Quantitative and qualitative ¹H, ¹³C, and ¹⁵N NMR spectroscopic investigation of the urea–formaldehyde resin synthesis

Steinhof

Kibrik

Scherr

et al. 2014

Magnetic Reson in Chemistry

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Urea-formaldehyde resins are bulk products of the chemical industry. Their synthesis involves a complex reaction network. The present work contributes to its elucidation by presenting results from detailed NMR spectroscopic studies with different methods. Besides (1)H NMR and (13)C NMR, (15)N NMR spectroscopy is also applied. (15)N-enriched urea was used for the investigations. A detailed NMR signal assignment and a model of the reaction network of the hydroxymethylation step of the synthesis are presented. Because of its higher spectral dispersion and the fact that all key reactions directly involve the nitrogen centers, (15)N NMR provides a much larger amount of detail than do (1)H and (13)C NMR spectroscopy. Symmetric and asymmetric dimethylol urea can be clearly distinguished and separated from monomethylol urea, trimethylol urea, and methylene-bridged urea. The existence of hemiformals of methylol urea is confirmed. 1,3,5-Oxadiazinan-4-on (uron) and its derivatives were not found in the reaction mixtures investigated here but were prepared via alternative routes. The molar ratios of formaldehyde to urea were 1, 2, and 4, the pH values 7.5 and 8.5, and the reaction temperature 60 °C.

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Section: Chemical Reaction Systemmentioning

confidence: 99%

Quantitative and qualitative ¹H, ¹³C, and ¹⁵N NMR spectroscopic investigation of the urea–formaldehyde resin synthesis

Steinhof

Kibrik

Scherr

et al. 2014

Magnetic Reson in Chemistry

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“…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.…”

Section: Ether Bridgesmentioning

confidence: 98%

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

Steinhof

Scherr

Hasse

2015

Magnetic Reson in Chemistry

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Quantitative on-line NMR spectroscopy is applied to study equilibria and reaction kinetics of the reaction of formaldehyde with 1,3-dimethylurea. This reaction system serves as a model system for the much more complex but industrially relevant urea-formaldehyde system. The aim is to study individual reactions and intermediates. The 1,3-dimethylurea-formaldehyde system undergoes only four reactions and, unlike urea-formaldehyde, does not form polymers. The following reactions are studied in detail: (1) the hydroxymethylation, (2) the formation of hemiformals of the hydroxymethylated intermediate, and (3) two condensation reactions of which the first leads to methylene bridges, the other to ether bridges. NMR spectroscopic chemical shift data of the reacting species are provided for the (1) H, (13) C, and (15) N domains. Equilibrium data of reactions (1), (2), and (3) are determined by quantitative (1) H and (13) C NMR spectroscopy at molar ratios of formaldehyde to 1,3-dimethylurea between 1:2 and 16:1 at a pH value of 8.5. Reaction kinetic experiments using an NMR spectrometer coupled to a batch reactor led to a reaction kinetic model parametrized with true species concentrations. The model takes into account reactions (1), (2), and (3). It describes the reaction system well for molar ratios of 1:1, 2:1, and 4:1, temperatures of 303 to 333K, and pH values from 5.0 to 9.5. Dilution experiments with a micro mixer coupled to the NMR spectrometer are conducted to estimate the time to equilibrium of reaction (2) of which the time constant is significantly lower than those of reactions (1) and (3). Copyright © 2015 John Wiley & Sons, Ltd.

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“…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].…”

Section: Procedures For Quantitative 13 C-nmr Determinationmentioning

confidence: 99%

“…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.…”

Section: Introductionmentioning

confidence: 99%

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

Wang

Cao

et al. 2018

Polymers

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Abstract:The structural changes during three-step synthesis of low-molar ratio urea-formaldehyde (UF) resin were tracked by quantitative 13 C nuclear magnetic resonance ( 13 C NMR) analysis and electrospray ionization-mass spectrometry (ESI-MS). Condensations that produced polymers were found to be linked by ether bonds in addition to hydroxymethylation reactions at the first alkaline stage with a formaldehyde to urea ratio of 2:1. Considerable formation of branched methylene linkages, with the highest content among all the condensed structures, was the key feature of the acidic stage. Notable changes were observed for the chemical structures and molecular masses of the resin components after the formaldehyde to urea molar ratio was lowered to 1.2 by adding post-urea at the final alkaline stage. Specifically, most of the branched hydroxymethyl groups on the polymers were cleaved, resulting in a significant decrease in the branching degree of the polymers. The performance degradation of the UF resin was attributed to this debranching effect and the production of components with low molecular masses. Based on the observations, the curing pattern of low molar ratio UF resin was postulated and branched polymeric formaldehyde catcher bearing urea-reactivity was proposed.

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Proof of ether-bridged condensation products in UF resins by 2D NMR spectroscopy

Cited by 11 publications

References 52 publications

Quantitative and qualitative ¹H, ¹³C, and ¹⁵N NMR spectroscopic investigation of the urea–formaldehyde resin synthesis

Quantitative and qualitative ¹H, ¹³C, and ¹⁵N NMR spectroscopic investigation of the urea–formaldehyde resin synthesis

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

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

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Proof of ether-bridged condensation products in UF resins by 2D NMR spectroscopy

Cited by 11 publications

References 52 publications

Quantitative and qualitative 1H, 13C, and 15N NMR spectroscopic investigation of the urea–formaldehyde resin synthesis

Quantitative and qualitative 1H, 13C, and 15N NMR spectroscopic investigation of the urea–formaldehyde resin synthesis

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

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

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Quantitative and qualitative ¹H, ¹³C, and ¹⁵N NMR spectroscopic investigation of the urea–formaldehyde resin synthesis

Quantitative and qualitative ¹H, ¹³C, and ¹⁵N NMR spectroscopic investigation of the urea–formaldehyde resin synthesis