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.
Key wo reis Urea formaldehyde resin Reaction pH Molar ratio Molecular weight I3 C NMR spectra Bond strength . Formaldehyde emission
SummaryUrea formaldehyde resins were formulated with combination variables of three reaction pH (1.0, 4.8, and 8.0) and four molar ratios of formaldehyde to urea (2.5, 3.0, 3.5, and 4.0). The resins were prepared by placing all formaldehyde and water in reaction kettle and pH was adjusted with sulfuric acid and sodium hydroxide, respectively. Urea was added in 15 equal parts at 1-minute intervals. The proportion of high molecular weight products in the resin increased substantially äs the reaction pH decreased. Furthermore, the F/U molar interacted with reaction pH to effect resin molecular weight. At acidic pH, the high molecular weight products increased äs F/U ratio decresed; while, at alkaline pH, little difference was evident between the high or low molecular weight products at various F/U ratios. The formation of a high percentage of uron derivatives under strong acidic conditions also indicated these resins differed considerable from conventional UF resins formulated in the past. Panels bonded with resins catalyzed at strong acidic conditions resulted in lowest formaldehyde emission but slightly lower bonding strength. Of the three pH conditions evaluated in the study, both weak and strong acid catalysts Systems are not commonly used in conventional UF resin formulation. Based on the bond strength and formaldehyde emission data, however, the weak acid catalysts seems to provide the best compromise between the strong acid and the conventional alkaline-acid catalyst System currently used for formulating UF resin wood adhesives.
Procedure
Resin preparationAll UF resins were prepared in the laboratory. Each resin preparation was replicated one time. To prepare each resin, all formaldehyde and water were placed in a reaclion kettle and pH was adjusted with sulfuric acid and sodium hydroxide, respectively.Holzforschung / Vol. 48 / 1994 / No. 6
The random chemical structures of melamine–formaldehyde resins, including methylated melamine–formaldehyde resins and urea–melamine formaldehyde resins, were investigated by 13C‐NMR spectroscopy (Fourier transform). All the combined formaldehydes, methylol and methyl ether groups, methylene structures, and dimethylene ether structures were assigned. A 13C chemical shift of methylene carbon occurred by substitution of other constituents of the methylene group for a proton of the adjacent monosubstituted nitrogen atom, as shown in a 13C‐NMR spectrum of urea–formaldehyde resins. It was found that the chemical shift of each corresponding carbon of both melamine resins and urea–melamine resins was almost superimposed with that of urea resins.
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